Manabe, Syukuro, and Anthony J Broccoli, January 2020: Beyond Global Warming: How Numerical Models Revealed the Secrets of Climate Change, Princeton, NJ: Princeton University Press, 193pp.
Successful projection of the distribution of surface temperature change increases our confidence in climate models. Here we evaluate projections of global warming from almost 30 years ago using the observations made during the past half century.
Tsushima, Y, and Syukuro Manabe, May 2013: Assessment of radiative feedback in climate models using satellite observations of annual flux variation. Proceedings of the National Academy of Sciences, 110(19), DOI:10.1073/pnas.1216174110. Abstract
In the climate system, two types of radiative feedback are in operation.
The feedback of the first kind involves the radiative damping
of the vertically uniform temperature perturbation of the troposphere
and Earth’s surface that approximately follows the Stefan–
Boltzmann lawof blackbody radiation. The second kind involves the
change in the vertical lapse rate of temperature, water vapor, and
clouds in the troposphere and albedo of the Earth’s surface. Using
satellite observations of the annual variation of the outgoing flux of
longwave radiation and that of reflected solar radiation at the top
of the atmosphere, this study estimates the so-called “gain factor,”
which characterizes the strength of radiative feedback of the second
kind that operates on the annually varying, global-scale perturbation
of temperature at the Earth’s surface. The gain factor is computed
not only for all sky but also for clear sky. The gain factor of socalled
“cloud radiative forcing” is then computed as the difference
between the two. The gain factors thus obtained are compared with
those obtained from 35 models that were used for the fourth and
fifth Intergovernmental Panel on Climate Change assessment. Here,
we show that the gain factors obtained from satellite observations
of cloud radiative forcing are effective for identifying systematic
biases of the feedback processes that control the sensitivity of simulated
climate, providing useful information for validating and improving
a climate model.
Fu, Qiang, Syukuro Manabe, and C M Johanson, August 2011: On the warming in the tropical upper troposphere: Models versus observations. Geophysical Research Letters, 38, L15704, DOI:10.1029/2011GL048101. Abstract
IPCC (Intergovernmental Panel on Climate Change) AR4 (Fourth Assessment Report)
GCMs (General Circulation Models) predict a tropical tropospheric warming that increases with
height, reaches its maximum at ~200 hPa, and decreases to zero near the tropical tropopause.
This study examines the GCM-predicted maximum warming in the tropical upper troposphere
using satellite MSU (microwave sounding unit)-derived deep-layer temperatures in the tropical
upper- and lower-middle troposphere for 1979-2010. While satellite MSU/AMSU observations
generally support GCM results with tropical deep-layer tropospheric warming faster than
surface, it is evident that the AR4 GCMs may exaggerate the increase in static stability between
tropical middle and upper troposphere during the last three decades.
Using the historical surface temperature data set compiled by Climatic Research Unit of University of East Anglia and Hadley Centre of UK, this study examines the seasonal and latitudinal profile of the surface temperature change observed during the last several decades. It reveals that the recent change in zonal mean surface air temperature is positive at practically all latitudes. In the Northern Hemisphere, the warming increases with increasing latitude and is large in the Arctic Ocean during much of the year except in summer, when it is small. At the Antarctic coast and in the northern part of the Circumpolar Ocean (near 55°S), where limited data are available, the changes appear to be small during most seasons, though the warming is notable at the coast in winter. This warming is, however, much less than the warming over the Arctic Ocean. The seasonal variation of the surface temperature change appears to be broadly consistent with the result from a global warming experiment, which was conducted some time ago using a coupled atmosphere-ocean-land model.
Based upon the results obtained from coupled ocean-atmosphere models of various complexities, this review explores the role of ocean in global warming. It shows that ocean can play a major role in delaying global warming and shaping its geographical distribution. It is very encouraging that many features of simulated change of the climate system have begun to agree with observation. However, it has been difficult to confirm the apparent agreement because the density and frequency of the observation are insufficient in many oceanic regions of the world, in particular, in the Circumpolar Ocean of the Southern Hemisphere. It is therefore essential to intensify our effort to monitor not only at the surface but also in the subsurface layers of oceans.
Tsushima, Y, A Abe-Ouchi, and Syukuro Manabe, March 2005: Radiative damping of annual variation in global mean surface temperature: comparison between observed and simulated feedback. Climate Dynamics, 24(6), DOI:10.1007/s00382-005-0002-y. Abstract PDF
The sensitivity of the global climate is essentially determined by the radiative damping of the global mean surface temperature anomaly through the outgoing radiation from the top of the atmosphere (TOA). Using the TOA fluxes of terrestrial and reflected solar radiation obtained from the Earth radiation budget experiment (ERBE), this study estimates the magnitude of the overall feedback, which modifies the radiative damping of the annual variation of the global mean surface temperature, and compares it with model simulations. Although the pattern of the annually varying anomaly is quite different from that of the global warming, the analysis conducted here may be used for assessing the systematic bias of the feedback that operates on the CO2 -induced warming of the surface temperature. In the absence of feedback effect, the outgoing terrestrial radiation at the TOA approximately follows the Stefan-Boltzmann's fourth power of the planetary emission temperature. However, it deviates significantly from the blackbody radiation due to various feedbacks involving water vapor and cloud cover. In addition, the reflected solar radiation is altered by the feedbacks involving sea ice, snow and cloud, thereby affecting the radiative damping of surface temperature. The analysis of ERBE reveals that the radiative damping is weakened by as much as 70% due to the overall effect of feedbacks, and is only 30% of what is expected for the blackbody with the planetary emission temperature. Similar feedback analysis is conducted for three general circulation models of the atmosphere, which was used for the study of cloud feedback in the preceding study. The sign and magnitude of the overall feedback in the three models are similar to those of the observed. However, when it is subdivided into solar and terrestrial components, they are quite different from the observation mainly due to the failure of the models to simulate individually the solar and terrestrial components of the cloud feedback. It is therefore desirable to make the similar comparison not only for the overall feedback but also for its individual components such as albedo- and cloud-feedbacks. Although the pattern of the annually-varying anomaly is quite different from that of global warming, the methodology of the comparative analysis presented here may be used for the identification of the systematic bias of the overall feedback in a model. A proposal is made for the estimation of the best guess value of climate sensitivity using the outputs from many climate models submitted to the Intergovernmental Panel on Climate Change.
By use of a coupled ocean-atmosphere-land model, this study explores the changes of water availability, as measured by river discharge and soil moisture, that could occur by the middle of the 21st century in response to combined increases of greenhouse gases and sulphate aerosols based upon the "IS92a" scenario. In addition, it presents the simulated change in water availability that might be realized in a few centuries in response to a quadrupling of CO2 concentration in the atmosphere. Averaging the results over extended periods, the radiatively forced changes, which are very similar between the two sets of experiments, were successfully extracted. The analysis indicates that the discharges from Arctic rivers such as the Mackenzie and Ob' increase by up to 20% (of the pre-Industrial Period level) by the middle of the 21st century and by up to 40% or more in a few centuries. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra rivers increase substantially. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller, with both positive and negative signs. For soil moisture, the results of this study indicate reductions during much of the year in many semiarid regions of the world, such as the southwestern region of North America., the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. As a percentage, the reduction is particularly large during the dry season. From middle to high latitudes of the Northern Hemisphere, soil moisture decreases in summer but increases in winter.
It has been suggested that, unless a major effort is made, the atmospheric concentration of carbon dioxide may rise above four times the pre-industrial level in a few centuries. Here we use a coupled atmosphere-ocean-land model to explore the response of the global water cycle to such a large increase in carbon dioxide, focusing on river discharge and soil moisture. Our results suggest that water is going to be more plentiful in those regions of the world that are already `water-rich'. However, water stresses will increase significantly in regions and seasons that are already relatively dry. This could pose a very challenging problem for water-resource management around the world. For soil moisture, our results indicate reductions during much of the year in many semi-arid regions of the world, such as the southwestern region of North America, the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. In some of these regions, soil moisture values are reduced by almost a factor of two during the dry season. The drying in semi-arid regions is likely to induce the outward expansion of deserts to the surrounding regions. Over extensive regions of both the Eurasian and North American continents in high and middle latitudes, soil moisture decreases in summer but increases in winter, in contrast to the situation in semi-arid regions. For river discharge, our results indicate an average increase of ~ 15% during the next few centuries. The discharges from Arctic rivers such as the Mackenzie and Ob' increase by much larger fractions. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra also increase considerably. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller.
This study evaluates the equilibrium response of a coupled ocean-atmosphere model to the doubling, quadrupling, and halving of CO2 concentration in the atmosphere. Special emphasis in the study is placed upon the response of the thermohaline circulation in the Atlantic Ocean to the changes in CO2 concentration of the atmosphere. The simulated intensity of the thermohaline circulation (THC) is similar among three quasi-equilibrium states with the standard, double the standard, and quadruple the standard amounts of CO2 concentration in the atmosphere. When the model atmosphere has half the standard concentration of CO2, however, the THC is very weak and shallow in the Atlantic Ocean. Below a depth of 3 km, the model oceans maintain very thick layer of cold bottom water with temperature close to -2 °C, preventing the deeper penetration of the THC in the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere, sea ice extends beyond the Antarctic Polar front, almost entirely covering the regions of deepwater ventilation. In addition to the active mode of the THC, there exists another stable mode of the THC for the standard, possibly double the standard (not yet confirmed), and quadruple the standard concentration of atmospheric carbon dioxide. This second mode is characterized by the weak, reverse overturning circulation over the entire Atlantic basin, and has no ventilation of the entire subsurface water in the North Atlantic Ocean. At one half the standard CO2 concentration, however, the intensity of the first mode is so weak that it is not certain whether there are two distinct stable modes or not. The paleoceanographic implications of the results obtained here are discussed as they relate to the signatures of the Cenozoic changes in the oceans.
Davey, M K., M Huddleston, Kenneth R Sperber, P Braconnot, F O Bryan, D Chen, R Colman, C Cooper, U Cubasch, P Delecluse, D G DeWitt, L Fairhead, G M Flato, C Tony Gordon, T Hogan, M Ji, , A Kitoh, Thomas R Knutson, M Latif, H Le Treut, Tim Li, Syukuro Manabe, C R Mechoso, Gerald A Meehl, Scott B Power, E Roeckner, L Terray, A Vintzileos, R Voss, Bin Wang, W M Washington, I Yoshikawa, J-Y Yu, S Yukimoto, and S E Zebiak, 2002: STOIC: A study of coupled model climatology and variability in tropical ocean regions. Climate Dynamics, 18(5), 403-420. Abstract PDF
We describe the behavior of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose; nevertheless several common features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with interannual Niño 3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress.
Using the results obtained from a coupled ocean-atmosphere-land model with medium computational resolution, we investigated how the hydrology of the continents changes in response to the combined increases of greenhouse gases and sulfate aerosols in the atmosphere, which are determined based upon the IS92a scenario. In order to extract the forced response from natural, internal variability, the difference between the mean of an eight-member ensemble of numerical experiments and a control experiment are used for the present analysis. The global mean surface air temperature of the coupled model increases by about 2.3°C above the preindustrial level by the middle of the 21st century. Accompanying the warming, the global mean rates of both precipitation and evaporation increase by 5.2%, yielding the average increase in the rate of runoff by approximately 7.3%. The increase in the rate of runoff simulated by the model is particularly large in high northern latitudes, where the runoff from some rivers such as the Mackenzie and Ob´ may increase by as much as 20%. Runoff from many European rivers increases by more than 20%. Runoff also increases substantially in some tropical rivers such as the Amazon and Ganges. However, the percentage changes in simulated runoff from many other tropical rivers and middle latitude rivers are smaller with both positive and negative signs. In middle and high latitudes in the Northern Hemisphere, soil moisture tends to decrease in summer, whereas it increases in winter. However, in many semi-arid regions in subtropical and middle latitudes, soil moisture is reduced during most of a year. These semi-arid regions include the southwestern part of North America, the northeastern part of China in the Northern Hemisphere, and the region in the vicinity of the Kalahari Desert and southern part of Australia in the Southern Hemisphere. Since a semi-arid region usually surrounds a desert, the reduction of soil moisture in such a region often results in the expansion of the desert. Soil moisture is also reduced during the dry season in many semi-arid regions. For example, it is reduced in the savannahs of Africa and South America during winter and early spring in the Southern Hemisphere. In the Northern Hemisphere, it is reduced at the Mediterranean coast of Europe in summer.
Abstract An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.
This lecture discusses the low-frequency variability of surface temperature using a coupled ocean-atmosphere-land-surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the coupled model.
Tsushima, Y, and Syukuro Manabe, 2001: Influence of cloud feedback on annual variation of global mean surface temperature. Journal of Geophysical Research, 106(D19), 22,635-22,646. Abstract PDF
The goal of this study is to estimate the cloud radiative feedback effect on the annual variation of the global mean surface temperature using radiative flux data from the Earth Radiation Budget Experiment. We found that the influence of the cloud feedback upon the change of the global mean surface temperature is quite small, though the increase of the temperature is as much as 3.3 K from January to July. On a global scale, we found no significant relationship between either solar reflectivity of clouds or effective cloud top height and the annual cycle of surface temperature. The same analysis was repeated using the output from three general circulation models, which explicitly predict microphysical properties of cloud cover. On a global scale, both solar cloud reflectivity and cloud top height increase significantly with the increase of surface temperature, in contrast to the observation. The comparative analysis conducted here could be used as an effective test for evaluating the cloud feedback process of a model.
Hall, A, and Syukuro Manabe, 2000: Effect of water vapor feedback on internal and anthropogenic variations of the global hydrologic cycle. Journal of Geophysical Research, 105(D5), 6935-6944. Abstract PDF
Using two versions of the GFDL coupled ocean-atmosphere model, one where water vapor anomalies are allowed to affect the longwave radiation calculation and one where they are not, we examine the role of water vapor feedback in internal precipitation variability and greenhouse-gas-forced intensification of the hydrologic cycle. Without external forcing, the experiment with water vapor feedback produces 44% more annual-mean, global-mean precipitation variability than the one without. We diagnose the reason for this difference: In both experiments, global-mean surface temperature anomalies are associated with water vapor anomalies. However, when water vapor interacts with longwave radiation, the temperature anomalies are associated with larger anomalies in surface downward longwave radiation. This increases the temperature anomaly damping through latent heat flux, creating an evaporation anomaly. The evaporation anomaly, in turn, leads to an anomaly of nearly the same magnitude in precipitation. In the experiment without water vapor feedback, this mechanism is absent. While the interaction between longwave and water vapor has a large impact on the global hydrologic cycle internal variations, its effect decreases as spatial scales decrease, so water vapor feedback has only a very small impact on grid-scale hydrologic variability. Water vapor feedback also affects the hydrologic cycle intensification when greenhouse gas concentrations increase. By the 5th century of global warming experiments where CO2 is increased and then fixed at its doubled value, the global-mean precipitation increase is nearly an order of magnitude larger when water vapor feedback is present. The cause of this difference is similar to the cause of the difference in internal precipitation variability: When water vapor feedback is present, the increase in water vapor associated with a warmer climate enhances downward longwave radiation. To maintain surface heat balance, evaporation increases, leading to a similar increase in precipitation. This effect is absent in the experiment without water vapor feedback. The large impact of water vapor feedback on hydrologic cycle intensification does not weaken as spatial scales decrease, unlike the internal variability case. Accurate representations of water vapor feedback are therefore necessary to simulate global-scale hydrologic variability and intensification of the hydrologic cycle in global warming.
Hall, A, and Syukuro Manabe, 2000: Suppression of ENSO in a coupled model without water vapor feedback. Climate Dynamics, 16(5), 393-403. Abstract PDF
We examine 800-year time series of internally generated variability in both a coupled ocean-atmosphere model where water vapor anomalies are not allowed to interact with longwave radiation and one where they are. The ENSO-like phenomenon in the experiment without water vapor feedback is drastically suppressed both in amplitude and geographic extent relative to the experiment with water vapor feedback. Surprisingly, the reduced amplitude of ENSO-related sea surface temperature anomalies in the model without water vapor feedback cannot be attributed to greater longwave damping of sea surface temperature. (Differences between the two experiments in radiative feedback due to clouds counterbalance almost perfectly the differences in radiative feedback due to water vapor.) Rather, the interaction between water vapor anomalies and longwave radiation affects the ENSO-like phenomenon through its influence on the vertical structure of radiative heating: Because of the changes in water vapor associated with it, a given warm equatorial Pacific sea surface temperature anomaly is associated with a radiative heating profile that is much more gravitationally unstable when water vapor feedback is present. The warm sea surface temperature anomaly therefore results in more convection in the experiment with water vapor feedback. The increased convection, in turn, is related to a larger westerly wind-stress anomaly, which creates a larger decrease in upwelling of cold water, thereby enhancing the magnitude of the original warm sea surface temperature anomaly. In this manner, the interaction between water vapor anomalies and longwave radiation magnifies the air-sea interactions at the heart of the ENSO phenomenon; without this interaction, the coupling between sea surface temperature and wind stress is effectively reduced, resulting in smaller amplitude ENSO episodes with a more limited geographical extent.
This study examines the responses of the simulated modern climate of a coupled ocean-atmosphere model to the discharge of freshwater into the North Atlantic Ocean. Two numerical experiments were conducted. In the first numerical experiment in which freshwater is discharged into high North Atlantic latitudes over the period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens. This weakening reduces surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic Continent of the Southern Hemisphere. Upon termination of the water discharge at the 500th year, the THC begins to reintensify, gaining its original intensity in a few hundred years. As a result, the climate of the northern North Atlantic and surrounding regions resumes its original distribution. However, in the Pacific sector of the Circumpolar Ocean of the Southern Hemisphere, the initial cooling and recovery of surface air temperature is delayed by a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces a multidecadal variation in the intensities of the THC and convective activities, which generate large multidecadal fluctuations of both sea surface temperature and salinity in the northern North Atlantic. Such oscillation yields almost abrupt changes of climate with rapid rise and fall of surface temperature in a few decades. In the second experiment, in which the same amount of freshwater is discharged into the subtropical North Atlantic over the period of 500 years, the THC and climate evolve in a manner of qualitatively similar to the first experiment. However, the magnitude of the THC response is 4-5 times smaller. It appears that freshwater is much less effective in weakening the THC if it is discharged outside high North Atlantic latitudes. The results from numerical experiments conducted earlier indicate that the intensity of the THC could also weaken in response to a future increase of atmospheric CO2, thereby moderating the CO2-induced warming over the northern North Atlantic and surrounding regions.
Hall, A, and Syukuro Manabe, 1999: The role of water vapor feedback in unperturbed climate variability and global warming. Journal of Climate, 12(8), 2327-2346. Abstract PDF
To understand the role of water vapor feedback in unperturbed surface temperature variability, a version of the Geophysical Fluid Dynamics Laboratory coupled ocean-atmosphere model is integrated for 1000 yr in two configurations, one with water vapor feedback and one without. For all spatial scales, the model with water vapor feedback has more low-frequency (timescale ≥ 2 yr) surface temperature variability than the one without. Thus water vapor feedback is positive in the context of the model's unperturbed variability. In addition, water vapor feedback is more effective the longer the timescale of the surface temperature anomaly and the larger its spatial scale.
To understand the role of water vapor feedback in global warming, two 500-yr integrations were also performed in which CO2 was doubled in both model configurations. The final surface global warming in the model with water vapor feedback is 3.38°C, while in the one without it is only 1.05°C. However, the model's water vapor feedback has a larger impact on surface warming in response to a doubling of CO2 than it does on internally generated, low-frequency, global-mean surface temperature anomalies. Water vapor feedback's strength therefore depends on the type of temperature anomaly it affects. The authors found that the degree to which a surface temperature anomaly penetrates into the troposphere is a critical factor in determining the effectiveness of its associated water vapor feedback. The more the anomaly penetrates, the stronger the feedback. It is also shown that the apparent impact of water vapor feedback is altered by other feedback mechanisms, such as albedo and cloud feedback. The sensitivity of the results to this fact is examined.
Finally, the authors compare the local and global-mean surface temperature time series from both unperturbed variability experiments to the observed record. The experiment without water vapor feedback does not have enough global-scale variability to reproduce the magnitude of the variability in the observed global-mean record, whether or not one removes the warming trend observed over the past century. In contrast, the amount of variability in the experiment with water vapor feedback is comparable to that of the global-mean record, provided the observed warming trend is removed. Thus, the authors are unable to simulate the observed levels of variability without water vapor feedback.
The standard version of the coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory (GFDL) of NOAA has at least two stable equilibria. One has a realistic and active thermohaline circulation (THC) with sinking regions in the northern North Atlantic Ocean. The other has a reverse THC with extremely weak upwelling in the North Atlantic and sinking in the Circumpolar Ocean of the Southern Hemisphere. Although the model has the seasonal variation of insolation, the structure of these two stable equilibria are very similar to those of a previous GFDL model without the seasonal variation. It is noted that the inactive mode of the reverse THC mentioned above is not a stable equilibrium for another version of the same coupled model which has a large coefficient of vertical subgrid scale diffusion. Although the reverse THC cell was produced in the Atlantic Ocean by a massive discharge of freshwater, it began to transform back to the original direct THC as soon as the freshwater discharge was terminated. It appears that there is a critical value of diffusivity, above which two stable equilibria do not exist. Based upon paleoceanographic evidence, we suggest that the stable state of the reverse THC mentioned above did not prevail during the cold periods of Younger Dryas event which occurred during the last deglacial period. Instead, it is likely that the THC weakened temporarily, but reintensified before it reached the state of the reverse THC with no deep water formation in the North Atlantic Ocean.
This article discusses the role of the THC in climate, based upon the results of several numerical experiments which use a coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of NOAA, USA. The first part of the article explores the mechanism which is responsible for the abrupt climate change such as the Younger Dryas event using the coupled model. In response to the freshwater discharge into high north Atlantic latitudes over a period of 500 years, the THC in the Atlantic Ocean weakens, reducing surface air temperature over the northern north Atlantic Ocean, the Scandinavian Peninsula, and the circumpolar ocean and Antarctic Continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces the multidecadal fluctuation in the intensity of the THC, which generates the almost abrupt change of climate. It is noted that similar but much weaker oscillation of the THC is also evident in the control integration of the coupled model without freshwater forcing. The irregular oscillation of the THC mentioned above appears to be related to the fluctuation of the Subarctic Gyre and associated east Greenland current, yielding the evolution of the surface salinity anomaly which resembles that of "great salinity anomaly". The second part of this article describes the response of a coupled ocean-atmosphere model to the doubling and quadrupling of atmospheric carbon dioxide over centuries time-scale. In one integration, the CO2 concentration increases by 1%/year (compounded) until it reaches 4 x the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO2 concentration also increases at the rate of 1%/year until it reaches 2 x the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO2-quadrupling integration is the gradual disappearance of thermohaline circulation in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the north Atlantic by the 250 years of the integration and remains very weak until the 900th year. However, it begins to restore the original intensity by the 1600th year. In the CO2-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years, but almost recovers its original intensity by the 500th year. The weakening of the THC moderates temporarily the greenhouse warming over the north Atlantic Ocean and its vicinity. In both numerical experiments described above, the initial weakening of the THC results from the capping of oceanic surface by relatively fresh, low-density water, which surpresses the convective cooling of water in the sinking region of the THC.
Stouffer, Ronald J., and Syukuro Manabe, 1999: Response of a coupled ocean-atmosphere model to increasing atmospheric carbon dioxide: Sensitivity to the rate of increase. Journal of Climate, 12(8), 2224-2237. Abstract PDF
The influence of differing rates of increase of the atmospheric CO2 concentration on the climatic response is investigated using a coupled ocean-atmosphere model. Five transient integrations are performed, each using a different constant exponential rate of CO2 increase ranging from 4% yr-1 to 0.25% yr-1. By the time of CO2 doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally more than 35% of the equilibrium response. The land-sea contrast in the warming, which is evident in the equilibrium results, is larger in all the transient experiments. The land-sea difference in the response increases with the rate of increase in atmospheric CO2 concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO2 concentration in all the transient integrations, confirming earlier work. The results also indicate that the slower the rate of increase, the larger the weakening of the THC by the time of doubling. Two of the transient experiments are continued beyond the time of CO2 doubling with the CO2 concentration maintained at that level. The amount of weakening of the THC after the CO2 stops increasing is smaller in the experiment with the slower rate of CO2 increase, indicating that the coupled system has more time to adjust to the forcing when the rate of CO2 increase is slower. After a period of slow overturning, the THC gradually recovers and eventually regains the intensity found in the control integration, so that the equilibrium THC is very similar in the control and doubled CO2 integrations. Considering only the sea level changes due to the thermal expansion of seawater, the integration with the slowest rate of increase in CO2 concentration (i.e., 0.25% yr-1 ) has the largest globally averaged sea level rise by the time of CO2 doubling (about 42 cm). However, only a relatively small fraction of the equilibrium sea level rise of 1.9 m is realized by the time of doubling in all the transient integrations. This implies that sea level continues to rise long after the CO2 concentration stops increasing, as the warm anomaly penetrates deeper into the ocean.
This study investigates the temporal and spatial variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765, integrations of the coupled model for 300 years were performed for three scenarios: increasing greenhouse gases only, increasing sulfate-aerosol loading only and the combination of both radiative forcings. The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20th century. Analysis of this integration indicates that both summer dryness and winter wetness occur in middle-to-high latitudes of North America and southern Europe. These features were identified in earlier studies. However, in the southern part of North America where the percentage reduction of soil moisture during summer is quite large, soil moisture is decreased for nearly the entire annual cycle in response to greenhouse warming. A similar observation applies to other semi-arid regions in subtropical to middle latitudes such as central Asia and the area surrounding the Mediterranean Sea. On the other hand, annual mean runoff is greatly increased in high latitudes because of increased poleward transport of moisture in the warmer model atmosphere.
An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature. The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason. Also, according to the above criterion, the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades. It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture. Therefore, we are unlikely to notice definitive CO2 -induced continental summer dryness until several decades into the 21st century.
In this report, global coupled ocean-atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in Pacific Ocean SSTs over the past century. The leading mode of internally generated decadal (>7 yr) variability in the model resembles the observed decadal variability in terms of pattern and amplitude. In the model, the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and ENSO timescales, suggesting that the decadal variability has a similar "delayed oscillator" mechanism to that on the ENSO timescale. The westward phase propagation of the heat content anomalies, however, is slower and centered slightly farther from the equator (~12° vs 9° N) for the decadal variability. Cool SST anomalies in the midlatitude North Pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies.
An index of observed SST over a broad triangular region of the tropical and subtropical Pacific indicates a warming rate of +0.41°C (100 yr)-1 since 1900, +1.2°C (100 yr)-1 since 1949, and +2.9°C (100 yr)-1 since 1971. All three warming trends are highly unusual in terms of their duration, with occurrence rates of less than 0.5% in a 2000-yr simulation of internal climate variability using a low-resolution model. The most unusual is the trend since 1900 (96-yr duration); the longest simulated duration of a trend of this magnitude is 85 yr. This suggests that the observed trends are not entirely attributable to natural (internal) variability alone, although natural variability could potentially account for much of the observed trends. To quantitatively explore the possible role of greenhouse gases and aerosols in the observed warming trends, two simulations (using different initial conditions) of twentieth-century climate change due to these two radiative forcings were analyzed. These simulate an accelerated warming trend [~2°C (100 yr)-1] in the triangular Pacific region beginning around the 1960s and suggest that nearly all of the recent warming in the region could be attributable to such a thermal forcing. In summary, the authors' model results indicate that the observed warming trend in the eastern tropical Pacific is not likely to be solely attributable to internal (natural) climate variability. Instead, it is likely that a sustained thermal forcing, such as the increase of greenhouse gases in the atmosphere, has been at least partly responsible for the observed warming.
Knutson, Thomas R., and Syukuro Manabe, 1998: Model assessment of decadal variability and trends in the tropical Pacific Ocean In The Ninth Symposium on Global Change Studies and Namias Symposium on the Status and Prospects for Climate Prediction, Boston, MA, American Meteorological Society, 216-219.
Martinson, D G., David S Battisti, R Bradley, J Cole, R Fine, M Ghil, Y Kushnir, Syukuro Manabe, M S McCartney, P McCormick, Michael J Prather, E Sarachik, P P Tans, L Thompson, and Michael Winton, 1998: Decade-to-Century-Scale Climate Variability and Climate Change: A Science Strategy, Washington, D.C.: National Research Council, 42 pp.
A 1995 report of the Intergovernmental Panel on Climate Change provides a set of illustrative anthropogenic CO2 emission models leading to stabilization of atmospheric CO2 concentrations ranging from 350 to 1,000 p.p.m. (refs 1 - 4). Ocean carbon-cycle models used in calculating these scenarios assume that oceanic circulation and biology remain unchanged through time. Here we examine the importance of this assumption by using a coupled atmosphere-ocean model of global warming for the period 1765 to 2065. We find a large potential modification to the ocean carbon sink in a vast region of the Southern Ocean where increased rainfall leads to surface freshening and increased stratification. The increased stratification reduces the downward flux of carbon and the loss of heat to the atmosphere, both of which decrease the oceanic uptake of anthropogenic CO2 relative to a constant-climate control scenario. Changes in the formation, transport and cycling of biological material may counteract the reduced uptake, but the response of the biological community to the climate change is difficult to predict on present understanding. Our simulation suggests that such physical and biological changes might already be occurring, and that they could substantially affect the ocean carbon sink over the next few decades.
Broccoli, Anthony J., and Syukuro Manabe, 1997: Mountains and midlatitude aridity In Tectonic Uplift and Climate Change, New York, NY, Plenum Press, 89-121.
Pronounced oscillations of ocean temperature and salinity occur in the Greenland Sea in a 2000 year integration of a coupled ocean-atmosphere model. The oscillations, involving both the surface and subsurface ocean layers, have a timescale of approximately 40-80 years, and are associated with fluctuations in the intensity of the East Greenland Current. The Greenland Sea temperature and salinity variations are preceded by large-scale changes in near-surface salinity in the Arctic, which appear to propagate out of the Arctic through the East Greenland Current. These anomalies then propagate around the subpolar gyre into the Labrador Sea and the central North Atlantic. These oscillations are coherent with previously identified multi-decadal fluctuations in the intensity of the North Atlantic thermohaline circulation. The oscillations in the Greenland Sea are related to atmospheric variability. Negative (cold) anomalies of surface air temperature are associated with negative (cold) sea surface temperature (SST) anomalies in the Greenland Sea, with amplitudes up to 2°C near Greenland declining to several tenths of a degree C over northwestern Europe. The cold SST anomalies and intensified East Greenland Current are also associated with enhanced northerly winds over the Greenland Sea.
Hall, A, and Syukuro Manabe, 1997: Can local linear stochastic theory explain sea surface temperature and salinity variability?Climate Dynamics, 13(3), 167-180. Abstract PDF
Sea surface temperature (SST) and salinity (SSS) time series from four ocean weather stations and data from an integration of the GFDL coupled ocean-atmosphere model are analyzed to test the applicability of local linear stochastic theory to the mixed-layer ocean. According to this theory, mixed-layer variability away from coasts and fronts can be explained as a 'red noise' response to the 'white noise' forcing by atmospheric disturbances. At one weather station, Papa (northeast Pacific), this stochastic theory can be applied to both salinity and temperature, explaining the relative redness of the SSS spectrum. Similar results hold for a model grid point adjacent to Papa, where the relationships between atmospheric energy and water fluxes and actual changes in SST and SSS are what is expected from local linear stochastic theory. At the other weather stations, this theory cannot adequately explain mixed-layer variability. Two oceanic processes must be taken into account: at Panulirus (near Bermuda), mesoscale eddies enhance the observed variability at high frequencies. At Mike and India (North Atlantic), variations in SST and SSS advection, indicated by the coherence and equal persistence of SST and SSS anomalies, contribute to much of the low frequency variability in the model and observations. To achieve a global perspective, TOPEX altimeter data and model results are used to identify regions of the ocean where these mechanisms of variability are important. Where mesoscale eddies are as energetic as at Panulirus, indicated by the TOPEX global distribution of sea level variability, one would expect enhanced variability on short time scales. In regions exhibiting signatures of variability similar to Mike and India, variations in SST and SSS advection should dominate at low frequencies. According to the model, this mode of variability is found in the circumpolar ocean and the northern North Atlantic, where it is associated with the irregular oscillations of the model's thermohaline circulation.
This study investigates changes in surface air temperature (SAT), hydrology and the thermohaline circulation due to the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and thermohaline circulation are also briefly discussed.
Knutson, Thomas R., Syukuro Manabe, and D Gu, 1997: Simulated ENSO in a global coupled ocean-atmosphere model: Multidecadal amplitude modulation and CO2 sensitivity. Journal of Climate, 10(1), 138-161. Abstract PDF
An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr. experiments with a low-resolution (R15) global coupled ocean-atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO's life cycle is qualitatively similar to the "delayed oscillator" ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different mutidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from Delta 18O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2-7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean-atmosphere system. In two 1000-yr CO2 sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO2. This decreased variability is due in part to CO2-induced changes in the model's time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO2. The frequency of ENSO in the model does not appear to be strongly influenced by increased CO2. Since the multidecadal fluctuations in the model ENSO's amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO2, the results suggest that the impact of increased CO2 on ENSO is unlikely to be clearly distinguishable from the climate system "noise" in the near future - unless ENSO is substantially more sensitive to increased CO2 than indicated in the present study.
Manabe, Syukuro, 1997: Early development in the study of greenhouse warming: The emergence of climate models. Ambio, 26(1), 47-51. Abstract PDF
Following the pioneering contributions of Arrhenius, Callendar and others, climate models emerged as a very promising tool for the study of greenhouse warming. In the early 1960s, a one-dimensional, radiative-convective equilibrium model was developed as the first step towards the development of a three-dimensional model of climate. Incorporating not only the radiative but also the convective heat exchange between the earth's surface and the atmosphere, the model overcame the difficulty encountered by the earlier approach of surface radiative heat balance in estimating the magnitude of greenhouse warming. By the 1970s, a three-dimensional, general circulation model (GCM) of the atmosphere, coupled to a very idealized ocean of swamp-like wet surface, had been used for studies of greenhouse warming. Despite many drastic simplifications, the GCM was very effective for elucidating the physical mechanisms that control global warming and served as a stepping stone towards the use of more comprehensive, coupled ocean-atmosphere GCMs for the study of this problem.
Manabe, Syukuro, and Ronald J Stouffer, 1997: Climate variability of a coupled ocean-atmosphere-land surface model: Implication for the detection of global warming (Walter Orr Roberts Lecture). Bulletin of the American Meteorological Society, 78(6), 1177-1185. Abstract PDF
This lecture evaluates the low-frequency variability of surface air temperature that was obtained from a 1000-yr integration of a coupled ocean-atmosphere-land surface model. The model simulates reasonably well the variability of local and global mean surface air temperature (SAT) at decadal timescales. The physical mechanisms responsible for this variability are explored. Based upon an analysis of the time series of the simulated global mean SAT, it is indicated that the warming trend of ~0.5 degrees C century-1 since the end of the last century was not generated internally through the interaction among the atmosphere, ocean, and land surface. Instead, it appears to have been induced by a sustained change in the thermal forcing such as that resulting from changes in atmospheric greenhouse gas concentration, solar irradiance, and aerosol loading.
This study explores the responses of a coupled ocean-atmosphere model to the discharge of freshwater into the North Atlantic Ocean. In the first numerical experiment in which freshwater is discharged into high North Atlantic latitudes over a period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens, reducing surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. With the exception of the Pacific sector of the Circumpolar Ocean of the southern hemisphere, where the surface air temperature recovery is delayed, the climate of the northern North Atlantic and surrounding regions rapidly resumes its original distribution. The evolution of the ocean-atmosphere system described above resembles the Younger Dryas event as inferred from the comprehensive analysis of ice cores and deep-sea and lake sediments. In the second experiment, in which the same amount of freshwater is discharged into the subtropical North Atlantic again over a period of 500 years, the THC and climate evolve in a manner qualitatively similar to the first experiment. However, the magnitude of the THC response is 4-5 times smaller. It appears that freshwater is much less effective in weakening the THC if it were discharged outside high North Atlantic latitudes.
Delworth, Thomas L., and Syukuro Manabe, 1996: Climate variability and land surface processes In From Atmospheric Circulation to Global Change - Celebration of the 80th Birthday of Prof. YE Duzheng, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing: China, China Meteorological Press, 477-502. Abstract
The coupled ocean-atmosphere-land climate system is characterized by substantial amounts of variability at a wide range of spatial and temporal scales. This natural variability of climate increases the difficulty of detecting climate change attributable to increasing greenhouse gas concentrations. A key issue in climate research is obtaining a better description of this variability and the physical mechanisms responsible for it. One of the important physical processes contributing to this variability is the interaction between the land surface and the atmosphere. Through its effect on the surface energy flux components, the land surface can exert a pronounced effect on the variability of the atmosphere. The potential importance of such interactions for climate variability is examined through the use of numerical modeling studies. The physical mechanisms governing the time scales of soil moisture variability in the model are outlined, and observational evidence is presented supporting this analysis. In addition, it is shown that interactions between soil wetness and the atmosphere can both increase the total variability of the atmosphere and lengthen the time scales of near-surface atmospheric fluctuations.
Manabe, Syukuro, and Ronald J Stouffer, 1996: Low-frequency variability of surface air temperature in a 1000-year integration of a coupled atmosphere-ocean-land surface model. Journal of Climate, 9(2), 376-393. Abstract PDF
This study analyzes the variability of surface air temperature (SAT) and sea surface temperature (SST) obtained from a 1000-yr. integration of a coupled atmosphere-ocean-land surface model, which consists of general circulation models of the atmosphere and oceans and a heat and water budget model of land surface.
It also explores the role of oceans in maintaining the variability of SAT by comparing the long-term integration of the coupled model with those of two simpler models. They are 1) a "mixed layer model," that is, the general circulation model of the atmosphere combined with a simple slab model of the mixed layer ocean, and 2) a "fixed SST model," that is, the same atmosphere model overlying seasonally varying, prescribed SST.
With the exception of the tropical Pacific, both the coupled and mixed layer models are capable of approximately simulating the standard deviations of observed annual and 5-yr. mean anomalies of local SAT. The standard deviation tends to be larger over continents than over oceans, in agreement with the observations. Over most continental regions, the standard deviations of annual, 5-yr. and 25-yr. mean SATs in the fixed SST model are slightly less than but comparable to the corresponding standard deviations in the coupled model, suggesting that a major fraction of low-frequency local SAT variability over continents of the coupled model is generated in situ.
Over the continents of both the coupled and the mixed layer models, the spectral density of local SAT is nearly independent of frequency. On the other hand, the spectral density of local SAT over most of the oceans of both models increases very gradually with decreasing frequency apparently influenced by the thermal inertia of mixed layer oceans. However, both SST and SAT spectra in the coupled model are substantially different from those in the mixed layer near the Denmark Strait and in some regions of the circumpolar ocean of the Southern Hemisphere where water mixes very deeply. In these regions, both SST and SAT are much more persistent in the coupled than in the mixed layer models, and their spectral densities are much larger at multi-decadal and/or centennial timescales.
It appears significant that not only the coupled model but also the mixed layer model without ocean currents can approximately simulate the power spectrum of observed, global mean SAT at decadal to interdecadal time scales. However, neither model generates a sustained, long-term warming trend of significant magnitude such as that observed since the end of the last century.
Stouffer, Ronald J., and Syukuro Manabe, 1996: The role of the oceans in the variability of surface air temperature as found in a 1000 year integration of a coupled atmosphere-ocean model In Proceedings of the Workshop on Dynamics and Statistics of Secular Climate Variations, Calverton, MD, Center for Ocean-Land-Atmosphere Studies, Report 26, 27-31.
Vinnikov, K Y., A Robock, Ronald J Stouffer, and Syukuro Manabe, 1996: Vertical patterns of free and forced climate variations. Geophysical Research Letters, 23(14), 1801-1804. Abstract PDF
Observations of the vertical structure of atmospheric temperature changes over the past three decades show that while the global-average lower atmosphere has warmed, the upper troposphere and lower stratosphere have cooled. While these changes may be due to observed anthropogenic increases of greenhouse gases, decreases of lower stratospheric ozone, and increases of tropospheric aerosols, the changes may also have been caused by natural unforced internal fluctuations of the climate system. Here we use the results of a 1000-year simulation from a mathematical model of the coupled ocean- atmosphere-land system performed without any changes in external forcing, so that we may consider its variations as a surrogate for free, internally-generated, natural fluctuations of the climate system. When the global mean surface air temperature is warm in the model, the lower troposphere, upper troposphere and lower stratosphere are also warm over most of the Earth, in contrast to the observations of the last three decades and to model simulations of the forced climate response due to increased greenhouse gases. The observed temperature change of the past three decades is therefore unlikely to have been caused solely by natural internal variations of the climate system, thereby strengthening the argument that these changes can at least partly be attributed to anthropogenic activities.
Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1996: The effect of soil moisture on the short-term climate and hydrology change - A numerical experiment In Atmospheric Circulation to Global Change - Celebration of the 80th Birthday of Prof. YE Duzheng, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing: China, China Meteorological Press, 147-173. Abstract
This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model with a limited computational domain and idealized geography.
The soil at three latitude bands, namely 30° - 60° N, and 15° S - 15° N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, if the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30° - 60° N case which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0° - 30° N case.
Although both the 30° - 60° N and 15° S - 15° N latitude regions occur under rainbelts, the soil moisture anomaly for the 15° S - 15° N case does not persist as long as it does for the 30° - 60° N case. This is because in the 15 degrees S - 15° N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30° - 60° N case.
The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in changes of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.
Broccoli, Anthony J., Syukuro Manabe, J F B Mitchell, and L Bengtsson, 1995: Comments on "Global climate change and tropical cyclones": Part II. Bulletin of the American Meteorological Society, 76(11), 2243-2245. PDF
Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1995: North Atlantic Interdecadal variability in a coupled model In Natural Climate Variability on Decade-to-Century Time Scales, Washington, DC, National Academy Press, 432-439; 440-441. Abstract
A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 40 to 50 years. The fluctuations appear to be driven by density anomalies in the sinking region of the thermohaline circulation combined with much smaller density anomalies of opposite sign in the broad, rising region. Anomalies of sea surface temperature associated with this oscillation induce surface air temperature anomalies over the northern North Atlantic, the Arctic, and northwestern Europe. The spatial pattern of sea surface temperature anomalies bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic.
Knutson, Thomas R., and Syukuro Manabe, 1995: Time-mean response over the tropical Pacific to increased CO2 in a coupled ocean-atmosphere model. Journal of Climate, 8(9), 2181-2199. Abstract PDF
The time-mean response over the tropical Pacific region to a quadrupling of CO2 is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4 degrees - 5 degrees C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with CO2 increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase. Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with a quadrupling of CO2 is surprising in light of the increased time-mean condensation heating over the "warm pool" region. Three aspects of the model response are important for interpreting this result. 1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. 2) The dynamical cooling term, - omega delta theta/ delta p, is enhanced due to increased time-mean static stability ( - delta theta/delta p). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making - delta theta/ delta p larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 3) The weaker surface zonal winds and wind stress in the equatorial Pacific are consistent with the reduced zonal SST gradient. The SST gradient is damped by the west-east differential in evaporative surface cooling (with greater evaporative cooling in the west than in the east). This evaporative damping increases with increasing temperature, owing to the temperature dependence of saturation mixing ratios, which leads to a reduction in the SST gradient in the warmer climate.
Temperature records from Greenland ice cores suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean-atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean-atmosphere system recorded in ice and deep-sea cores. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and it neighbourhood, but is relatively small in the rest of the world.
Manabe, Syukuro, Ronald J Stouffer, and Michael J Spelman, 1995: Interaction between polar climate and global warming In Fourth Conference on Polar Meteorology and Oceanography, Boston, MA, American Meteorological Society, J1-J9.
To improve understanding of the mechanisms]responsible for CO2-induced, midcontinental summer dryness obtained by earlier modeling studies, several integrations were performed using a GCM with idealized geography. The simulated reduction of soil moisture in middle latitudes begins in late spring and is caused by the excess of evaporation over precipitation. The increase of carbon dioxide and the associated increase of atmospheric water vapor enhances the downward flux of terrestrial radiation at the continental surface at all latitudes. However, due mainly to the To improve understanding of the mechanisms responsible for CO2-induced, midcontinental summer dryness obtained by earlier modeling studies, several integrations were performed using a GCM with idealized geography. The simulated reduction of soil moisture in middle latitudes begins in late spring and is caused by the excess of evaporation over precipitation. The increase of carbon dioxide and the associated increase of atmospheric water vapor enhances the downward flux of terrestrial radiation at the continental surface at all latitudes. However, due mainly to the CO2-induced change in midtropospheric relative humidity, the increase in the downward flux of terrestrial radiation is larger in the equatorward side of the rain belt, making more energy available there for both sensible and latent heat. Since the saturation vapor pressure at the surface increases nonlinearly with surface temperature, a greater fraction of the additional radiative energy is realized as latent heat flux at the expense of sensible heat. Therefore, evaporation increases more than precipitation over the land surface in the equatorward side of the rain belt during spring and early summer and initiates the drying of the soil there. As the rain belt moves poleward from spring to summer, the soil moisture decreases in middle latitudes, reducing the rate of evaporation. This reduction of evaporation, in turn, causes a corresponding decrease of precipitation in middle latitudes, keeping the soil dry throughout the summer.
In high latitudes, there is also a tendency for increased summer dryness. As noted in our previous studies, this feature mainly results from the earlier removal of highly reflective snow cover in spring, which enhances the evaporation in the late spring, lengthening the period of drying during the summer season. A similar mechanism also operates in middle latitudes, but its contribution is relatively small. The drying of soil is also enhanced by the land surface - cloud interaction in both middle and high latitudes. Owing to the reduction of cloud cover that results from the decrease of relative humidity in the lower troposphere, solar radiation absorbed by the continental surface increases, thereby enhancing evaporation and further reducing the soil moisture in summer.
Although there is additional radiative energy available at the surface during winter, a greater fraction of it occurs as sensible heat rather than latent heat due to the colder surface temperature, thereby causing evaporation to increase less than precipitation. Because of the increased evaporation from the oceanic surface upstream whose temperature is warmer than the continental region in winter, precipitation over most of the continent increases substantially.
The impact of a CO2-induced global warming on ENSO-like fluctuations in a global coupled ocean-atmosphere GCM is analyzed using two multi-century experiments. In the 4xCO2 experiment, CO2 increases by a factor of four in the first 140 years and then remains constant at 4xCO2 for another 360 years; in the control experiment, CO2 remains constant at 1xCO2 for 1000 years. The standard deviation of tropical Pacific SST fluctuations (7°N-7°S, 173°E-120°W; 2 to 15 year timescales) is 24% lower in the 4xCO2 experiment than in the control experiment; for the model's Southern Oscillation Index, a 19% decrease occurs, whereas for central tropical Pacific rainfall, a 3% increase occurs. An important feature of the control simulation is the internally generated modulation of variability on a multi-century timescale, which is comparable in magnitude to the changes occurring with 4xCO2. We conclude that despite an order 5 K warming of the tropical Pacific, and order 50% increase in time-mean atmospheric water vapor under 4xCO2 conditions, ENSO-like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude.
Knutson, Thomas R., and Syukuro Manabe, 1994: Impact of increasing CO2 on the Walker circulation and ENSO-like phenomena in a coupled ocean-atmosphere model In The Sixth Conference on Climate Variations, Boston, MA, American Meteorological Society, 80-81.
Manabe, Syukuro, and Ronald J Stouffer, 1994: Multiple-century response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide. Journal of Climate, 7(1), 5-23. Abstract PDF
To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the CO2 concentration increases by 1% yr-1 (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO2 concentration also increases at the rate of 1% yr-1 until it reaches twice the initial value at the 70th year and remains unchanged thereafter.
One of the most notable features of the CO2-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the first 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere.
In the CO2-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by thegradual increase in surface salinity that accompanies the intensification of the thermohaline circulation.
During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future change.
This study investigates the response of a climate model to a 1% per year increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system, with a global computational domain, smoothed geography, and seasonal variation of insolation. The simulated increase of sea-surface temperature is very slow in the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. Extending this work, we investigated the transient responses of the coupled model to the doubling and quadrupling of atmospheric CO2, over the period of several centuries. During the entire 500-yr. period of the experiment, the global mean surface air temperature increases almost 3.5°C when CO2 is doubled, and 7°C when it is quadrupled. In the latter experiment, the thermal structure and dynamics of the model oceans undergo drastic changes, such as cessation of the thermohaline circulation in most of the model oceans, and substantial deepening of the thermocline, especially in the North Atlantic. These changes prevent the ventilation of the deeper layer of the oceans and, if they occurred in reality, could have a profound impact on the carbon cycle and biogeochemistry of the coupled ocean-atmosphere system.
Stouffer, Ronald J., Syukuro Manabe, and K Y Vinnikov, 1994: Model assessment of the role of natural variability in recent global warming. Nature, 367, 634-636. Abstract
Since the late nineteenth century, the global mean surface air temperature has been increasing at the rate of about 0.5°C per century, but our poor understanding of low-frequency natural climate variability has made it very difficult to determine whether the observed warming trend is attributable to the enhanced greenhouse effect associated with increased atmospheric concentrations of greenhouse gases. Here we evaluate the observed warming trend using a 1,000-year time series of global temperature obtained from a mathematical model of the coupled ocean-atmosphere-land system. We find that the model approximately reproduces the magnitude of the annual to interdecadal variation in global mean surface air temperature. But throughout the simulated time series no temperature change as large as 0.5°C per century is sustained for more than a few decades. Assuming that the model is realistic, these results suggest that the observed trend is not a natural feature of the interaction between the atmosphere and oceans. Instead, it may have been induced by a sustained change in the thermal forcing, such as that resulting from changes in atmospheric greenhouse gas concentrations and aerosol loading.
Broccoli, Anthony J., and Syukuro Manabe, 1993: Climate model studies of interactions between ice sheets and the atmosphere-ocean system In Ice in the Climate System, W. R. Peltier, ed., NATO ASI Series I, Vol. 12, Berlin, Germany, Springer-Verlag, 271-290. Abstract PDF
A number of climate modeling studies have been conducted at the Geophysical Fluid Dynamics Laboratory to study the interaction of continental ice sheets with the climate system. This paper reviews some of the primary results from these studies. Substantial changes in the atmospheric circulation, location and intensity of storm tracks, precipitation distribution, sea surface temperature, sea ice extent, and soil moisture occur in response to the ice sheets of the last glacial maximum. Estimates of the mass budgets of these ice sheets suggest that they are not in equilibrium with the simulated LGM climate, although questions regarding the refreezing of surface meltwater make this result uncertain. Results from climate model experiments with and without orography suggest that orographic uplift could have produced a climate slightly more favorable for ice sheet initiation.
A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of sea surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.
The coupled ocean-atmosphere-land climate system is characterized by substantial amounts of variability on a wide range of spatial and temporal scales. This natural variability of climate increases the difficulty of detecting climate change attributable to increasing greenhouse gas concentrations. A key issue in climate research is obtaining a better description of this variability and the physical mechanisms responsible for it. One of the important physical processes contributing to this variability is the interaction between the land surface and the atmosphere. Through its effect on the surface energy flux components, the land surface can exert a pronounced effect on the variability of the atmosphere. The potential importance of such interactions for climate variability is examined through the use of numerical modeling studies. The physical mechanisms governing the time scales of soil moisture variability in the model are outlined, and observational evidence is presented supporting this analysis. In addition, it is shown that interactions between soil wetness and the atmosphere can both increase the total variability of the atmosphere and lengthen the time scales of near-surface atmospheric fluctuations.
Several studies have addressed the likely effects of CO2-induced climate change over the coming decades, but the longer-term effects have received less attention. Yet these effects could be very significant, as persistent increases in global mean temperatures may ultimately influence the large-scale processes in the coupled ocean-atmosphere system that are thought to play a central part in determining global climate. The thermohaline circulation is one such process-Broecker has argued that it may have undergone abrupt changes in response to rising temperatures and ice-sheet melting at the end of the last glacial period. Here we use a coupled ocean-atmosphere climate model to study the evolution of the world's climate over the next few centuries, driven by doubling and quadrupling of the concentration of atmospheric CO2. We find that the global mean surface air temperature increases by about 3.5 and 7°C, respectively, over 500 years, and that sea-level rise owing to thermal expansion alone is about 1 and 2 m respectively (ice-sheet melting could make these values much larger). The thermal and dynamical structure of the oceans changes markedly in the quadrupled-CO2 climate-in particular, the ocean settles into a new stable state in which the thermohaline circulation has ceased entirely and the thermocline deepens substantially. These changes prevent the ventilation of the deep ocean and could have a profound impact on the carbon cycle and biogeochemistry of the coupled system.
The role of mountains in maintaining extensive midlatitude arid regions in the Northern Hemisphere was investigated using simulations from the GFDL Global Climate Model with and without orography. In the integration with mountains, dry climates were simulated over central Asia and the interior of North America, in good agreement with the observed climate. In contrast, moist climates were simulated in the same regions in the integration without mountains. During all seasons but summer, large amplitude stationary waves occur in response to the Tibetan Plateau and Rocky Mountains. The midlatitude dry regions are located upstream of the troughs of these waves, where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited. In summer, this mechanism contributes to the dryness of interior North America as a stationary wave trough remains east of the Rockies, but is not effective in Eurasia due to seasonal changes in the atmospheric circulation. The dryness of interior Eurasia in summer results, in part, from the south Asian monsoon circulation induced by the Tibetan Plateau. Its rising branch is centered above the southeastern Tibetan Plateau, and its salient features are a cyclonic flow at low levels (the "south Asian low") and an anticyclonic flow in the upper troposphere. This circulation is associated with a northward displacement of the storm track and a flow of relatively dry, subsiding air across much of central Asia. In addition, land surface-atmosphere feedback contributes to the dryness of all midlatitude dry regions. Although the effect of this feedback is small in winter, it is responsible for more than half of the reduction in summer precipitation. Orography also substantially reduces the moisture transport across the continental interiors. The results from this experiment suggest that midlatitude dryness is largely due to the existence of orography. This is an alternative to the traditional explanation that distance from oceanic moisture sources, accentuated locally by the presence of mountain barriers upwind, is the major cause of midlatitude dry regions. Paleoclimatic evidence of less aridity during the late Tertiary, before substantial uplift of the Rocky Mountains and Tibetan Plateau is believed to have occurred, supports this possibility.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Response of a coupled ocean-atmosphere-land surface model to a gradual increase of atmospheric carbon dioxide In The Global Role of Tropical Rainfall, Hampton, Virginia, Deepak Publishing, 93-103. Abstract
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal variation of insolation. It is found that the simulated warming of sea surface temperature is very slow over the northern North Atlantic and the circumpolar ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies.
The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model, but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basins is increased markedly.
In qualitative agreement with the results of equilibrium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Transient response of a coupled ocean-atmosphere-land surface model to increasing atmospheric carbon dioxide In Advances in Theoretical Hydrology: A Tribute to Jim Dooge, The Netherlands, Elsevier Science Publishers, 159-173. Abstract
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal veriation of insolation. It is found that the simulated increase of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions identified above, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies. In most of the Northern Hemisphere, the seasonal dependence of surface air temperature change is also similar to the equilibrium response. For example, the temperature increase is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter, whereas it is at a minimum in summer. However, the increase of surface air temperature and its seasonal variation is very small in the Circumpolar Ocean of the Southern Hemisphere and the northern North Atlantic.
The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basin increases markedly.
In qualitative agreement with the results of equilibium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
This study investigates the seasonal variation of the transient response of a coupled ocean-atmosphere model
to a gradual increase (or decrease) of atmospheric carbon dioxide. The model is a general circulation model of
the coupled atmosphere-ocean-Iand surface system with a global computational domain, smoothed geography,
and seasonal variation of insolation.
It was found that the increase of surface air temperature in response to a gradual increase of atmospheric
carbon dioxide is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter. On the
other hand, the Arctic warming is at a minimum in summer. In sharp contrast to the situation in the Arctic
Ocean, the increase of surface air temperature and its seasonal variation in the circumpolar ocean of the Southern
Hemisphere are very small because of the vertical mixing of heat over a deep water column.
In response to the gradual increase of atmospheric carbon dioxide, soil moisture is reduced during the June-July-
August period over most of the continents in the Northern Hemisphere with the notable exception of the
Indian subcontinent, where it increases. The summer reduction of soil moisture in the Northern Hemisphere
is relatively large over the region stretching from the northern United States to western Canada, eastern China,
southern Europe, Scandinavia, and most of the Russian Republic. During the December-January-February
period, soil moisture increases in middle and high latitudes of the Northern Hemisphere. The increase is relatively
large over the western portion of the Russian Republic and the central portion of Canada. On the other hand,
it is reduced in the subtropics, particularly over Southeast Asia and Mexico.
Because of the reduction (or delay) in the warming of the oceanic surface due to the thermal inertia of the
oceans, the increase of the moisture supply from the oceans to continents is reduced, thereby contributing to
the reduction of both soil moisture and runoff over the continents in middle and high latitudes of the Northern
Hemisphere. This mechanism enhances the summer reduction of soil moisture and lessens its increase during
winter in these latitudes.
The changes in surface air temperature and soil moisture in response to the gradual reduction of atmospheric
CO2 are opposite in sign but have seasonal and geographical distributions that are broadly similar to the response
to the gradual CO2 increase described above.
Broccoli, Anthony J., and Syukuro Manabe, 1991: Will global warming increase the frequency of tropical cyclones? In Fifth Conference on Climate Variations, Boston, MA, American Meteorological Society, 46.
MacCracken, M, Syukuro Manabe, and Ronald J Stouffer, 1991: Working Group 2: A critical appraisal of model simulations In Greenhouse-Gas-Induced Climatic Change: A Critical Appraisal of Simulations and Observations, The Netherlands, Elsevier Science Publishers, 583-591.
Manabe, Syukuro, 1991: Transient responses of a coupled ocean-atmosphere-land surface model to a gradual change of atmospheric CO2 In Global Change, Proceedings of the first Demetra meeting held at Chianciano Terme, Italy from 28 to 31 October 1991, Environment and Quality of Life, EUR 15158 EN, Directorate-General Science, Research and Development, European Commission, 16-17.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1991: Transient responses of a coupled ocean-atmosphere-land surface model to gradual changes of atmospheric CO2 In Global Change, Proceedings of the first Demetra meeting held at Chianciano Terme, Italy from 28 to 31 October 1991, Environment and Quality of Life, EUR 15158 EN, Directorate-General Science, Research and Development, European Commission, 82-93.
This study investigates the response of a climate model to a gradual increase or decrease of atmospheric carbon dioxide. The model is a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal variation of insolation. To offset the bias of the coupled model toward settling into an unrealistic state, the fluxes of heat and water at the ocean-atmosphere interface are adjusted by amounts that vary with season and geography but do not change from one year to the next. Starting from a quasi-equilibrium climate, three numerical time integrations of the coupled model are performed with gradually increasing, constant, and gradually decreasing concentrations of atmospheric carbon dioxide.
It is noted that the simulated response of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where vertical mixing of water penetrates very deply. However, in most of the Northern Hemisphere and low latitudes of the Southern Hemisphere, the distribution of the change in surface air temperature of the model at the time of doubling (or halving) of atmospheric carbon dioxide resembles the equilibrium response of an atmospheric-mixed layer ocean model to CO2 doubling (or halving). For example, the rise of annual mean surface air temperature in response to the gradual increase of atmospheric carbon dioxide increases with latitudes in the Northern Hemisphere and is larger over continents than oceans.
When time-dependent response of the model oceans to the increase of atmospheric carbon dioxide is compared with the corresponding response to the CO2 reduction at an identical rate, the penetration of the cold anomaly in the latter case is significantly deeper than that of the warm anomaly in the former case. The lack of symmetry in the penetration depth of a thermal anomaly between the two cases is associated with the difference in static stability, which is due mainly to the change in the vertical distribution of salinity in high latitudes and temperature changes in middle and low latitudes.
Despite the difference in penetration depth and accordingly, the effective thermal inertia of the oceans between two experiments, the time-dependent response of the global mean surface air temperature in the CO2 reduction experiment is similar in magnitude to the corresponding response in the CO2 growth experiment. In the former experiment with a colder climate, snow and sea ice with high surface albedo cover a much larger area, thereby enhancing their positive feedback effect upon surface air temperature. On the other hand, surface cooling is reduced due to the larger effective thermal inertia of the oceans. Because of the compensation between these two effects, the magnitude of surface air temperature response turned out to be similar between the two experiments.
Stouffer, Ronald J., Syukuro Manabe, and Kirk Bryan, 1991: Climatic response to a gradual increase of atmospheric carbon dioxide In Greenhouse-Gas-Induced Climatic Change: A Critical Appraisal of Simulations and Observations, The Netherlands, Elsevier Science Publishers, 129-136. Abstract
The transient response of a coupled ocean-atmosphere model to an increase of carbon dioxide has been the subject of several studies (Bryan et al., 1982; Spelman and Manabe, 1984; Bryan and Spelman, 1985; Schlesinger and Jiang, 1988; Schlesinger et al., 1985; Bryan et al., 1988; Manabe et al., 1990; Washington and Meehl, 1989). The models used in these studies explicitly incorporate the effect of heat transport by ocean currents and are different from the model used by Hansen et al. (1988). Here we evaluate the climatic influence of increasing atmospheric carbon dioxide using a coupled model recently developed at the NOAA Geophysical Fluid Dynamics Laboratory. The model response exhibits a marked and unexpected interhemispheric asymmetry. In the circumpolar ocean of the Southern Hemisphere, a region of deep vertical mixing, the increase of surface air temperature is very slow. In the Northern Hemisphere of the model, the rise of surface air temperature is faster and increases with latitude, with the exception of the northern North Atlantic, where it is relatively slow because of the weakening of the thermohaline circulation.
The importance of clouds in the upper troposphere (cirrus) for the sensitivity of the Earth's climate e.g., requires that these clouds be modeled accurately in general circulation model (GCM) studies of the atmosphere. Bearing in mind the lack of unambiguous quantitative information on the geographical distribution and properties of high clouds, the simulated distribution of upper tropospheric clouds in a spectral GCM is compared with several satellite-derived datasets that pertain to high clouds only, for both winter and summer seasons. In the model, clouds are assumed to occupy an entire grid box whenever the relative humidity exceeds 99%: otherwise the grid box is assumed to be free of cloud. Despite the simplicity of the cloud prediction scheme, the geographical distribution of the maxima in the model's upper tropospheric cloud cover coincides approximately with the regions of the observed maxima in the high cloud amount and their frequency of occurrence (e.g., intertropical convergence zone and the monsoon areas). These areas exhibit a minimum in the outgoing longwave radiation (OLR; Nimbus-7) and are also coincident with regions of heavy precipitation. The model, with its relatively simple cloud formation scheme, appears to capture the principal large-scale features of the tropical convective processes that are evident in the satellite and precipitation datasets, wherein the intense, upward motion is accompanied by condensation and the spreading of thick upper tropospheric layers of high relative humidity and.cloudiness in the vicinity of the tropical rainbelt regions.
Broccoli, Anthony J., and Syukuro Manabe, 1990: Can existing climate models be used to study anthropogenic changes in tropical cyclone climate?Geophysical Research Letters, 17(11), 1917-1920. Abstract PDF
The utility of current generation climate models for studying the influence of greenhouse warming on the tropical storm climatology is examined. A method developed to identify tropical cyclones is applied to a series of model integrations. The global distribution of tropical storms is simulated by these models in a generally realistic manner. While the model resolution is insufficient to reproduce the fine structure of tropical cyclones, the simulated storms become more realistic as resolution is increased. To obtain a preliminary estimate of the response of the tropical cyclone climatology, CO2 was doubled using models with varying cloud treatments and different horizontal resolutions. In the experiment with prescribed cloudiness, the number of storm‐days, a combined measure of the number and duration of tropical storms, undergoes a statistically significant increase in the doubled‐CO2 climate. In contrast, a smaller but significant reduction of the number of storm‐days is indicated in the experiment with cloud feedback. In both cases the response is independent of horizontal resolution. While the inconclusive nature of these experimeital results highlights the uncertainties that remain in examining the details of greenhouse gas induced climate change, the ability of the models to qualitatively simulate the tropical storm climatology suggests that they are appropriate tools for this problem.
Simulations from a global climate model with and without orography have been used to investigate the role of mountains in maintaining extensive arid climates in middle latitudes of the Northern Hemisphere. Dry climates similar to those observed were simulated over central Asia and western interior North America in the experiment with mountains, whereas relatively moist climates were simulated in these areas in the absence of orography. The experiments suggest that these interior regions are dry because general subsidence and relatively infrequent storm development occur upstream of orographically induced stationary wave troughs. Downstream of these troughs, precipitation-bearing storms develop frequently in association with strong jet streams. In contrast, both atmospheric circulation and precipitation were more zonally symmetric in the experiment without mountains. In addition, orography reduces the moisture transport into the continental interiors from nearby oceanic sources. The relative soil wetness of these regions in the experiment without mountains is consistent with paleoclimatic evidence of less aridity during the late Tertiary, before substantial uplift of the Rocky Mountains and Tibetan Plateau is believed to have occurred.
The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insolation. An equilibrium climate of the coupled model climate during the 60-year period after the doubling is compared with the result from a control integration of the model without the doubling. The increase of surface air temperature in middle and high latitudes is slower in the Southern Hemisphere than the Northern Hemisphere. The large thermal inertia of the ocean-dominated hemisphere is partly responsible for this difference. The effective thermal inertia of the oceans becomes particularly large in high southern latitudes. Owing to the absence of meridional barriers at the latitudes of the Drake Passage, a wind-driven, deep cell of meridional circulation is maintained in the Circumpolar Ocean of the model. In addition, a deep reverse cell develops in the immediate vicinity of the Antarctic Continent. The thermal advection by these cells and associated convective overturning result in a very efficient mixing of heat in the 2-km thick upper layer and increase the effective thermal inertia of the ocean, thereby contributing to the slowdown of the CO2- induced warming of the near-surface layer of the Circumpolar Ocean of the model. It is surprising that, during the last 15 years of the 60-year experiment, sea surface temperatures in the Circumpolar Ocean actually reduce with time. Because of the increase in precipitation caused by the enhanced penetration of warm, moisture-rich air aloft into high latitudes, the surface halocline of the Circumpolar Ocean intensifies, thereby suppressing the convective mixing between the surface layer and the warmer underlying water. Thus, sea surface temperature is reduced in the Circumpolar Ocean towards the end of the experiment. In the Northern Hemisphere, the CO2-induced warming of the lower troposphere increases with increasing latitudes and is at a maximum near the North Pole due partly to the albedo feedback process involving sea ice and snow cover. The warming of the upper ocean layer also increases with increasing latitudes up to about 65 degrees N where the absorption of solar radiation increases markedly due to the poleward retreat of sea ice. Over the Arctic Ocean, the warming is very large in the surface layer of the model atmosphere, whereas it is very small in the underlying water. Both sea ice and a stable surface halocline act as thermal insulators and are responsible for the large air-sea contrast of the warming in this region. In short, the CO2- induced warming of the sea surface has a large interhemispheric asymmetry, in qualitative agreement with the results from a previous study conducted by use of a coupled model with a sector computational domain and an idealized geography. This asymmetry induces an atmospheric response which is quite different between the two hemispheres.
The temporal variability of soil wetness and its interactions with the atmosphere were studied using a general circulation model of the atmosphere. It was found that time series of soil wetness computed by the model contain substantial amounts of variance at low frequencies. Long time-scale anomalies of soil moisture resemble the red noise response of the soil layer to white noise rainfall forcing. The dependence of the temporal variability of soil moisture on potential evaporation and precipitation is discussed.
Mitchell, J F., Syukuro Manabe, T Tokioka, and V Meleshko, 1990: Equilibrium climate change In Climate Change: The IPCC Scientific Assessment, Cambridge, UK, Cambridge University Press, 131-172.
The subject of possible hydrologic change in response to a CO2-induced warming of the earth's atmosphere has received increasing attention at various scientific institutions in North America and Europe. Because changes of precipitation and evaporation could have a major impact on various aspects of our environment, scientific investigations have been concentrating upon the geographical details of CO2-induced changes of hydrology. In order to better determine the effects of this warming, global circulation models provide a means of obtaining results that can be employed in the analysis of climatic changes.
Bryan, Kirk, and Syukuro Manabe, 1989: Ocean circulation in warm and cold climates In Climatic Change Influences on Oceanic Circulation, Amsterdam; The Netherlands, Kluwer Academic Publishers, 951-966.
The influence of land surface processes on near-surface atmospheric variability on seasonal and interannual time scales is studied using output from two integrations of a general circulation model. In the first experiment, of 50 year s duration, soil moisture is predicted, thereby taking into consideration interactions between the surface moisture budget and the atmosphere. In the second experiment, of 25 years duration, the seasonal cycle of soil moisture is prescribed at each grid point based upon the results of the first integration, thereby suppressing these interactions. The same seasonal cycle of soil moisture is prescribed for each year of the second integration. Differences in atmospheric variability between the two integrations are due to interactions between the surface moisture budget and the atmosphere. Analyses of monthly data indicate that the surface moisture budget interacts with the atmosphere in such a way as to lengthen the time scales of fluctuation of near-surface relative humidity and temperature, as well as to increase the total variability of the atmosphere. During summer months at middle latitudes, the persistence of near-surface relative humidity, as measured by correlations of monthly mean relative humidity between successive months, increases from near zero in the experiment with prescribed soil moisture to as large as 0.6 in the experiment with interactive soil moisture, which corresponds to an e-folding time of approximately two months. The standard deviation of monthly mean relative humidity during summer is substantially larger in the experiment with interactive soil moisture than in the experiment with prescribed soil moisture. Surface air temperature exhibits similar changes, but of smaller magnitude. Soil wetness influences the atmosphere by altering the partitioning of the outgoing energy flux at the surface into latent and sensible heat compon ents. Fluctuations of soil moisture result in large variations in these fluxes, and thus significant variations in near surface relative humidity and temperature. Because anomalies of monthly mean soil moisture are characterized by seasonal and interannual time scales, they create persistent anomalous fluxes of latent and sensible heat, thereby increasing the persistence of near-surface atmospheric relative humidity and temperature.
Manabe, Syukuro, 1989: Studies of glacial climates by coupled atmosphere-ocean models: how useful are coupled models? In Global Changes of the Past, R. S. Bradley, ed., Boulder, CO, UCAR/Office for Interdisciplinary Earth Studies, 421-448.
The transient response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide has been the subject of several studies. The models used in these studies explicitly incorporate the effect of heat transport by ocean currents and are different from the model used by Hansen et al. Here we evaluate the climatic influence of increasing atmospheric carbon dioxide using a coupled model recently developed at the NOAA Geophysical Fluid Dynamics Laboratory. The model response exhibits a marked and unexpected interhemispheric asymmetry. In the circumpolar ocean of the Southern Hemisphere, a region of deep vertical mixing, the increase of surface air temperature is very slow. In the Northern Hemisphere of the model, the warming of surface air is faster and increases with latitude, with the exception of the northern North Atlantic, where it is relatively slow because of the weakening of the thermohaline circulation.
Bryan, Kirk, and Syukuro Manabe, 1988: Ocean circulation in warm and cold climates In Physically-based Modelling and Simulation of Climate Change, Part II, Dordrecht, Holland, Kluwer Academic Publishers, 951-966.
Bryan, Kirk, Syukuro Manabe, and Michael J Spelman, 1988: Interhemispheric asymmetry in the transient response of a coupled ocean-atmosphere model to a CO2 forcing. Journal of Physical Oceanography, 18(6), 851-867. Abstract PDF
Numerical experiments are carried out using a general circulation model of a coupled ocean-atmosphere system with idealized geography, exploring the transient response of climate to a rapid increase of atmospheric carbon dioxide. The computational domain of the model is bounded by meridians 120° apart, and includes two hemispheres. The ratio of land to sea at each latitude corresponds to the actual land-sea ratio for the present geography of the Earth. At the latitude of the Drake Passage the entire sector is occupied by ocean.
In the equivalent of the Northern Hemisphere the ocean delays the climate response to increased atmospheric carbon dioxide. The delay is of the order of several decades, a result corresponding to previous modeling studies. At high latitudes of the equivalent of the ocean-covered Southern Hemisphere, on the other hand, there is no warming at the sea surface, and even a slight cooling over the 50-year duration of the experiment. Two main factors appear to be involved. One is the very large ratio of ocean to land in the Southern Hemisphere. The other factor is the very deep penetration of a meridional overturning associated with the equatorward Ekman transport under the Southern Hemisphere westerlies. The deep cell delays the response to carbon-dioxide warming by upwelling unmodified waters from great depth. This deep cell disappears when the Drake Passage is removed from the model.
Delworth, Thomas L., and Syukuro Manabe, 1988: Influence of potential evaporation on the variabilities of simulated soil wetness and climate. Journal of Climate, 1(5), 523-547. Abstract PDF
An atmospheric general circulation model with prescribed sea surface temperature and cloudiness was integrated for 50 years to study atmosphere-land surface interactions. The temporal variability of model soil moisture and precipitation has been studied in an effort to understand the interactions of these variables with other components of the climate system. Temporal variability analysis has shown that the spectra of monthly mean precipitation over land are close to white at all latitudes, with total variance decreasing poleward. In contrast, the spectra of soil moisture are red and become more red with increasing latitude. As a measure of this redness, half of the total variance of a composite tropical soil moisture spectrum occurs at periods longer than nine months, while at high latitudes, half of the total variance of a composite soil moisture spectrum occurs at periods longer than 22 months. The spectra of soil moisture also exhibit marked longitudinal variations.
These spectral results may be viewed in light of stochastic theory. The formulation of the GFDL soil moisture parameterization is mathematically similar to a stochastic process. According to this model, forcing of a system by an input white noise variable (precipitation) will yield an output variable (soil moisture) with a red spectrum, the redness of which is controlled by a damping term (potential evaporation). Thus, the increasingly red nature of the soil moisture spectra at higher latitudes is a result of declining potential evaporation values at higher latitudes. Physically, soil moisture excesses are dissipated more slowly at high latitudes, where the energy available for evaporation is small.
Some of the longitudinal variations in soil moisture spectra result from longitudinal variations in potential evaporation, while others are explicable in terms of the value of the ratio of potential evaporation to precipitation. Regions where this value is less than one are characterized by frequent runoff and short time scales of soil moisture variability. By preventing excessive positive anomalies of soil moisture, the runoff process hastens the return of soil moisture values to their mean state, thereby shortening soil moisture time scales.
Through the use of a second GCM integration with prescribed soil moisture, it was shown that interactive soil moisture may substantially increase summer surface air temperature variability. Soil moisture interacts with the atmosphere primarily through the surface energy balance. The degree of soil saturation strongly influences the partitioning of outgoing energy from the surface between the latent and sensible heat fluxes. Interactive soil moisture allows larger variations of these fluxes, thereby increasing the variance of surface air temperature. Because the flux of latent heat is directly proportional to potential evaporation under conditions of sufficient moisture, the influence of soil moisture on the atmosphere is greatest when the potential evaporation value is large. This occurs most frequently in the tropics and summer hemisphere extratropics.
Two stable equilibria have been obtained from a global model of the coupled ocean-atmosphere system developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The model used for this study consists of general circulation models of the atmosphere and the world oceans and a simple model of land surface. Starting from two different initial conditions, "asynchronous" time integrations of the coupled model, under identical boundary conditions, lead to two stable equilibria. In one equilibrium, the North Atlantic Ocean has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there is no thermohaline circulation, and an intense halocline exists in the surface layer at high latitudes. In both integrations, the air-sea exchange of water is adjusted to remove a systematic bias of the model that supresses the thermohaline circulation in the North Atlantic. Nevertheless, these results raise the intriguing possibility that the coupled system may have at least two equilibria. They also suggest that the thermohaline overturning in the North Atlantic is mainly responsible for making the surface salinity of the northern North Atlantic higher than that of the northern North Pacific. Finally, a discussion is made on the paleoclimatic implications of these results for the large and abrupt transition between the Allerod and Younger Dryas events which occurred about 11,000 years ago.
The influence of the cloud feedback process upon the sensitivity of climate is investigated by comparing the behavior of two versions of a climate model with predicted and prescribed cloud cover. The model used for this study is a general circulation model of the atmosphere coupled with a mixed layer model of the oceans. The sensitivity of each version of the model is inferred from the equilibrium response of the model to a doubling of the atmospheric concentration of carbon dioxide.
It is found that the cloud feedback process in the present model enhances the sensitivity of the model climate. In response to the increase of atmospheric carbon dioxide, cloudiness increases around the tropopause and is reduced in the upper troposphere, thereby raising the height of the cloud layer in the upper troposphere. This rise of the high cloud layer implies a reduction of the temperature of the cloud top and, accordingly, of the upward terrestrial radiation from the top of the model atmosphere. Thus, the heat loss from the atmosphere-earth system of the model is reduced. As the high cloud layer rises, the vertical distribution of cloudiness changes, thereby affecting the absorption of solar radiation by the model atmosphere. At most latitudes the effect of reduced cloud amount in the upper troposphere overshadows that of increased cloudiness around the tropopause, thereby lowering the global mean planetary albedo and enhancing the CO2 induced warming.
On the other hand, the increase of low cloudiness in high latitudes raises the planetary albedo and thus decreases the CO2 induced warming of climate. However, the contribution of this negative feedback process is much smaller than the effect of the positive feedback process involving the change of high cloud.
The model used here does not take into consideration the possible change in the optical properties of clouds due to the change of their liquid water content. In view of the extreme idealization in the formulation of the cloud feedback process in the model, this study should be regarded as a study of the mechanisms involved in this process rather than the quantitative assessment of its influence on the sensitivity of climate.
Broccoli, Anthony J., and Syukuro Manabe, 1987: The effects of the Laurentide ice sheet on North American climate during the last glacial maximum. Géographie Physique et Quaternaire, XLI(2), 291-299. Abstract PDF
A climate model, consisting of an atmospheric general circulation model coupled with a simple model of the oceanic mixed layer, is used to investigate the effects of the continental ice distribution of the last glacial maximum (LGM) on North American climate. This model has previously been used to simulate the LGM climate, producing temperature changes reasonably in agreement with paleoclimatic data. The LGM distribution of continental ice according to the maximum reconstruction of HUGHES et al. (1981) is used as input to the model. In response to the incorporation of the expanded continental ice of the LGM, the model produces major changes in the climate of North America. The ice sheet exerts an orographic effect on the tropospheric flow, resulting in a splitting of the midlatitude westerlies in all seasons but summer. Winter temperatures are greatly reduced over a wide region south of the Laurentide ice sheet, although summer cooling is less extensive. An area of reduced soil moisture develops in the interior of North America just south of the ice margin. At the same time, precipitation increases in a belt extending from the extreme southeastern portion of the ice sheet eastward into the North Atlantic. Some of these findings are similar to paleoclimatic inferences based on geological evidence.
Broccoli, Anthony J., and Syukuro Manabe, 1987: The influence of continental ice, atmospheric CO2, and land albedo on the climate of the last glacial maximum. Climate Dynamics, 1, 87-99. Abstract PDF
The contributions of expanded continental ice, reduced atmospheric CO2, and changes in land albedo to the maintenance of the climate of the last glascial maximum (LGM) are examined. A series of experiments is performed using an atmosphere-mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination. The model used has been shown to produce a reasonably realistic simulation of the reduced temperature of the LGM (Manabe and Broccoli 1985b). By comparing the results from pairs of experiments, the effects of each of these environmental changes can be determined.The contributions of expanded continental ice, reduced atmospheric CO2, and changes in land albedo to the maintenance of the climate of the last glacial maximum (LGM) are examined. A series of experiments is performed using an atmosphere-mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination. The model used has been shown to produce a reasonably realistic simulation of the reduced temperature of the LGM (Manabe and Broccoli 1985b). By comparing the results from pairs of experiments, the effects of each of these environmental changes can be determined.
Expanded continental ice and reduced atmospheric CO2 are found to have a substantial impact on global mean temperature. The ice sheet effect is confined almost exclusively to the Northern Hemisphere, while lowered CO2 cools both hemispheres. Changes in land albedo over ice-free areas have only a minor thermal effect on a global basis. The reduction of CO2 content in the atmosphere is the primary contributor to the cooling of the Southern Hemisphere. The model sensitivity to both the ice sheet and CO2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum. Expanded continental ice and reduced atmospheric CO2 are found to have a substantial impact on global mean temperature. The ice sheet effect is confined almost exclusively to the Northern Hemisphere, while lowered CO2 cools both hemispheres. Changes in land albedo over ice-free areas have only a minor thermal effect on a global basis. The reduction of CO2 content in the atmosphere is the primary contributor to the cooling of the Southern Hemisphere. The model sensitivity to both the ice sheet and CO2effects is characterized by a high latitude amplification and a late autumn and early winter maximum.
Substantial changes in Northern Hemisphere tropospheric circulation are found in response to LGM boundary contitions during winter. An amplified flow pattern and enhanced westerlies occur in the vicinity of the North American and Eurasian ice sheets. These alterations of the troposopheric circulation are primarily the result of the ice sheet effect, with reduced CO2 contributing only a slight amplification of the ice sheet-induced pattern.
Manabe, Syukuro, and Richard T Wetherald, 1987: Large-scale changes of soil wetness induced by an increase in atmospheric carbon dioxide. Journal of the Atmospheric Sciences, 44(8), 1211-1235. Abstract PDF
The change in soil wetness in response to an increase of atmospheric concentration of carbon dioxide is investigated by two versions of a climate model which consists of a general circulation model of the atmosphere and a static mixed layer ocean. In the first version of the model, the distribution of cloud cover is specified whereas it is computed in the second version incorporating the interaction among cloud cover, radiative transfer and the atmospheric circulation. The CO2-induced changes of climate and hydrology are evaluated based upon a comparison between two quasi-equilibrium climates of a model with a normal and an above normal concentration of atmospheric carbon dioxide. It is shown that, in response to a doubling (or quadrupling) of atmospheric carbon dioxide, soil moisture is reduced in summer over extensive midcontinental regions of both North America and Eurasia in middle and high latitudes. Based upon the budget analysis of heat and water, the physical mechanisms responsible for the CO2-induced changes of soil moisture are determined for the following four regions: northern Canada, northern Siberia, the Great Plains of North America and southern Europe. It is found that, over northern Canada and northern Siberia, the CO2-induced reduction of soil moisture in summer results from the earlier occurrence of the snowmelt season followed by a period of intense evaporation. Over the Great Plains of North America, the earlier termination of the snowmelt season also contributes to the reduction of soil moisture during the summer season. In addition, the rainy period of late spring ends earlier, thus enhancing the CO2-induced reduction of soil moisture in summer. In the model with variable cloud cover, the summer dryness over the Great Plains is enhanced further by a reduction of cloud amount and precipitation in the lower model atmosphere. This reduction of cloud amount increases the solar energy reaching the continental surface and the rate of potential evaporation. Both the decrease of precipitation and the increase of potential evaporation further reduce the soil moisture during early summer and help to maintain it at a low level throughout the summer. Over southern Europe, the CO2-induced reduction of soil wetness occurs in a qualitatively similar manner, although the relative magnitude of the contribution from the change in snowmelt is smaller. During winter, soil moisture increases poleward of 30°N in response to an increase of atmospheric carbon dioxide. Because of the CO2-induced warming, a greater fraction of the total precipitation occurs as rainfall rather than snowfall. The warmer atmosphere also causes the accumulated snow cover to melt during winter. Both processes act to increase the soil moisture in all four regions during the winter season. The increase of soil moisture is enhanced further in high latitudes due to the increase of precipitation resulting from the penetration of warm, moisture-rich air into higher latitudes. The CO2-induced warming of the lower model troposphere increases with increasing latitude. The present analysis suggests that the changes of soil wetness described in this investigation are controlled by the latitudinal profile of the warming and are very broad scale, mid-continental phenomena.
The geographical distribution of the change in soil wetness in response to an increase in atmospheric carbon dioxide was investigated by using a mathematical model of climate. Responding to the increase in carbon dioxide, soil moisture in the model would be reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, western Europe, northern Canada, and Siberia. These results were obtained from the model with predicted cloud cover and are qualitatively similar to the results from several numerical experiments conducted earlier with prescribed cloud cover.
The role of cloud cover in determining the sensitivity of climate has been a source of great uncertainty. This article reviews the distributions of cloud cover change from several climate sensitivity experiments conducted at the Geophysical Fluid Dynamics Laboratory of NOAA (GFDL) and other institutions. Two of the sensitivity experiments conducted at GFDL used a general circulation model with a limited computational domain and idealized geography, whereas three other experiments were conducted by the use of a global model with realistic geography. A thermal forcing imposed was either a change of solar constant or that of the CO2-concentration in the atmosphere. It was found that in all five cases, clouds were decreased in the moist, convectively active regions such as the tropical and middle latitude rainbelts, whereas they increased in the stable region near the model surface from middle to higher latitudes. In addition, cloud also increased in the lower model stratosphere and generally decreased in the middle and upper troposphere for practically all latitudes.
A comparison of the cloud changes obtained from investigations carried out at other institutions reveals certain qualitative (but not necessarily quantitative) similarities to the GFDL results. These similarities include a general reduction of tropospheric cloud cover especially in the vicinity of the rainbelts, a general increase of lower stratospheric cloud cover for almost all latitudes and an increase of low stratiform cloud in high latitudes.
Bryan, Kirk, and Syukuro Manabe, 1985: A coupled ocean-atmosphere and the response to increasing atmospheric CO2 In Coupled Ocean-Atmosphere Models, Amsterdam; The Netherlands, Elsevier Science Publishers, 1-6. PDF
Manabe, Syukuro, and Anthony J Broccoli, 1985: A comparison of climate model sensitivity with data from the last glacial maximum. Journal of the Atmospheric Sciences, 42(23), 2643-2651. Abstract PDF
attempt has been made to use paleoclimatic data from the last glacial maximum to evaluate the sensitivity of two versions of an atmosphere/mixed-layer ocean model. Each of these models has been used to study the CO2-induced changes in climate. The models differ in their treatment of cloudiness, with one using a fixed cloud distribution and the other using a simple parameterization to predict clouds. The models also differ in the magnitude of their response to a doubling of atmospheric CO2, with the variable cloud model being nearly twice as sensitive as the fixed cloud version. Given the distributions of continental ice sheets, surface albedo, and the reduced carbon dioxide concentration of the ice age, the climate of the last glacial maximum (LGM) is simulated by each model and compared with the corresponding simulation of the present climate. Both models generate differences in sea surface temperature and surface air temperature which compare favorably with estimates of the actual differences in temperature between the LGM and the present. However, it is difficult to determine which version of the model is more realistic in simulating the ice age climate for two reasons: 1) the differences between the two models are relatively small; and 2) there are substantial uncertainties in the paleoclimatic data. Nevertheless, the similarity between the LGM simulations and the available paleoclimatic data suggests that the estimates of CO2-induced climate change obtained from these models may not be too far from reality.
The climate influence of the land ice that existed 18,000 years before present (18K B.P.) is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean. Simulated climates are obtained from two versions of the model: one with the land ice distribution of the present and the other with that of 18K B.P. In the northern hemisphere the tropospheric flow field is strongly influenced by the Laurentide ice sheet and features a split flow straddling the ice sheet, with a strong jet stream forming the southern branch. The northern branch of the flow brings very cold air over the North Atlantic Ocean, where thick sea ice is maintained. The distribution of sea surface temperature (SST) difference between the two experiments in the northern hemisphere resembles the difference between the SST at 18K B.P. and at present, as estimated by the CLIMAP Project (1981). The 18K B.P. ice sheets have very little influence upon atmospheric temperature and SST in the southern hemisphere. This is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation. Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North American and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.
Manabe, Syukuro, and Kirk Bryan, 1985: CO2 -induced change in a coupled ocean-atmosphere model and its paleoclimatic implications. Journal of Geophysical Research, 90(C6), 11,689-11,707. Abstract PDF
The climatic effects of very large changes of CO2 concentration in the atmosphere are explored using a general circulation model of the coupled ocean-atmosphere system. As a simplification the model has an annual mean insolation and a highly idealized geography. A series of climatic equilibria are obtained for cases with 1/2, 1/sq rt 2, 1, 2, 4, and 8 times the present CO2 concentration in the atmosphere. The results from these six numerical experiments indicate the climatic signatures of large CO2 changes in the atmosphere and in the abyssal and surface waters of the ocean. As the CO2 concentration in the model atmosphere increased from 1 to 8 times the normal value, the meridional gradient of surface air temperature decreased, while that of upper tropospheric temperature increased in agreement with the results of earlier CO2 climate sensitivity studies. However, the intensity and latitudinal placement of the atmospheric jet hardly changed. Despite the reduction of meridional temperature gradient, the meridional density gradient of water at the ocean surface changed little because of the increase of thermal expansion coefficient of seawater with increasing temperature. Thus the intensity of thermohaline circulation in the ocean model does not diminish as expected in the range from 1 to 8 times the normal atmospheric CO2 concentration. As was shown in an earlier study, the CO2-induced changes in the deep sea follow the change of sea surface temperature in high latitudes and thus are much larger than the globally averaged changes of sea surface temperature. The model predicts that the area mean rates of precipitation, evaporation, and runoff increase with increasing CO2 concentration in the atmosphere. The latitudes of the arid zone and the high surface pressure belt in the subtropics are almost constant in the entire range of 1-8 times normal CO2. In general, the climatic signature obtained from the model appears to be consistent with a CO2 hypothesis for the climatic changes in the Cenozoic with the following exception: the tropical sea surface temperature in the model has a small but significant increase with increasing atmospheric CO2 concentration, while tropical sea surface temperature as deduced from the isotopic record appears to have no systematic trend during the Tertiary. It is found that the climate corresponding to one-half normal CO2 is markedly different from the normal and high-CO2 cases. Sea ice extends to middle latitudes, and the thermohaline circulation in the model ocean loses its intensity and is largely confined to an area between the sea ice margin and the equator. The poleward heat transport by ocean currents is very small in high latitudes, markedly reducing the surface air temperature there. It is suggested that a similar process, which enhances the positive albedo feedback effect of sea ice, played a key role in reducing surface air temperatures over the North Atlantic during the last glacial maximum.
Manabe, Syukuro, and Anthony J Broccoli, 1984: Ice-age climate and continental ice sheets: some experiments with a general circulation model. Annals of Glaciology, 5, 100-105. Abstract PDF
The climatic influence of the land ice which existed 18 ka BP is investigated using a climate model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration. The model consists of an atmospheric general circulation model coupled with a static mixed layer ocean model. Simulated climates are obtained from each of two versions of the model: one with the land-ice distribution of the present and the other with that of 18 ka BP.
In the northern hemisphere, the difference in the distribution of sea surface temperature (SST) between the two experiments resembles the difference between the SST at 18 ka BP and at present as estimated by CLIMAP Project Members (1981). In the northern hemisphere a substantial lowering of air temperature also occurs in winter, with a less pronounced cooling during summer. The mid-tropospheric flow field is influenced by the Laurentide ice sheet and features a split jet stream straddling the ice sheet and a long wave trough along the east coast of North America. In the southern hemisphere of 18 ka BP, the ice sheet has little influence on temperature. An examination of hemispheric heat balances indicates that this is because only a small change in interhemispheric heat transport exists, as the in situ radiative compensation in the northern hemisphere counterbalances the effective reflection of solar radiation by continental ice sheets.
Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North America and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.
Manabe, Syukuro, and Anthony J Broccoli, 1984: Influence of the Climap ice sheet on the climate of a general circulation model: implications for the Milankovitch theory In Milankovitch and Climate, Part 2, Amsterdam, The Netherlands, Reidel Publishing Co, 789-799. PDF
Spelman, Michael J., and Syukuro Manabe, 1984: Influence of oceanic heat transport upon the sensitivity of a model climate. Journal of Geophysical Research, 89(C1), 571-586. Abstract PDF
The influence of oceanic heat transport on the sensitivity of climate to an increase of the atmospheric CO2 concentration is studied by comparing the CO2-induced changes of two mathematical models. The first model is a general circulation model of the coupled ocean-atmosphere system which includes ocean currents. In the second model the oceanic component of the first model is replaced by a simple mixed layer without ocean currents. Both models have limited computational domain with idealized geography and annual mean insolation. For each model, the sensitivity of climate is evaluated from the difference between the equilibrium climates of the normal CO2 and 4 times the normal CO2 concentrations. The results indicate that the presence of ocean currents reduces the sensitivity of surface air temperature because of the difference in magnitude of the surface albedo feedback effect. The poleward transport of heat by ocean currents raises the surface temperature at high latitudes, shifts poleward the margins of snow and sea ice, decreases the contribution of the albedo feedback effect, and reduces the sensitivity of climate. The equilibrium response of climate is compared with the transient response of climate to a sudden increase of atmospheric CO2 content. According to this comparison, the latitudinal dependence of the equilibrium response of zonally averaged surface temperature is qualitatively similar to the transient response approximately 25 years after the time of the sudden CO2 increase. This result suggests that the distribution of the zonally averaged temperature change in response to a gradual increase of atmospheric carbon dioxide also resembles the distribution of the equilibrium response provided that the characteristic time scale of the CO2 increase is longer than 25 years.
Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1984: The effect of soil moisture on a short-term climate and hydrology change--A numerical experiment. Monthly Weather Review, 112(3), 474-490. Abstract PDF
This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model with a limited computational domain and idealized geography.
The soil at three latitude bands, namely 30 degrees N-60 degrees N, 0-30 degrees N, and 15 degrees S-15 degrees N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, if the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30 degrees N-60 degrees N case which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0-30 degrees N case. The soil at three latitude bands, namely 30 degrees N-60 degrees N, 0-30 degrees N, and 15 degrees S-15 degrees N is initially saturated with moisture.
Although both the 30 degrees N-60 degrees N and 15 degrees S-15 degrees N latitude regions occur under rainbelts, the soil moisture anomaly for the 15 degrees S-15 degrees N case does not persist as long as it does for the 30 degrees N-60 degrees N case. This is because in the 15 degrees S-15 degrees N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30 degrees N-60 degrees N case.
The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in changes of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.
Manabe, Syukuro, 1983: Carbon dioxide and climate. Advances in Geophysics, 25, 39-82. PDF
Manabe, Syukuro, 1983: Oceanic influence on climate: studies with mathematical models of the joint ocean-atmosphere system In Large-scale Oceanographic Experiments in the WCRP, Vol. 2. Geneva, Switzerland, World Meteorological Organization, 1-27.
Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1983: A model study of the short-term climatic and hydrologic effects of sudden snow-cover removal. Monthly Weather Review, 111(5), 1013-1024. Abstract PDF
This paper describes the results from a set of numerical experiments which stimulate the effect of a large-scale removal of snow cover in middle and high latitudes during the early spring season. This is done through use of a simplified general circulation model with a limited computational domain and idealized geography.
It is found that removal of snow cover reduces the water available to the soil through snowmelt and decreases soil moisture in this region during the following seasons. Furthermore, it also reduces surface albedo in this region and increases absorption of insolation by the ground surface. This, in turn, heats the ground surface and allows more evaporation to occur. However, the change of evaporation is relatively small owing to the low values of surface temperature in high latitudes. Therefore, the negative anomaly of soil moisture induced initially by the removal of snow cover persists for the entire spring and summer seasons.
The removal of snow cover also affects the thermal and dynamical structure of the atmosphere. It is found that the increase of surface temperature extends into the upper troposphere thereby reducing both meridional temperature gradient and zonal wind in high latitudes.
The ocean's role in the delayed response of climate to increasing atmospheric carbon dioxide has been studied by means of a detailed three-dimensional climate model. A near-equilibrium state is perturbed by a fourfold, step-function increase in atmospheric carbon dioxide. The rise in the sea surface temperature was initially much more rapid in the tropics than at high latitudes. However, the fractional response, as normalized on the basis of the total difference between the high carbon dioxide and normal carbon dioxide climates, becomes almost uniform at all latitudes after 25 years. Because of the influence of a more rapid response over continents, the normalized response of the zonally averaged surface air temperature is faster and becomes nearly uniform with respect to latitude after only 10 years.
Hahn, D G., and Syukuro Manabe, 1982: Observational network and climate monitoring In Proceedings of the Sixth Annual Climate Diagnostics Workshop, Washington, DC, NOAA, 229-235.
Manabe, Syukuro, 1982: Simulation of climate by general circulation models with hydrologic cycles In Land Surface Processes in Atmospheric General Circulation Models, UK, Cambridge University Press, 19-66.
Manabe, Syukuro, and Ronald J Stouffer, 1982: Seasonal and latitudinal variation of the CO2-induced change in a climate of an atmosphere-mixed-layer ocean model. In Carbon Dioxide Effects Research and Assessment, DOE/CONF-8106214, UC-11, Washington, DC, U.S. Dept. of Energy, 79-94.
Smagorinsky, Joseph, Kirk Bryan, and Syukuro Manabe, et al., 1982: CO2/Climate Review Panel In Carbon Dioxide and Climate: A Second Assessment, Washington, DC, National Academy Press, 1-72.
Manabe, Syukuro, and D G Hahn, 1981: Simulation of atmospheric variability. Monthly Weather Review, 109(11), 2260-2286. Abstract PDF
A spectral atmospheric circulation model is time-integrated for approximately 18 years. The model has a global computational domain and realistic geography and topography. The model undergoes an annual cycle as daily values of seasonally varying insolation and sea surface temperature are prescribed without any interannual variation. It has a relatively low computational resolution with 15 spectral components retained in both zonal and meridional directions. Analysis of the results from the last 15 years of the time integration indicates that, in middle and high latitudes, the model approximately reproduces the observed geographical distribution of the variability (i.e., standard deviation) of daily, monthly and yearly mean surface pressure and temperature.
In the tropics, the model tends to underestimate the variability of surface pressure, particularly at longer time scales. This result suggests the importance of processes with long time scales such as ocean-atmosphere interaction, in maintaining the variability of the atmosphere in low latitudes.
It is shown that the global mean values of standard deviation of daily, 5-daily, 10-daily, monthly, seasonal and annual mean surface pressure of the model atmosphere may be approximately fitted by a corresponding set of standard deviations of a red noise time series with a decay time scale of slightly longer than four days. However, it appears that the temporal variation of surface pressure also includes minor contributions from disturbances with much longer decay time scales.
In general, the model tends to underestimate the persistence (or decay time scale) of atmospheric disturbances. However, it reproduces some of the features of the observed geographical distribution of decay time scale of the surface pressure fluctuations in middle and high latitudes.
The observed standard deviation of annual, hemispheric mean surface air temperature also is compared with model results. Although a clearcut evaluation of model performance is somewhat hampered by observational uncertainty, it appears that the model's value amount to a substantial fraction of the corresponding standard deviation derived from observational studies.
To investigate the hydrologic changes of climate in response to an increase of CO2-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO2 -induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in the air.
It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO2 -concentration in air. This CO2-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.
To investigate the hydrologic changes of climate in response to an increase of CO2-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO2 -induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in the air.
It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO2 -concentration in air. This CO2-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.
Hahn, D G., and Syukuro Manabe, 1980: Simulation of atmospheric variability In Proceedings of the Fourth Annual Climate Diagnostics Workshop, Rockville, MD, NOAA, 398-399.
Manabe, Syukuro, and Ronald J Stouffer, 1980: Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere. Journal of Geophysical Research, 85(C10), 5529-5554. Abstract PDF
This study investigates the response of a global model of the climate to the quadrupling of the CO2 concentration in the atmosphere. The model consists of (1) a general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a simple mixed layer model of the oceans. It has a global computational domain and realistic geography. For the computation of radiative transfer, the seasonal variation of insolation is imposed at the top of the model atmosphere, and the fixed distribution of cloud cover is prescribed as a function of latitude and of height. It is found that with some exceptions, the model succeeds in reproducing the large-scale characteristics of seasonal and geographical variation of the observed atmospheric temperature. The climatic effect of a CO2 increase is determined by comparing statistical equilibrium states of the model atmosphere with a normal concentration and with a 4 times the normal concentration of CO2 in the air. It is found that the warming of the model atmosphere resulting from the CO2increase has significant seasonal and latitudinal variation. Because of the absence of an albedo feedback mechanism, the warming over the Antarctic continent is somewhat less than the warming in high latitudes of the northern hemisphere. Over the Arctic Ocean and its surroundings, the warming is much larger in winter than summer, thereby reducing the amplitude of seasonal temperature variation. It is concluded that this seasonal asymmetry in the warming results from the reduction in the coverage and thickness of the sea ice. The warming of the model atmosphere results in an enrichment of the moisture content in the air and an increase in the poleward moisture transport. The additional moisture is picked up from the tropical ocean and is brought to high latitudes where both precipitation and runoff increase throughout the year. Further, the time of rapid snowmelt and maximum runoff becomes earlier.
Manabe, Syukuro, and Richard T Wetherald, 1980: On the distribution of climate change resulting from an increase in CO2 content of the atmosphere. Journal of the Atmospheric Sciences, 37(1), 99-118. Abstract PDF
A study of the climatic effect of doubling or quadrupling of CO2 in the atmosphere has been continued by the use of a simple general circulation model with a limited computational domain, highly idealized geography, no seasonal variation of insolation, and a simplified interaction between cloud and radiative transfer.
The results from the numerical experiments reveal that the response of the model climate to an increase of CO2 content in air is far from uniform geographically. For example, one can identify the high-latitude region of the continent where the runoff rate increases markedly, a zonal belt of decreasing soil moisture around 42 degrees latitude, and a zone of enhanced wetness along the east coast of the subtropical portion of the model continent.
The general warming and the increase of moisture content of air, which results from a CO2 increase, contributes to the large reduction of the meridional temperature gradient in the lower model troposphere because of 1) poleward retreat of highly reflective snow cover and 2) large increase in the poleward transport of latent heat. The reduction of the meridional temperature gradient appears to reduce not only the eddy kinetic energy, but also the variance of temperature in the lower model troposphere. The penetration of moisture into higher latitudes in the CO2-rich warm climate is responsible for the large increase of the rates of precipitation and runoff in high latitudes of the model.
This study discusses how the sensitivity of climate may be affected by the variation of cloud cover based on the results from numerical experiments with a highly simplified, three-dimensional model of the atmospheric general circulation. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized geography, no heat transport by ocean currents and no seasonal variation. Two versions of the model are constructed. The first version includes prognostic schemes of cloud cover and its radiative influences, and the second version uses a prescribed distribution of cloud cover for the computation of radiative transfer. Two sets of equilibrium climates are obtained from the long-term integrations of both versions of the model for several values of the solar constant. Based on the comparison between the variable and the fixed cloud experiments, the influences of cloud cover variation on the response of a model climate to an increase of the solar constant are identified.
It is found that, in response to an increase of the solar constant, cloudiness diminishes in the upper and middle troposphere at most latitudes and increases near the earth's surface and the lower stratosphere of the model particularly in higher latitudes. Because of the changes described above, the total cloud amount diminishes in the region equatorward of 50 degrees latitude with the exception of a narrow sub-tropical belt. However, it increases in the region poleward of this latitude. In both regions, the area mean change in the net incoming solar radiation, which is attributable to the cloud-cover change described above, is approximately compensated by the corresponding change in the outgoing terrestrial radiation at the top of the model atmosphere. For example, equatorward of 50 degrees latitude, the reduction of both cloud amount and effective cloud-top height contributes to the increase in the area-mean flux of outgoing terrestrial radiation and compensates for the increase in the flux of net incoming solar radiation caused by the reduction of cloud amount. Poleward of 50 degrees latitude, the increase of cloudiness contributes to the reduction of both net incoming solar and outgoing terrestrial fluxes at the top of the model atmosphere. Although the effective cloud-top height does not change as it does in lower latitudes, the changes of these fluxes approximately compensate each other because of the smallness of insolation in high latitudes. Owing to the compensations mentioned above, the changes of cloud cover have a relatively minor effect on the sensitivity of the area-mean climate of the model.
Kurbatkin, G P., Syukuro Manabe, and D G Hahn, 1979: The moisture content of the continents and the intensity of summer monsoon circulation. Soviet Meteorology and Hydrology, 11, 5-11. Abstract
By means of a spectral model of the atmosphere developed by the Geophysical Fluid Dynamics Laboratory/NOAA, we studied the effect of changes in the moisture content of the continents on the rate of summer monsoon circulation in the middle latitudes. The model includes the annual climate cycle and the hydrology of the atmosphere and the continents. An analysis of the numerical experiments demonstrated that drainage of the continents may lead to a decrease in precipitation not only over the continents, but also over the ocean; drainage may simulataneously increase the rate of planet-wide summer monsoon circulation in the middle latitudes, which may be an important condition in the annual climate cycle for summer radiant heating of the ocean.
Manabe, Syukuro, 1979: Effect of increasing the CO2 concentration on the climate of a general circulation model In Carbon Dioxide Effects Research and Assessment Program: Workshop on the Global Effects of Carbon Dioxide from Fossil Fuels, U.S. Dept. of Energy, Office of Health & Environmental Research, Washington, DC, Springfield, VA, NTIS, 100-101.
Manabe, Syukuro, Kirk Bryan, and Michael J Spelman, 1979: A global ocean-atmosphere climate model with seasonal variation for future studies of climate sensitivity. Dynamics of Atmospheres and Oceans, 3, 393-426.
Manabe, Syukuro, D G Hahn, and J L Holloway, Jr, 1979: Climate simulations with GFDL spectral models of the atmosphere: Effect of spectral truncation In Report of the JOC Study Conference on Climate Models: Performance, Intercomparison and Sensitivity Studies,, Vol. I, Global Atmospheric Research Programme, Joint Organizing Committee, GARP Publications No. 22., World Meteorological Organization, 41-94. PDF
An increase in the CO2-content of the atmosphere resulting from man's activity could have a significant effect on the climate in the near future. We describe here some new results from a study of the response of a mathematical model of the climate to an increase in the CO2-content of the air.
Wetherald, Richard T., and Syukuro Manabe, 1979: Sensitivity studies of climate involving changes in CO2 concentration In Man's Impact on Climate, New York, NY, Elsevier/North-Holland, Inc., 57-64. Abstract
Attempts are made to estimate the temperature changes resulting from increasing the present CO2 concentration by the use of: (a) a one-dimensional radiative convective equilibrium model and, (b) a simplified three-dimensional general circulation model. The following assumptions are made in the 3-D model: a limited computational domain, an idealized topography, zero surface heat capacity, no heat transport by ocean currents and an annual mean insolation.
In general, the CO2 increase raises the temperature of the model troposphere, whereas, it lowers that of the model stratosphere for both the 1-D and 3-D models. It is found that the tropospheric warming is somewhat larger for the 3-D model as compared with that obtained from the 1-D radiative convective equilibrium model. In particular, the increase of surface temperature in the 3-D model in high latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective heating to the lowest layer. It is also found that increasing the CO2 concentration significantly increases the overall intensity of the hydrologic cycle of the 3-D model. However, this does not necessarily imply the increase of wetness everywhere in the model region. In particular, the sign of wetness change depends upon the geographical location within the model domain.
Manabe, Syukuro, and D G Hahn, 1977: Simulation of the tropical climate of an ice age. Journal of Geophysical Research, 82(27), 3889-3911. Abstract PDF
Numerical time integrations of a general circulation model of the atmosphere are performed with both modern and ice age boundary conditions. It is shown that the climate of continental portions of the tropics in the ice age simulation is much drier than that of the modern climate simulation. According to comparisons of results from the two experiments, tropical continental aridity of the ice age results from stronger surface outflow from (or weaker surface inflow into) continents. The intensification of outflow from (or weakening of inflow into) tropical continental regions results from the fact that in response to ice age boundary conditions, atmospheric temperature is reduced more over continents than over oceans. With the exception of high latitudes, boundary condition differences between the two experiments consist mainly of changes of the prescribed values of sea surface temperature and continental albedo. In order to evaluate the relative contributions of these changes in producing continental tropical aridity in the ice age simulation, a third numerical experiment is time-integrated in which a hybrid combination of ice age sea surface temperatures and modern continental albedo values is prescribed. From intercomparisons between results from this and the previous two experiments it is shown that the effect of increased continental albedo is mainly responsible for the weak Asian monsoon in the ice age simulation.
Hahn, D G., and Syukuro Manabe, 1976: Reply. Journal of the Atmospheric Sciences, 33(11), 2258-2262. PDF
Manabe, Syukuro, and Jerry D Mahlman, 1976: Simulation of seasonal and interhemispheric variations in the stratospheric circulation. Journal of the Atmospheric Sciences, 33(11), 2185-2217. Abstract PDF
This paper describes the stratosphere as simulated by the time integration of a global model of the atmosphere as developed at the Geophysical Fluid Dynamics Laboratory of NOAA.
It is shown that the model is capable of simulating a number of the features of the seasonal variation in the stratosphere. For example, it qualitatively reproduces the seasonal reversals of zonal wind direction in the mid-stratosphere between westerlies in winter and the zonal easterlies prevailing during the summer season. In the mid-latitude region of the lower model stratosphere, zonal mean temperature is highest in the winter when solar radiation is weak. At the cold equatorial tropopause of the model, the seasonal variation of temperature is also quite different from that which would be expected from the seasonal variation of solar radiation. These results are in qualitative agreement with the observed variation.
Attempts are made to identify the factors which are responsible for the various aspects of the seasonal variation of the model stratosphere, based upon detailed budget analyses of angular momentum, heat and eddy kinetic energy. It is found that, with the exception of the high latitude regions, the seasonal variation of temperature in the lower model stratosphere is essentially controlled by dynamical effects rather than by the seasonal variation of local heating due to solar radiation.
The stratosphere as simulated by the global model has large interhemispheric asymmetries in the shape of the polar westerly vortex, the magnitudes and the distributions of eddy kinetic energy, and the meridional circulation in the winter hemisphere. Interhemispheric asymmetries in orography are apparently responsible for the interhemispheric differences in the quasi-stationary component of energy flux from the troposphere to the stratosphere of the model, and thus account for many of the asymmetries in the stratospheric circulation. In particular, the simulated stratospheric Aleutian anticyclone is shown to be related to the presence of the strong quasi-stationary tropospheric jet stream off the east coast of Asia.
Some of the important shortcomings of the model in simulating the stratosphere include an exaggeration of the magnitudes of the various components of the eddy kinetic energy budget at the top computational level (10 mb) of the model and an overestimation of the intensity of the polar westerly vortex. Also, the model fails to reproduce the mid-winter "sudden stratospheric warming" phenomenon and the quasi-biennial wind reversal in the equatorial stratosphere. It is suggested that the performance of the model at the top level suffers from the coarseness in the vertical finite-difference resolution and the lid boundary condition imposed at the top of the model atmosphere.
A numerical experiment has been carried out with a joint model of the ocean and atmosphere. The 12-level model of the world ocean predicts the fields of horizontal velocity, temperature and salinity. It includes the effects of bottom topography, and a simplified model of polar pack ice. The numerical experiment allows the joint ocean-atmosphere model to seek an equilibrium over the equivalent of 270 years in the ocean time scale. The initial state of the ocean is uniform stratification and complete rest. Although the final temperature distribution is more zonal than it should be, the major western boundary currents and the equatorial undercurrent are successfully predicted. The calculated salinity field has the correct observed range, and correctly indicates that the Atlantic is saltier than the Pacific. It also predicts that the surface waters of the North Pacific are less saline than the surface waters of the South Pacific in accordance with observations. The pack ice model predicts heavy ice in the Arctic Ocean, and only very light pack ice along the periphery of the Antarctic Continent.
The poleward heat transport of the model is very sensitive to the strength of the circulation in the vertical-meridional plane. The heat transport is strongest in the trade wind belt where Ekman drift and thermohaline forces act together to cause a net flow of surface water toward the poles. At higher latitudes in the westerly belt the wind and thermohaline forces on the meridional circulation tend to oppose each other. As a result, the heat transport is weaker. Heat balance computations made from observed data consistently show that the maximum heat transport by ocean currents is shifted 10 degrees - A numerical experiment has been carried out with a joint model of the ocean and atmosphere. The 12-level model of the world ocean predicts the fields of horizontal velocity, temperature and salinity. It includes the effects of bottom topography, and a simplified model of polar pack ice. The numerical experiment allows the joint ocean-atmosphere model to seek an equilibrium over the equivalent of 270 years in the ocean time scale. The initial state of the ocean is uniform stratification and complete rest. Although the final temperature distribution is more zonal than it should be, the major western boundary currents and the equatorial undercurrent are successfully predicted. The calculated salinity field has the correct observed range, and correctly indicates that the Atlantic is saltier than the Pacific. It also predicts that the surface waters of the North Pacific are less saline than the surface waters of the South Pacific in accordance with observations. The pack ice model predicts heavy ice in the Arctic Ocean, and only very light pack ice along the periphery of the Antarctic Continent.
The poleward heat transport of the model is very sensitive to the strength of the circulation in the vertical-meridional plane. The heat transport is strongest in the trade wind belt where Ekman drift and thermohaline forces act together to cause a net flow of surface water toward the poles. At higher latitudes in the westerly belt the wind and thermohaline forces on the meridional circulation tend to oppose each other. As a result, the heat transport is weaker. Heat balance computations made from observed data consistently show that the maximum heat transport by ocean currents is shifted 10 degrees - 20 degrees equatorward relative to the maximum poleward heat transport by the atmosphere in middle latitudes. The effect of the zonal wind in enhancing poleward heat transport at low latitudes and suppressing it in middle latitudes is offered as an explanation.
Hahn, D G., and Syukuro Manabe, 1975: The role of mountains in the south Asian monsoon circulation. Journal of the Atmospheric Sciences, 32(8), 1515-1541. Abstract PDF
An 11-level numerical model of the atmospheric circulation which has a prescribed seasonal variation of insolation and sea surface temperatures is integrated with respect to time for approximately three model years. The model is global in domain and incorporates a smoothed mountain topography. In order to investigate the role that mountains play in the south Asian monsoon circulation, a second numerical experiment, exactly the same as the first except that all mountains are removed, is integrated with respect to time from 25 March through July.
Analysis of the model with mountains reveals that the large-scale circulation associated with the south Asian monsoon is well simulated. However, the onset of the monsoon is approximately 10-15 days later than normal, and the atmosphere over the western Pacific seems to be dynamically too active, while the atmosphere over the northern reaches of the Bay of Bengal and northern India is relatively inactive.
Comparison of the simulation with mountains with the simulation without mountains reveals that the presence of mountains is instrumental in maintaining the south Asian low pressure system as the continental low forms far to the north and east in the simulation without mountain topography. In the model with mountains, much higher temperatures are maintained in the middle and upper troposphere over the Tibetan Plateau, a region where upward motion and latent heating dominate. Without mountains, downward motion and sensible heating by the earth's surface dominate in this region. In the simulation with mountains, high temperatures over Tibet produce a low pressure envelope over these mountains which extends southward over the plains of south Asia. The low pressure belt being located farther south than in the simulation without mountains produces a stronger north-south pressure gradient which enables moist southerly flow at the surface to penetrate farther northward into Asia. Many of the features of the monsoon break persist in the model without mountains as copious precipitation extends northward only to south India. Clearly, mountain effects help to extend a monsoon climate farther north onto the Asian continent.
The evolution of the south Asian monsoon is also influenced by the effects of mountains. Near the time of onset in the model with mountains, the subtropical jet abruptly jumps northward from a latitude just south of Tibet, 25 degrees N, to a mean summertime position along 45 degrees N. In the model without mountains, the subtropical jet gradually moves northward over a period of about two months, finally reaching a summertime position approximately 10 degrees farther south than in the model with mountains. At the time of onset in the model with mountains, humid southerly flow near the earth's surface suddenly extends northward from equatorial latitudes to the south Asian low pressure belt centered at 30 degrees N. In the model without mountains, humid southerly flow extends northward from equatorial regions, but it doesn's extend as far northward as northern and central India. These differences are attributed to mechanical and thermodynamical effects of the Tibetan Plateau.
Manabe, Syukuro, 1975: Cloudiness and the radiative, convective equilibrium In The Changing Global Environment, S. F. Singer, ed., Amsterdam, The Netherlands, D. Reidel Publishing Co., 175-176. Abstract
The dependence of the temperature of the Earth's surface upon the cloud cover at various altitudes is estimated. The effect of contrail on the surface temperature is discussed.
Manabe, Syukuro, 1975: The use of comprehensive general circulation modelling for studies of the climate and climate variation In The Physical Basis of Climate and Climate Modelling, Report of the International Study Conference, GARP Publications Series No. 16, World Meteorological Organization, 148-162. PDF
Manabe, Syukuro, and J L Holloway, Jr, 1975: The seasonal variation of the hydrologic cycle as simulated by a global model of the atmosphere. Journal of Geophysical Research, 80(12), 1617-1649. Abstract PDF
A numerical model of the atmosphere with a seasonal variation of insolation and sea surface temperature is time integrated for over 3 simulation years on a finite difference grid network having a nearly uniform horizontal resolution of approximately 265 km. There are 11 levels in the model from 80 m to 31 km above the ground. The model has realistic continents with smoothed topography. In addition to wind, temperature, pressure, and water vapor, the model simulates rainfall, snowfall, and evaporation at the surface. The runoff rate and the rates of change of soil moisture and snow depth are determined by a budget of liquid water, snow, and heat at the ground surface. The simulated precipitation and other hydrologic quantities are compared with those derived from observed data. In addition, the correspondence between the distribution of precipitation rate and those of other relevant quantities, such as sea level pressure and kinetic energy of transient disturbances, is examined. To obtain an overall impression of the climate simulation, a map of Köppen climate types, which is obtained from simulated temperatures and precipitation rates throughout the year, is compared with a similar map based upon observed climatic data. It is shown that the model locates the major arid regions of the earth, such as the Sahara Desert in northern Africa and the deserts of central Asia and Australia. Furthermore, the tropical rain belt and its seasonal movement are well reproduced. The model approximately simulated the changeover from dry winter to wet summer conditions in southern Asia and the seasonal reversal of the monsoon wind system over Asia. The wet zone in middle and high latitudes, such as in Canada, Europe, and western Siberia, is also simulated by the model. Examination of the distribution of runoff over continents of the model reveals that the watersheds of many important rivers of the world are correctly positioned in the simulation. In general, the quality of the simulation tends to deteriorate in the neighborhood of major mountain ranges. Furthermore, the rate of precipitation and that of runoff over continental regions in the model tropics is much larger than estimates of the actual rates of these quantities. Nevertheless, this study demonstrates that the model is capable of reproducing many of the basic features of the seasonal variation of hydrology and climate on a global scale.
A joint ocean-atmosphere model covering the entire globe has been constructed at the Geophysical Fluid Dynamics Laboratory (GFDL) of NOAA. This model differs from the earlier version of the joint model of Bryan and Manabe both in global domain and inclusion of realistic rather than idealized topography. This part of the paper describes the structure of the atmospheric portion of the joint model and discusses the atmospheric circulation and climate that emerges from the time integration of the model. The details of the oceanic part are given by Bryan et al. (1974), hereafter referred to as Part II.
The atmospheric part of the model incorporates the primitive equations of motion in a spherical coordinate system. The numerical problems associated with the treatment of mountains are minimized by using the "sigma" coordinate system in which pressure, normalized by surface pressure, is the vertical coordinate. For vertical finite differencing, nine levels are chosen so as to represent the planetary boundary layer and the stratosphere as well as the troposphere. For horizontal finite differencing, the regular latitude-longitude grid is used. To prevent linear computational instability in the time integration, Fourier filtering is applied in the longitudinal direction to all prognostic variables in higher latitudes such that the effective grid size of the model is approximately 500 km everywhere.
For the computation of radiative transfer, the distribution of water vapor, which is determined by the prognostic system of water vapor is used. However, the distribution of carbon dioxide, ozone and cloudiness are prescribed as a function of latitude and height and assumed to be constant with time. The temperature of the ground surface is determined such that it satisfies the condition of heat balance.
The prognostic system of water vapor includes the contribution of three-dimensional advection of water vapor and condensation in case of supersaturation. To simulate moist convection, a highly idealized procedure of moist convective adjustment is introduced. The prediction of soil moisture and snow depth is based upon the budget of water, snow and heat. Snow cover and sea ice are assumed to have much larger albedos than soil surface or open sea, and have a very significant effect upon the heat balance of the surface of the model.
Starting from the initial conditions of an isothermal and dry atmosphere at rest, the long-term integration of the joint model is conducted with the economical method adopted by Bryan and Manabe in their earlier study. The climate that emerges from this integration includes some of the basic features of the actual climate. However, it has many unrealistic features, which underscores the necessity of further increasing the computational resolution of horizontal finite differencing.
In order to identify the effect of the ocean currents upon climate, the joint model climate is compared with another climate obtained from the time integration of a so-called "A-model" in which oceanic regions are occupied by wet swampy surfaces without any heat capacity. Based upon the comparison between these two climates, the possible effects of oceanic heat transport on the climate are discussed. For example, the results show that the total poleward transport of energy is affected little by the oceanic heat transport. Although ocean currents significantly contribute to the transport, the atmospheric transport of energy in the presence of the latter decreases by approximately the same magnitude. Therefore, the total transport in the joint model differs little from that in the A-model. Further comparison between the two models indicates that ocean currents significantly affect not only the horizontal distribution of surface temperature of both oceans and continents but also the global distribution of precipitation.
Manabe, Syukuro, and Richard T Wetherald, 1975: The effects of doubling CO2 concentration on the climate of a general circulation model. Journal of the Atmospheric Sciences, 32(1), 3-15. Abstract PDF
An attempt is made to estimate the temperature changes resulting from doubling the present CO2 concentration by the use of a simplified three-dimensional general circulation model. This model contains the following simplifications: a limtied computational domain, an idealized topography, no heat transport by ocean currents, and fixed cloudiness. Despite these limitations, the results from this computation yield some indication of how the increase of CO2 concentration may affect the distribution of temperature in the atmosphere. It is shown that the CO2 increase raises the temperature of the model troposphere, whereas it lowers that of the model stratosphere. The tropospheric warming is somewhat larger than that expected from a radiative-convective equililbrium model. In particular, the increase of surface temperature in higher latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective heating to the lowest layer. It is also shown that the doubling of carbon dioxide significantly increases the intensity of the hydrologic cycle of the model.
Wetherald, Richard T., and Syukuro Manabe, 1975: The effects of changing the solar constant on the climate of a general circulation model. Journal of the Atmospheric Sciences, 32(11), 2044-2059. Abstract PDF
A study is conducted to evaluate the response of a simplified three-dimensional model climate to changes of the solar constant. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized topography, no heat transport by ocean currents, no seasonal variation, and fixed cloudiness.
It is found that the temperature of the model troposphere increases with increasing solar radiation. The greatest increase occurs in the surface layer of higher latitudes due to the effects of the snow-cover feedback mechanism as well as the suppression of vertical mixing by a stable lower troposphere. This result is found to be qualitatively similar to that obtained from previous studies with one-dimensional zonal mean models.
One of the most interesting features of this investigation is the extreme sensitivity of the intensity of the computed hydrologic cycle to small changes of the solar constant. Current estimates indicate a 27% increase of the former as compared with a 6% increase of the latter. This large intensification of the hydrologic cycle in the model atmosphere results from the increase in the rate of evaporation which is caused by the following changes: 1) reduction of the Bowen ratio due to the nonlinear increase of saturation vapor pressure with increasing temperature at the earth's surface, and 2) decrease in the net upward terrestrial surface radiation resulting from the increase in the moisture content in air and from the reduction of the lapse rate (both of which increase the downward terrestrial radiation and increase the energy available for evaporation).
It is shown that the latitude of maximum snowfall retreats poleward as the solar constant is increased. Furthermore, the total amounts of snowfall and snow accumulation decrease markedly with increasing insolation due to the poleward shift of the region of subfreezing surface temperature away from the zone of maximum baroclinic instability.
Manabe, Syukuro, D G Hahn, and J L Holloway, Jr, 1974: The seasonal variation of the tropical circulation as simulated by a global model of the atmosphere. Journal of the Atmospheric Sciences, 31(1), 43-83. Abstract PDF
A mathematical model of the atmosphere with a seasonal variation of insolation and sea surface temperatures is integrated numerically with respect to time over three model years. The model has a global computational domain and a realistic distribution of mountains. It contains a highly idealized parameterization of convection, i.e., dry and moist convective adjustment.
It is found that the model accurately simulates the seasonal variation of the location of the tropical rainbelt as well as that of the flow field associated with it. Over the continental regions of the model, the tropical rainbelt tends to form very close to the equator, whereas, in certain oceanic regions, it has a tendency to form away from the equator. Based upon a comparison of these results with those of another numerical experiment, it is concluded that this tendency is not due to an inherent characteristic of the rainbelt of the model to avoid the equator in oceanic regions, but rather it is due to the equatorial belt of low sea surface temperatures which is not favorable for the formation of a rainbelt. Over the sea, the surface temperature distribution seems to be the primary factor in determining the location of the rainbelt and accompanying tropical disturbances.
The primary source of kinetic energy of the disturbances in the model tropics is the conversion of eddy available potential energy which is generated by the effects of moist convection. A secondary source is the energy supplied from middle latitudes through pressure interaction. This effect has a significant magnitude in the subtropics of the model. The belt of maximum eddy conversion moves from one summer hemisphere to the other with respect to season in a manner similar to the tropical rainbelt. On the other hand, the contribution of pressure interaction in the production of eddy kinetic energy is significant in the winter hemisphere and thus supplements the contribution of eddy conversion. In general, the rate of eddy conversion due to transient eddies is particularly large in areas of relatively warm sea surface temperatures, where the tropical rainbelt and its accompanying disturbances predominate.
Manabe, Syukuro, and T B Terpstra, 1974: The effects of mountains on the general circulation of the atmosphere as identified by numerical experiments. Journal of the Atmospheric Sciences, 31(1), 3-42. Abstract PDF
In order to identify the effects of mountains upon the general circulation of the atmosphere, a set of numerical experiments is performed by use of a general circulation model developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The numerical time integrations of the model are performed with and without the effects of mountains. By comparing the structure of the model atmospheres that emerged from these two numerical experiments, it is possible to discuss the role of mountains in maintaining the stationary and transient disturbances in the atmosphere.
The model adopted for this study has a global computational domain and covers both the troposphere and stratosphere. For the computation of radiative transfer, the distribution of incoming solar radiation in January is assumed. Over the ocean, the observed distribution of the sea surface temperature of February is assumed as a lower boundary condition of the model. Over the continental surface, temperature is determined such that the condition of heat balance at the ground surface is satisfied. The mountain topography is taken into consideration using the so-called s-coordinate system in which pressure normalized by surface pressure is used as a vertical coordinate. The grid size for the computation of horizontal finite differences is chosen to be about 250 km. Nine finite-difference levels are chosen in unequal pressure intervals so that these levels can represent not only the structure of the mid-troposphere but also that of the stratosphere and the planetary boundary layer.
The results of the numerical experiments indicate that it is necessary to consider the effects of mountains for the successful simulation of the stationary flow field in the atmosphere, particularly in the upper troposphere and stratosphere. As predicted by Bolin, the flow field in the upper troposphere of the mountain model has a stationary trough in the lees of major mountain ranges such as the Rocky Mountains and the Tibetan Plateau. To the east of the trough, an intense westerly flow predominates. In the stratosphere, an anticyclone develops over the Aleutian Arhcipelago. These features of the mountain model, which are missing in the model without mountains, are in good qualitative agreement with the features of the actual atmosphere in winter.
In the model troposphere, mountains increase markedly the kinetic energy of stationary disturbances by increasing the stationary component of the eddy conversion of potential energy, whereas mountains decrease the kinetic energy of transient disturbances. The sum of the stationary and transient eddy kinetic energy is affected little by mountains. In the model stratosphere, mountains increase the amplitude of stationary disturbances partly because they enhance the energy supply from the model troposphere to the stratosphere.
According to wavenumber analysis, the longitudinal scale of eddy conversion in the model atmosphere increases significantly due to the effects of mountains. This increase results mainly from the large increase of stationary eddy conversion which takes place at very low wavenumbers.
The results of the analysis reveal other important effects of mountains. For example, the probability of cyclogenesis in the model atmosphere increases significantly on the lee side of major mountain ranges where the core of the westerly jet is located. Also, mountains affect the hydrologic processes in the model atmosphere by modifying the field of three-dimensional advection of moisture, and alter the global distribution of precipitation very significantly. In general, the distribution of the model with mountains is less zonal and more realisitic than that of the model without mountains.
Holloway, Jr, J L., Michael J Spelman, and Syukuro Manabe, 1973: Latitude-longitude grid suitable for numerical time integration of a global atmospheric model. Monthly Weather Review, 101(1), 69-78. Abstract PDF
A simple, free-surface, barotropic model and a nine-level, baroclinic model are numerically time integrated on both latitude-longitude grids and on Kurihara-type grids to compare the results obtained from the two grid systems. The prognostic variables are Fourier space filtered in the longitudinal direction on the latitude-longitude grids to permit the use of the same time-step length on both grids.
With respect to geopotential height and zonal wind distributions and to the phase speed of wave propagation, the results from the barotropic model, time-integrated on a sector latitude-longitude grid, agree better with a high-resolution control run than those computed on a modified Kurihara grid, particularly at high latitudes. The barotropic model is also time-integrated on a hemispheric, latitude-longitude grid, and the results compare well with a high-resolution control. The latter comparison is performed on initial data having strong cross-polar flow.
The mean sea-level pressure distribution obtained from a 64-day time integration of the baroclinic model on a global, latitude-longitude grid is better than that derived from a similar model using a Kurihara grid of comparable resolution. For example, the tendency for the Kurihara grid model to predict excessive pressures in the north polar region is for the most part corrected by use of the latitude-longitude grid.
Mahlman, Jerry D., and Syukuro Manabe, 1972: Numerical simulation of the stratosphere: implications for related climate change problems In Climatic Impact Assessment Program, Proceedings of the Survey Conference, Washington, DC, Department of Transportation, 186-193. Abstract
Current results are presented from an atmospheric simulation model which extends to a height of about 30 km. The model is global and contains 11 vertical levels with a horizontal resolution of about 265 km. Realistic topography, an annual march of radiation, sea surface temperature, and water vapor effects are included.
Zonal-mean cross-sections of temperature and zonal wind are shown and compared with reality. The results indicate close agreement with observations except for a few important exceptions; e.g., the simulated stratospheric polar night vortex is about a factor of two stronger than the observed. Synoptic charts for the 38-millibar pressure level are shown for different seasons of the year. These reveal a satisfactory simulation of the stratospheric winter Aleutian anticyclone and polar vortex, as well as the summertime easterlies.
Special attention is directed toward the problems of using atmospheric simulation models to study mechanisms acting to redistribute trace substances. Numerical and physical problems of modeling tracer advection, sub-grid-scale transfer, sources, and sinks are discussed in relation to the climate change problem. On the basis of current experience, some speculations are offered on efforts toward solving problems in which the distribution of tracers affects, and is affected by, the dynamics of the stratosphere.
Wetherald, Richard T., and Syukuro Manabe, 1972: Response of the joint ocean-atmosphere model to the seasonal variation of the solar radiation. Monthly Weather Review, 100(1), 42-59. Abstract PDF
The effect of the seasonal variation of solar radiation is incorporated into the joint ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration, and the resulting system is integrated for the 1 1/2-yr model time. The purpose of this study is to analyze the response of the joint air-sea model to seasonal changes in the solar zenith angle rather than to obtain a true equilibrium state. Comparisons are also made with results previously presented for the case of annual mean conditions.
The most important feature that emerges as a direct result of this seasonal variation is a significant warming of the lower troposphere in high latitudes. This warming is found to be caused by (1) the removal of the snowpack during the summer season, which decreases the earth's albedo there during this time, and (2) a net rise in the temperature of the ocean surface in high latitudes as a result of the seasonal variation of convective activity in the surface layer of the ocean. The present results indicate that the snow cover effect is the primary factor responsible for this warming trend whereas the ocean effect is of secondary importance.
The main consequences of this high latitude warming include a reduction of the mean atmospheric north-south temperature gradient (and, therefore, a reduction of baroclinic instability in middle latitudes), a reduction of the mean oceanic meridional circulation, and a reduction of the atmospheric and oceanic poleward heat energy transports.
Holloway, Jr, J L., and Syukuro Manabe, 1971: Simulation of climate by a global general circulation model, I. Hydrologic cycle and heat balance. Monthly Weather Review, 99(5), 335-370. Abstract PDF
The primitive equations of motion in spherical cooridinates are integrated with respect to time on global grids with mean horizontal resolutions of 500 and 250 km. There are nine levels in the models from 80 m to 29 km above the ground. The models have realistic continents with smoothed topography and an ocean surface with February water temperatures prescribed. The insolation is for a Northern Hemipshere winter. In addition to wind, temperature, pressure, and water vapor, the models simulate precipitation, evaporation, soil moisture, snow depth, and runoff. The models were run long enough beyond a state of quasi-equilibrium for meaningful statistics to be obtained. Time means of meteorological and hydrological quantities computed by the models compare favorably with observed climatic means. For example, the thermal structure of the model atmosphere is very similar to that of the actual atmosphere except in the Northern Hemisphere stratosphere; and the simulated distributions of the major arid regions over continents and the distributions of the rain belts, both in the Tropics and in middle latitudes, are successfully simulated by the models described in this paper. The increase in the horizontal computational resolution improved the distributions of mean surface pressure and precipitation rate in particular.
Manabe, Syukuro, 1971: Estimates of future change of climate due to the increase of carbon dioxide concentration in the air In Man's Impact on the Climate, Cambridge, MA, The MIT Press, 249-264.
Manabe, Syukuro, 1971: General circulation of the atmosphere. EOS, 52(6), 313-320.
Manabe, Syukuro, 1970: Cloudiness and the radiative, convective equilibrium In Global Effects of Environmental Pollution, Dordrecht, Holland, D. Reidel Publishing Co., 156-157. Abstract
The dependence of the temperature of the earth's surface upon the cloud cover at various altitudes is estimated. The effect of contrail on the surface temperature is discussed.
Manabe, Syukuro, and J L Holloway, Jr, 1970: Climate modification and a mathematical model of atmospheric circulation In A Century of Weather Progress, Boston, MA, American Meteorological Society, 157-164. Abstract
The necessity of a mathematical model of the atmosphere for the study of climate modification is emphasized
The structure of the global circulation model of the atmosphere which has been developed at the Geophysical Fluid Dynamics Laboratory of ESSA is described very briefly. It is shown that this model is capable of simulating some of the fundamental features of the hydrologic cycle and climate.
As an elementary example of the study of climate modification by use of such a model, a very drastic modification of the model climate resulting from the removal of the mountain ranges is discussed.
The structure of the preliminary version of the joint ocean-atmosphere model and the results from the integration of this model are also described very briefly. Plans for improvement and use of the joint model for future investigations of climate modification are discussed.
Manabe, Syukuro, and J L Holloway, Jr, 1970: Simulation of the hydrologic cycle of the global atmospheric circulation by a mathematical model In World Water Balance, Proceedings of the Reading Symposium, International Association of Scientific Hydrology, 387-400. Abstract
A mathematical model of the global atmospheric circulation is constructed. The model is capable of simulating some of the fundamental features of the climate and the hydrological cycle in the earth's atmosphere. The potential usefulness of such a model for the study of hydrology and climatology on a global scale is discussed.
Manabe, Syukuro, J L Holloway, Jr, and H Stone, 1970: Tropical circulation in a time-integration of a global model of the atmosphere. Journal of the Atmospheric Sciences, 27(4), 580-613. Abstract PDF
An analysis is made of the structure and energetics of the tropical circulation which emerged from a numerical time integration of a global circulation model with realistic orography. An analysis is made of the structure and energetics of the tropical circulation which emerged from a numerical time integration of a global circulation model with realistic orography.
Near the earth's surface, the general features of the time mean flow field and the location of the inter-tropical convergence zone of the model compare favorably with those of the actual atmosphere. Along the convergence zone or shear line, disturbances with a characteristic scale of 2000-3000 km develop and cause heavy precipitation. They tend to develop in the geographical areas where the formation of actual tropical storms is most probable. In the upper troposphere of the model tropics, disturbances with planetary scale develop and are responsible for the maximum of eddy kinetic energy there. Near the earth's surface, the general features of the time mean flow field and the location of the inter-tropical convergence zone of the model compare favorably with those of the actual atmosphere. Along the convergence zone or shear line, disturbances with a characteristic scale of 2000-3000 km develop and cause heavy precipitation. They tend to develop in the geographical areas where the formation of actual tropical storms is most probable. In the upper troposphere of the model tropics, disturbances with planetary scale develop and are responsible for the maximum of eddy kinetic energy there.
In general, both the kinetic energy of the tropical cyclones and that of the planetary-scale disturbances in the model tropics are chiefly maintained by the conversion of available potential energy generated by the heat of condensation. However, the planetary-scale disturbances in the upper troposphere of the tropics seem to be also affected by various factors such as the interaction with higher latitudes and land-sea contrast. It is noteworthy that these disturbances transport angular momentum across the equator in the upper troposphere and strongly affect the budget of angular momentum in the model tropics.
Manabe, Syukuro, Joseph Smagorinsky, J L Holloway, Jr, and H Stone, 1970: Simulated climatology of a general circulation model with a hydrologic cycle. III. Effects of increased horizontal computational resolution. Monthly Weather Review, 98(3), 175-212. Abstract PDF
The results of a numerical time integration of a hemispheric general circulation model of the atmosphere with moist processes and a uniform earth's surface has already been published by Manabe, Smagorinsky, and Strickler. In this study, the integration is repeated after halving the midlatitude grid size from approximately 500 to 250 km.
This increase in the resolution of the horizontal finite differences markedly improves the features of the model atmosphere. For example, the system of fronts and the associated cyclone families in the high resolution atmosphere is much more realistic than that in the low resolution atmosphere. Furthermore, the general magnitude and the spectral distribution of eddy kinetic energy are in better agreement with the actual atmosphere as a result of the improvement in resolution.
In order to explain these improvements, an extensive analysis of the energetics of both the low and high resolution atmospheric models is carried out. It is shown that these improvements are due not only to the increase of the accuracy of the finite differences but also to the shift in the scale of dissipation by the nonlinear lateral viscosity toward a smaller scale resulting from the decrease in grid size. In the low resolution atmospheric model, the transfer of energy from eddy to zonal kinetic energy is missing because of excessive subgrid scale dissipation at medium wave numbers, whereas it has significant magnitude in the high resolution atmospheric model. It is speculated that further increase of resolution should improve the results because it tends to separate the characteristic scale of dissipation from that of the source of eddy kinetic energy.
The analysis of the energetics in wave number space clearly demonstrates the differences between the energetics of the different parts of the atmosphere. In middle latitudes there are essential differences between the energetics of the model troposphere and that of the model stratosphere. In the model troposphere, the eddy kinetic energy is produced by the conversion of eddy potential energy in the range of wave numbers from 2 to 8. Part of the energy thus produced is dissipated by the subgrid scale dissipation, and most of the remainder is decascaded to zonal kinetic energy. In the model stratosphere, where very long waves predominate, the eddy kinetic energy is generated in the range of wave numbers from 2 to 3 by the energy supplied from the troposphere. Most of this energy is then decascaded barotropically to zonal kinetic energy.
In the Tropics, eddy kinetic energy is mainly produced by the release of eddy available potential energy generated by the heat of condensation. Although the rate of conversion is maximum at very low wave numbers, the conversion spectrum extends to very high wave numbers.
A box diagram of the energetics of the high resolution moist model shows that the eddy available potential energy is generated by the heat of condensation as well as by energy transfer from the zonal available potential energy. Furthermore, it is noteworthy that the zonal kinetic energy is maintained not only by the barotropic exchange from the eddy kinetic energy but also from the conversions of zonal potential energy. The intensification of the direct tropical cell and the weakening of the indirect Ferrel cell in the middle latitudes caused by the moist processes are responsible for ths positive zonal conversion.
One of the highlights of the results from the integration of the high resolution moist model is the successful simulation of the evolution of fronts and the associated cyclone families. The influence of moist processes upon frontal structure as well as other synoptic features is investigated by comparing the moist model atmosphere with the dry model atmosphere without the effect of the selective heating of condensation. It is found that the heat of condensation significantly reduces the width fronts and the characteristic scale of cyclones in the lower troposphere.
Manabe, Syukuro, 1969: Climate and the ocean circulation - I. The atmospheric circulation and the hydrology of the earth's surface. Monthly Weather Review, 97(11), 739-774. Abstract PDF
The effect of the hydrology of the earth's surface is incorporated into a numerical model of the general circulation of the atmosphere developed at the Geophysical Fluid Dynamics Laboratory of the Environmental Science Services Ad ministration (ESSA). The primitive equation of motion is used for this study. The nine levels of the model are distributed so as to resolve the surface boundary layer and stratosphere. The depletion of solar radiation and the transfer of the terrestrial radiation are computed taking into consideration cloud and atmospheric absorbers such as water vapor, carbon dioxide, and ozone. The scheme treating the hydrology of our model involves the prediction of water vapor in the atmosphere and the prediction of soil moisture and snow cover. In order to represent the moisture-holding capacity of soil, the continent is assumed to be covered by boxes, which can store limited amounts of water. The ocean surface is idealized to be a completely wet surface without any heat capacity. The temperature of the earth's surface is determined in such a way that it satisfies the condition of heat balance. To facilitate the analysis and the interpretation of the results, a simple and idealized distribution of the ocean and the continental region is chosen for this study. The numerical integrations are performed for the annual mean distribution of solar insolation.
In general, the qualitative features of hydrologic and thermodynamic regimes at the earth's surface are successfully simulated. Particularly, the horizontal distribution of rainfall is in excellent qualitative agreement with the observations. For example, the typical subtropical desert, the break of the subtropical dry belt along the east coast of the continent, and the equatorial rain belt emerged as the result of numerical time integration. Some features of the spatial distributions of heat and water balance components at the earth's surface also agree well with those obtained by Budyko for the actual atmosphere.
Owing to the lack of seasonal variation of solar insolation and lack of poleward transport of heat by ocean currents in the model, excessive snow cover develops at higher latitudes. Accordingly, the temperature in the polar region is much lower than the annual mean temperature observed in the actual atmosphere.
This investigation constitutes a preliminary study preceding the numerical integration of the general circulation model of joint ocean-atmosphere interaction, in which the transport of heat by ocean currents plays an important role.
Manabe, Syukuro, 1969: Climate and the ocean circulation - II. The atmospheric circulation and the effect of heat transfer by ocean currents. Monthly Weather Review, 97(11), 775-805. Abstract PDF
A general circulation model of the joint ocean-atmosphere system is constructed by combining an ocean model and an atmospheric model. The quantities exchanged between the oceanic part and the atmospheric part of the joint model are momentum, heat, and water. Integration of the atmospheric part yields the surface wind stress, net radiation, sensible heat flux, rates of rainfall and snowfall, rates of evaporation and sublimation, and rates of runoff and iceberg formation, all of which constitute the upper boundary conditions for the oceanic part of the model. From the oceanic part, the thickness of ice and the distribution of sea-surface temperature, which constitute the lower boundary conditions for the atmospheric part of the model, are computed.
An approach toward a quasi-equilibrium state of the joint ocean-atmosphere system is attempted by numerical time integration of the joint model. Since the thermal relaxation time of the oceanic part of the model is much longer than that of the atmospheric part, a special technique for economizing the computation time is developed. Although a state of quasi-equilibrium is not reached satisfactorily, the time variation of the atmospheric "climate" is extremely slow toward the end of the time integration. A detailed analysis of the final solution at the end of the integration is carried out.
According to this analysis, the distributions of various heat balance components such as radiation flux and the turbulent flux of sensible and latent heat compare favorably with the corresponding distributions in the actual atmosphere estimated by Budyko and London.
By comparing the final state of the joint model atmosphere with the quasi-equilibrium state of the previous atmosphere without an active ocean, it is possible to identify the effect of an ocean circulation on the general circulation of the atmosphere. For example, the poleward transport of heat by an ocean circulation reduces the meridional gradient of atmospheric temperature and vertical wind shear in the troposphere. This reduction of vertical wind shear lowers the level of baroclinic instability and causes a general decrease in the magnitude of eddy kinetic energy in the atmosphere. The air mass modification by the energy exchange between the model ocean and atmosphere creates a favorable place for the development of cyclones off the east coast of the continent in high latitudes.
In the Tropics, the upwelling of relatively cold water at the Equator suppresses the intensity of rainfall in the oceanic region and increases it in the continental region. This increase significantly alters the hydrology of the tropical continent. In middle and subtropical latitudes, the advection of warm water by the subtropical gyre increases the flux of sensible and latent heat from the ocean to the atmosphere along the east coast of the continent and increases the intensity of precipitation in the coastal region. The subtropical desert of the joint model is more or less confined to the western half of the continent. In high latitudes, the advection of warm water by the subarctic gyre off the west coast of the continent increases the energy exchange and precipitation there. Most of these modifications contribute to make the hydrology of the joint model highly realistic despite the idealization of the land-sea configuration.
Manabe, Syukuro, and Kirk Bryan, 1969: Climate calculations with a combined ocean-atmosphere model. Journal of the Atmospheric Sciences, 26(4), 786-789. PDF
Hunt, B G., and Syukuro Manabe, 1968: Experiments with a stratospheric general circulation model: II. Large-scale diffusion of tracers in the stratosphere. Monthly Weather Review, 96(8), 503-539. Abstract PDF
The 18-level primitive equation, general circulation model described in Part I was usted to study the diffusion of two idealized tracers in the stratosphere. One tracer was designed to simulate broadly the behavior of the radioactive tungsten which escaped into the stratosphere following nulcear tests in the Tropics, the other was taken as a photochemical ozone distribution. Both the meridional circulation and the large-scale eddies were of primary importance for the polewards transport of the tracers in middle and high latitudes, but the supply of tracer for these eddies was principally maintained from the higher levels by the downward branches of the meridional circulation. Two meridional cells were found to occur in the stratosphere, a tropical direct cell and a higher latitude indirect cell, and these provided a natural explanation for many of the observed features of the tracer distributions in the actual atmosphere. The only major tropospheric-stratospheric exchange took place in the subtropics through the tropopause gap, the vertical eddies and the meridional circulation being of comparable magnitude for this exchange.
The synoptic situation in the atmosphere was found to be of fundamental importance for the large-scale diffusion of the tracers in middle latitudes, and the downgradient transport of tracers in the lower stratosphere was primarily accomplished by the upper level troughs of the planetary scale wave system.
Although the model used in this investigation was based on radiative conditions corresponding to annual mean insolation it appeared to be representative of winter conditions, and was in agreement with many observational features.
Schematic diagrams illustrating the principal features of the large-scale diffusion of the two tracers are given in figures 12 and 24.
Hunt, B G., and Syukuro Manabe, 1968: An investigation of thermal tidal oscillations in the earth's atmosphere using a general circulation model. Monthly Weather Review, 96(11), 753-766. Abstract PDF
An investigation of tidal oscillations in the earth's atmosphere has been made using an 18 vertical level, hemispheric general circulation model. This approach permitted these tides to be investigated without resorting to linearization of the governing differential equations, as is required by the conventional approach. In addition, it allows the tides to be studied in relation to a realistic atmosphere, and thus in their actual roles as small perturbations, at least in the lower atmosphere, on the basic meteorological fields. Day-to-day surface pressure variations in good agreement with observation were produced by the model, the diurnal and semidiurnal pressure amplitudes and phases also being close to the observed values. An investigation into the excitation mechnaism of the oscillation gave results supporting previous work in attributing the dominant cause of the tides to absorption of solar radiation by water vapor and ozone in the atmosphere. Contrary to previous studies, water vapor was found to be of primary importance in exciting both the diurnal and semidiurnal oscillations in the model atmosphere.
Generally speaking, the tidal wind and temperature variations obtained were also in agreement with observation and other theoretical work.
Manabe, Syukuro, 1968: The dependence of atmospheric temperature on the concentration of carbon dioxide In Global Effects of Environmental Pollution, Amsterdam, The Netherlands, D. Reidel Publishing Co., 25-29. Abstract
Numerical computations using a radiative, convective equilibrium model of the atmosphere predict an increase of 0.8 degrees C in temperature of the earth's surface by the end of this century, based on the anticipated increase in CO2.
Manabe, Syukuro, and B G Hunt, 1968: Experiments with a stratospheric general circulation model. I. Radiative and dynamic aspects. Monthly Weather Review, 96(8), 477-502. Abstract PDF
An 18-vertical level primitive equation general circulation model was developed from previous models of the Geophysical Fluid Dynamics Laboratory in order to study the lower stratosphere in detail. The altitude range covered was from the surface to 4 mb. (37.5 km.), the vertical resolution being optimized in the topopause region to permit a more accurate calculation of the vertical transport terms. A polar stereographic projection was used and the model was limited to a single hemisphere.
The model now resolves two distinct jet streams, one in the troposphere and the other in the middle polar stratosphere. The wind systems produce a 3-cell meridional structure in the troposphere, which evolves into a 2-cell structure in the stratosphere. However, the wind structure and associated features of the model in the troposphere had a general equatorward shift compared with observation.
A considerable improvement was also obtained in some features of the temperature distribution, in particular the local midlatitude temperature maximum in the lower stratosphere is well defined and shown to be dynamically maintained. The low temperature and sharpness of the equatorial tropopause temperature distribution are closely reproduced by the model, and these features are attributed to the action of the upwards branch of the direct meridional cell in the Tropics, as is the basic cause of the difference in height of the troposphere at low and high latitudes.
The energy balance of the lower stratosphere in the present model agrees better with observation than previous models did, and confirms earlier work that this region is maintained from the troposphere by a vertical flux of energy. A similar flux of energy is also required to maintain the middle stratosphere, even though this region generates kinetic energy internally, and it is concluded that it is only marginally possible that this region may be baroclinically unstable. It appears that forcing from below extends to higher altitudes in winter than previously suspected.
Stone, H, and Syukuro Manabe, 1968: Comparison among various numerical models designed for computing infrared cooling. Monthly Weather Review, 96(10), 735-741. Abstract PDF
The scheme of computing the temperature change due to long wave radiation, developed by Manabe and Strickler and incorporated into the general circulation models developed at the Geophysical Fluid Dynamics Laboratory of ESSA, is compared with a group of other numerical schemes for computing radiative temperature change e.g., the scheme of Rodgers and Walshaw). It is concluded that the GFDL radiation model has the accuracy comparable with other numerical models despite various assumptions adopted.
Manabe, Syukuro, 1967: General circulation of the atmosphere. EOS, 48(2), 427-431.
Manabe, Syukuro, and Joseph Smagorinsky, 1967: Simulated climatology of a general circulation model with a hydrologic cycle II. Analysis of the tropical atmosphere. Monthly Weather Review, 95(4), 155-169. Abstract PDF
The thermal and dynamical structure of the tropical atmosphere which emerged from the numerical integration of our general circulation model with a simple hydrologic cycle is analyzed in detail.
According to the results of our analysis, the lapse rate of zonal mean temperature in the model Tropics is super-moist-adiabatic in the lower troposphere, and is sub-moist-adiabatic above the 400-mb. level in qualitative agreement with the observed features in the actual Tropics. The flow field in the model Tropics also displays interesting features. For example, a zone of strong convergence and a belt of heavy rain develops around the equator. Synoptic-scale disturbances such as weak tropical cyclones and shear lines with strong convergence develop and are reminiscent of disturbances in the actual tropical atmosphere. The humid towers, which result from moist convective adjustment and condensation, develop in the central core of the regions of strong upward motion, sometimes reaching the level of the tropical tropopause and thus heating the upper tropical troposphere. This heating compensates for the cooling due to radiation and the meridional circulation.
According to the analysis of the energy budget of the model Tropics, the release of eddy available potential energy, which is mainly generated by the heat of condensation, constitutes the major source of eddy kinetic energy of disturbances prevailing in the model Tropics.
Manabe, Syukuro, and Richard T Wetherald, 1967: Thermal equilibrium of the atmosphere with a given distribution of relative humidity. Journal of the Atmospheric Sciences, 24(3), 241-259. Abstract PDF
Radiative convective equilibrium of the atmosphere with a given distribution of relative humidity is computed as the asymptotic state of an initial value problem.
The results show that it takes almost twice as long to reach the state of radiative convective equilibrium for the atmosphere with a given distribution of relative humidity than for the atmosphere with a given distribution of absolute humidity.
Also, the surface equilibrium temperature of the former is almost twice as sensitive to change of various factors such as solar constant, CO2 content, O3 content, cloudiness, than that of the latter, due to the adjustment of water vapor content to the temperature variation of the atmosphere.
According to our estimate, a doubling of the CO2 content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2C. Our model does not have the extreme sensitivity of atmospheric temperature changes of CO2 content which was adduced by Möller.
Manabe, Syukuro, Joseph Smagorinsky, and R F Strickler, 1965: Simulated climatology of a general circulation model with a hydrologic cycle. Monthly Weather Review, 93(12), 769-798. Abstract PDF
A numerical experiment with a general circulation model with a simple hydrologic cycle is performed. The basic framework of this model is identical with that adopted for the previous study [35] except for the incorporation of a simplified hydrologic cycle which consists of the advection of water vapor by large-scale motion, evaporation from the surface, precipitation, and an artificial adjustment to simulate the process of moist convection. This adjustment is performed only when the relative humidity reaches 100 percent and the lapse rate exceeds the moists adiabatic lapse rate. The radiative flux is computed for the climatological distribution of water vapor instead of using the distribution calculated by the prognostic equation of water vapor. A completely wet surface without any heat capacity is chosen as the lower boundary. The initial conditions consist of a completely dry and isothermal atmosphere. A state of quasi-equilibrium is obtained as a reault of the time integration of 187 days. A preliminary analysis of the result is performed for the 40-day period from 148th day to 187th day.
According to this analysis, the hemispheric mean of the rate of precipitation is about 1.06 m./yr. which is close to the estimate of the annual mean rainfall obtained by Budyko [5]. In the Tropics rainfall exceeds evaporation and in the subtropics the latter exceeds the former in qualitative agreement with observation. The difference between them, however, is too exaggerated, and an extremely large export of water vapor from the dry subtropics into the wet Tropics by the meridional circulation takes place. In the troposphere, relative humidity increases with decreasing altitude. In the stratosphere it is very low except at the tropical tropopause, and the mixing ratio of water vapor is extremely small in qualitative agreement with observation. Although water vapor is transported from the troposphere into the stratosphere, it is then transported toward low latitudes and condenses at the tropical tropopause where the temperature is very low and the relative humidity is high.
Based upon a harmonic analysis of the flow field and the surface pressure field, it is concluded that the effect of condensation tends to increase the wave number of the tropospheric flow and surface pressure field. Also, the incorporation of the moist process in the model seems to increase the intensity of meridional circulation in the Tropics. As a result of this increase, the transport of momentum and heat by the meridional circulation in the Tropics is much larger than that obtained from the previous study. In middle latitudes, the poleward transport of total energy in the moist-model atmosphere is less than that in the dry-model atmosphere because of the effect of the poleward transport of latent energy or the heat of condensation. # The latitudinal distributions of radiative fluxes at the top of the atmosphere and at the earth's surface coincide very well with those obtained by London [17] for the actual atmosphere. Bowen's ratio increases with increasing latitude and its magnitude coincides reasonably well with that obtained by Budyko [5] or Jacobs [11] for the ocean surface.
Smagorinsky, Joseph, R F Strickler, W E Sangster, Syukuro Manabe, J L Holloway, Jr, and G D Hembree, 1965: Prediction experiments with a general circulation model In Proceedings of IAMAP/WMO International Symposium - Dynamics of Large-scale Processes, Moscow, Russia, 70-134.
Smagorinsky, Joseph, Syukuro Manabe, and J L Holloway, Jr, 1965: Numerical results from a nine-level general circulation model of the atmosphere. Monthly Weather Review, 93(12), 727-768. Abstract PDF
The "primitive equations of motion" are adopted for this study. The nine levels of the model are distributed so as to resolve surface boundary layer fluxes as well as radiative transfer by ozone, carbon dioxide, and water vapor. The lower boundary is a kinematically uniform land surface without any heat capacity. The stabilizing effect of moist convection is implicitly incorporated into the model by requiring an adjustment of the lapse rate whenever it exceeds the moist adiabatic value. The numerical integrations are performed for the mean annual conditions over a hemisphere starting with an isothermal atmosphere at rest. The spatial distribution of gaseous absorbers is assumed to have the annual mean value of the actual atmosphere and to be constant with time.
A quasi-equilibrium is attained about which a cyclic energy variation occurs with an irregular period of about 2 weeks. The dominant wave number of the meridional component of the wind is 5 to 6 in the troposphere but is reduced to about 3 in the stratosphere. The gross structure and behavior of the tropopause and stratosphere below 30 km. agree reasonably well with observation. The meridional circulation obtained from the computation has a 3-cell structure in the troposphere and tends toward a 2-cell structure with increasing altitude in the stratosphere. Although the level of the jet stream as well as that of the maximum northward transport of momentum coincides with observation, the intensity of the jet stream turns out to be much stronger than the observed annual mean. In the stratosphere the temperature increases with increasing latitude because of the effect of large-scale motion. The magnitude of the increase, however, is smaller than that observed.
A detailed study of the vertical distribution of the budget of kinetic energy, of available potential energy, of heat, and of angular momentum is made. The mechanism for maintaining the kinetic energy of the jet stream and of the stratosphere is discussed. It is concluded that in the model the kinetic energy in the stratosphere is maintained against its conversion into potential energy and dissipation through interaction with the troposphere, which is in qualitative agreement with the results derived from an analysis of the actual atmosphere. In the troposphere, the conversion of potential energy reaches a maximum at about the 500-mb. level. This energy is then transferred to the level of the jet stream and to the surfce boundary layer by the so-called pressure interaction term, thus providing the source of kinetic energy for these two levels at which dissipation is predominant. As with the results of Phillips [27] and Smagorinsky [37], the ratio of eddy kinetic energy to zonal kinetic energy and that of eddy to zonal available potential energy are computed to be much smaller than those of the actual atmosphere.
Manabe, Syukuro, and R F Strickler, 1964: Thermal equilibrium of the atmosphere with a convective adjustment. Journal of the Atmospheric Sciences, 21(4), 361-385. Abstract PDF
The states of thermal equilibrium (incorporating an adjustment of super-adiabatic stratification) as well as that of pure radiative equilibrium of the atmosphere are computed as the asymptotic steady state approached in an initial value problem. Recent measurements of absorptivities obtained for a wide range of pressure are used, and the scheme of computation is sufficiently general to include the effect of several layers of clouds.
The atmosphere in thermal equilibrium has an isothermal lower stratosphere and an inversion in the upper stratosphere which are features observed in middle latitudes. The role of various gaseous absorbers (i.e., water vapor, carbon dioxide, and ozone), as well as the role of the clouds, is investigated by computing thermal equilibrium with and without one or two of these elements. The existence of ozone has very little effect on the equilibrium temperature of the earth's surface but a very important effect on the temperature throughout the stratosphere; the absorption of solar radiation by ozone in the upper and middle stratosphere, in addition to maintaining the warm temperature in that region, appears also to be necessary for the maintenance of the isothermal layer or slight inversion just above the tropopause. The thermal equilibrium state in the absence of solar insulation is computed by setting the temperature of the earth's surface at the observed polar value. In this case, the stratospheric temperature decreases monotonically with increasing altitude, whereas the corresponding state of pure radiative equilibrium has an inversion just above the level of the tropopause.
A series of thermal equilibriums is computed for the distributions of absorbers typical of different latitudes. According to these results, the latitudinal variation of the distributions of ozone and water vapor may be partly responsible for the latitudinal variation of the thickness of the isothermal part of the stratosphere. Finally, the state of local radiative equilibrium of the stratosphere overlying a troposphere with the observed distribution of temperature is computed for each season and latitude. In the upper stratosphere of the winter hemisphere, a large latitudinal temperature gradient appears at the latitude of the polar-night jet stream, while in the upper statosphere of the summer hemisphere, the equilibrium temperature varies little with latitude. These features are consistent with the observed atmosphere. However, the computations predict an extremely cold polar night temperature in the upper stratosphere and a latitudinal decrease (toward the cold pole) of equilibrium temperature in the middle or lower stratosphere for winter and fall. This disagrees with observation, and suggests that explicit introduction of the dynamics of large scale motion is necessary.
Manabe, Syukuro, and F Möller, 1961: On the radiative equilibrium and heat balance of the atmosphere. Monthly Weather Review, 89(12), 503-532. Abstract
In order to incorporate the effect of radiation into the numerical experiment of the general circulation of the atmosphere, a simplified scheme for computing the radiative temperature change is constructed. The effects included are long wave radiation by water vapor, carbon dioxide, and ozone and the absorption of solar radiation by these three gases. The absorptivities of these gases are determined based upon the recent results of laboratory experiments and those of theoretical computations. The effects of clouds are not included.
By use of this scheme the radiative equilibrium temperature is computed for various latitudes and seasons as asymptotic solutions of an initial value problem. To a certain degree the radiative equilibrium solutions reveal some of the typical characteristics of stratospheric temperature and tropopause height variations.
Radiative heat budgets of the atmosphere are also computed and compared with the results of the computations of radiative equilibrium. This comparison is helpful for understanding the role of radiative processes and also suggests the kinds of effect we should expect from other thermal processes in the atmosphere.
Möller, F, and Syukuro Manabe, 1961: Über das strahlungsgleischgewicht der atmosphäre. Zeitschrift für Meteorologie, 15(1-6), 3-8.
Syono, S, Kikuro Miyakoda, Syukuro Manabe, T Matsuno, S Murakami, and Y Okuta, 1959: Broad-scale and small-scale analyses of a situation of heavy precipitation over Japan in the last period of Baiu Season, 1957. Japan Journal of Physics, 2(2), 59-103.
Manabe, Syukuro, 1958: On the estimation of energy exchange between the Japan Sea and the atmosphere during winter based upon the energy budget of both the atmosphere and the sea. Journal of the Meteorological Society of Japan, 26(4), 123-133.
Manabe, Syukuro, 1957: On the modification of air-mass over the Japan Sea when the outburst of cold air predominates. Journal of the Meteorological Society of Japan, 35(6), 311-326.
Manabe, Syukuro, 1956: On the contribution of heat released by condensation to the change in pressure pattern. Journal of the Meteorological Society of Japan, 34(6), 308-320.
Kombayashi, M, Kikuro Miyakoda, M Aihara, Syukuro Manabe, and K Katow, 1955: The quantitative forecast of precipitation with the numerical prediction method. Journal of the Meteorological Society of Japan, 33(5), 205-216.
Manabe, Syukuro, 1955: A note on the barotropic forecast using a moving coordinate system. Journal of the Meteorological Society of Japan, 33(7), 164-168.
Manabe, Syukuro, 1955: On the development and energetics of the westerly waves. Journal of the Meteorological Society of Japan, 33(2), 76-88.
Syono, S, K Gambo, Kikuro Miyakoda, M Aihara, Syukuro Manabe, and K Katow, 1955: Report on the numerical prediction of the 500-mb contour height change with double Fourier series method. Journal of the Meteorological Society of Japan, 33(3), 133-139.
