Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.
Butchart, Neal, John Austin, and Y Yamashita, et al., March 2011: Multi-model climate and variability of the stratosphere. Journal of Geophysical Research: Atmospheres, 116, D05102, DOI:10.1029/2010JD014995. Abstract
The stratospheric climate and variability from simulations of
sixteen chemistry-climate models is evaluated. On average the polar night
jet is well reproduced though its variability is less well reproduced with a large
spread between models. Polar temperature biases are less than 5 K except
in the southern hemisphere (SH) lower stratosphere in spring. The accumulated
area of low temperatures responsible for polar stratospheric cloud formation
is accurately reproduced for the Antarctic but underestimated for
the Arctic. The shape and position of the polar vortex is well simulated, as
is the tropical upwelling in the lower stratosphere. There is a wide model spread
in the frequency of major sudden stratospheric warnings (SSWs), late biases
in the break-up of the SH vortex and a weak annual cycle in the zonal wind
in the tropical upper stratosphere. Quantitatively, “metrics” indicate a wide
spread in model performance for most diagnostics with systematic biases in
many, and poorer performance in the SH than in the northern hemisphere
(NH). Correlations were found in the SH between errors in the final warming,
polar temperatures, the leading mode of variability, and jet strength,
and in the NH between errors in polar temperatures, frequency of major SSWs
and jet strength. Models with a stronger QBO have stronger tropical upwelling,
and a colder NH vortex. Both the qualitative and quantitative analysis indicate a number of common and long standing model problems, particularly
related to the simulation of the SH and stratospheric variability.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Shu, J, and John Austin, et al., January 2011: Effects of sea surface temperature and greenhouse gas changes on the transport between the stratosphere and troposphere. Journal of Geophysical Research: Atmospheres, 116, D02124, DOI:10.1029/2010JD014520. Abstract
The effects of sea-surface temperature (SST) and greenhouse gas (GHG) changes on
the mean age-of-air and water vapor are investigated using a state-of-the-art general
circulation model (GCM), and general characteristics of tracer transport between the
stratosphere and troposphere are analyzed. Downward tracer transport in the
northern mid-latitude stratosphere is found to be faster than at southern mid-latitudes.
The global mean downward transport to the troposphere from stratosphere mainly
occurs during northern winter and the downward cross-tropopause transport is
weakest from August to October. The maximum troposphere mean (TM) age-of-air,
derived from an age tracer released near the stratopause (around 1 hPa), can reach
13yr and is much larger than the maximum stratosphere mean (SM) age-of-air
derived from an analogous age tracer released in the troposphere, with the SM
age-of-air in the northern hemisphere being younger than in the southern
hemisphere.
Increased SSTs tend to accelerate upward transport through the stratosphere and
slow downward transport in mid-latitudes and the tropical stratosphere. In the
context of effects of GHG increases and the associated SST increases on the
stratosphere mean age-of-air, the GHG effects dominate i.e. changes in the
stratospheric mean age-of-air caused by SST increases only are smaller than those
caused by combined changes in SSTs and GHGs. An increase in SSTs enhances the
upward Eliasen-Palm (EP) flux in the extra-tropics. A 7.7% enhancement of tropical
upwelling can be caused by a uniform 1.5 K SST increase. When both SST and
GHG values are increased to the 2100 conditions, the meridional heat flux decreases
in both winter hemispheres (and statistically significantly in the southern
hemisphere). Meanwhile, the EP flux in the northern hemisphere increases
significantly and the tropical upwelling is enhanced by 15% compared to the
present-day conditions.
Austin, John, and R John Wilson, September 2010: Sensitivity of polar ozone to sea surface temperatures and halogen amounts. Journal of Geophysical Research: Atmospheres, 115, D18303, DOI:10.1029/2009JD013292. Abstract
Coupled chemistry-climate model simulations are presented for the period 1951 to 2099, and for shorter periods. All the simulations include variations in the concentrations of the well-mixed greenhouse gases and chlorofluorocarbons. Run 1 also includes observed levels of bromine, and sea surface temperatures (SSTs) from a coupled ocean-atmosphere model. Run 2 includes observed levels of both bromine and SSTs. In Run 3 the bromine amount is reduced by 25\% but is otherwise as Run 2. The analysis concentrates on the period 1980 onwards when the Antarctic ozone hole developed. The results show that the ozone hole is sensitive to SSTs and bromine amounts. For the period 1990 to 2007, when the ozone hole was fully developed, the area of the ozone hole was simulated to be largest in Run 1 (7\% smaller than observed), compared with underpredictions of 13\% and 24\% for Runs 2 and 3 respectively. The sensitivity of the ozone hole to tropical SSTs is a manifestation of stratosphere-troposphere coupling. The effect of SSTs is shown to arise from changes in the strength of the Brewer-Dobson circulation which is weaker for the simulation with model SSTs. The sensitivity of the model results
to bromine indicates the need to include realistic bromine amounts and may explain in part the substantial model underpredictions of the ozone hole area in previous publications. Linear regression analysis of the results confirms that the ozone hole is sensitive to chlorine, bromine, SSTs and meridional heat flux, but insensitive to the solar cycle. The results also suggest that the ozone hole may not disappear entirely this century, and that a small, residual ozone hole may still be present after 2060.
Austin, John, H Struthers, J Scinocca, and Y Yamashita, et al., November 2010: Chemistry climate model simulations of spring Antarctic ozone. Journal of Geophysical Research: Atmospheres, 115, D00M11, DOI:10.1029/2009JD013577. Abstract
Coupled chemistry climate model simulations covering the recent past and
continuing throughout the 21st century have been completed with a range of different models. Common forcings are used for the halogen amounts and greenhouse gas concentrations, as expected under the Montreal Protocol (with amendments) and Intergovernmental Panel on climate Change A1b Scenario. The simulations of the Antarctic ozone hole are compared using commonly used diagnostics: the minimum ozone, the maximum area less than 220 DU and the ozone mass deficit below 220 DU. Despite the fact that the processes responsible for ozone depletion are reasonably well understood, a wide range of results is obtained. Comparisons with observations indicate that one of the reasons for the model underprediction in ozone hole area is the tendency for models to
underpredict, by up to a factor of two, the area of low temperatures
responsible for polar stratospheric cloud formation. Models also typically have species gradients that are too weak at the edge of the polar vortex, suggesting that there is too much mixing of air across the vortex edge. Other models show a high bias in total column ozone which restricts the size of the ozone hole (defined by a 220 DU threshold). The results of those models which agree best with observations are examined in more detail. For several models the ozone hole does not disappear this century but a small ozone hole of up to three million square kilometers continues to occur most springs even after 2070.
Austin, John, R John Wilson, and Y Yamashita, et al., November 2010: The decline and recovery of total column ozone using a multi-model time series analysis. Journal of Geophysical Research: Atmospheres, 115, D00M10, DOI:10.1029/2010JD013857. Abstract
Simulations of 15 coupled chemistry climate models, for the period 1960–2100, are
presented. The models include a detailed stratosphere, as well as including a realistic
representation of the tropospheric climate. The simulations assume a consistent set of
changing greenhouse gas concentrations, as well as temporally varying chlorofluorocarbon
concentrations in accordance with observations for the past and expectations for the future.
The ozone results are analyzed using a nonparametric additive statistical model.
Comparisons are made with observations for the recent past, and the recovery of ozone,
indicated by a return to 1960 and 1980 values, is investigated as a function of latitude.
Although chlorine amounts are simulated to return to 1980 values by about 2050, with
only weak latitudinal variations, column ozone amounts recover at different rates due to
the influence of greenhouse gas changes. In the tropics, simulated peak ozone amounts
occur by about 2050 and thereafter total ozone column declines. Consequently,
simulated ozone does not recover to values which existed prior to the early 1980s. The
results also show a distinct hemispheric asymmetry, with recovery to 1980 values in the
Northern Hemisphere extratropics ahead of the chlorine return by about 20 years. In the
Southern Hemisphere midlatitudes, ozone is simulated to return to 1980 levels only
10 years ahead of chlorine. In the Antarctic, annually averaged ozone recovers at
about the same rate as chlorine in high latitudes and hence does not return to 1960s values
until the last decade of the simulations.
Butchart, Neal, and John Austin, et al., October 2010: Chemistry–climate model simulations of twenty-first century stratospheric climate and circulation changes. Journal of Climate, 23(20), DOI:10.1175/2010JCLI3404.1. Abstract
The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 ± 0.07 K decade−1 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade−1 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twenty-first century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson circulation throughout the depth of the stratosphere, which reduces the mean age of air nearly everywhere at a rate of about 0.05 yr decade−1 in those models with this diagnostic. On average, the annual mean tropical upwelling in the lower stratosphere (70 hPa) increases by almost 2% decade−1, with 59% of this trend forced by the parameterized orographic gravity wave drag in the models. This is a consequence of the eastward acceleration of the subtropical jets, which increases the upward flux of (parameterized) momentum reaching the lower stratosphere in these latitudes.
Gerber, Edwin P., and John Austin, et al., September 2010: Stratosphere-troposphere coupling and annular mode variability in chemistry-climate models. Journal of Geophysical Research: Atmospheres, 115, D00M06, DOI:10.1029/2009JD013770. Abstract
The internal variability and coupling between the stratosphere and troposphere in CCMVal-2 chemistry-climate models are evaluated through analysis of the annular mode patterns of variability. Computation of the annular modes in long data sets with secular trends requires refinement of the standard definition of the annular mode, and a more robust procedure that allows for slowly varying trends is established and verified. The spatial and temporal structure of the models’ annular modes is then compared with that of reanalyses. As a whole, the models capture the key features of observed intraseasonal variability, including the sharp vertical gradients in structure between stratosphere and troposphere, the asymmetries in the seasonal cycle between the Northern and Southern hemispheres, and the coupling between the polar stratospheric vortices and tropospheric midlatitude jets. It is also found that the annular mode variability changes little in time throughout simulations of the 21st century. There are, however, both common biases and significant differences in performance in the models. In the troposphere, the annular mode in models is generally too persistent, particularly in the Southern Hemisphere summer, a bias similar to that found in CMIP3 coupled climate models. In the stratosphere, the periods of peak variance and coupling with the troposphere are delayed by about a month in both hemispheres. The relationship between increased variability of the stratosphere and increased persistence in the troposphere suggests that some tropospheric biases may be related to stratospheric biases and that a well-simulated stratosphere can improve simulation of tropospheric intraseasonal variability.
Gettelman, Andrew, and John Austin, et al., October 2010: Multi-model assessment of the upper troposphere and lower stratosphere: Tropics and global trends. Journal of Geophysical Research: Atmospheres, 115, D00M08, DOI:10.1029/2009JD013638. Abstract
The performance of 18 coupled Chemistry Climate Models (CCMs)
in the Tropical Tropopause Layer (TTL) is evaluated using qualitative and
quantitative diagnostics. Trends in tropopause quantities in the tropics and
the extra-tropical Upper Troposphere and Lower Stratosphere (UTLS) are
analyzed. A quantitative grading methodology for evaluating CCMs is extended
to include variability and used to develop four different grades for tropical
tropopause temperature and pressure, water vapor and ozone. Four of
the 18 models and the multi-model mean meet quantitative and qualitative
standards for reproducing key processes in the TTL. Several diagnostics are
performed on a subset of the models analyzing the Tropopause Inversion Layer
(TIL), Lagrangian cold point and TTL transit time. Historical decreases in
tropical tropopause pressure and decreases in water vapor are simulated, lending
confidence to future projections. The models simulate continued decreases
in tropopause pressure in the 21st century, along with »1K increases per century
in cold point tropopause temperature and 0.5-1ppmv per century increases
in water vapor above the tropical tropopause. TTL water vapor increases
below the cold point. In two models, these trends are associated with
35% increases in TTL cloud fraction. These changes indicate significant perturbations
to TTL processes, specifically to deep convective heating and humidity
transport. Ozone in the extra-tropical lowermost stratosphere has significant
and hemispheric asymmetric trends. O3 is projected to increase by
nearly 30% due to ozone recovery in the Southern Hemisphere (SH) and due to enhancements in the stratospheric circulation. These UTLS ozone trends
may have significant effects in the TTL and the troposphere.
Hegglin, Michaela I., and John Austin, et al., October 2010: Multi-model assessment of the upper troposphere and lower stratosphere: Extra-tropics. Journal of Geophysical Research: Atmospheres, D00M09, DOI:10.1029/2010JD013884. Abstract
A multi-model assessment of the chemistry-climate models (CCMs)
is conducted in the extra-tropical upper troposphere/lower stratosphere (UTLS)
for the first time. Process-oriented diagnostics are used to validate dynamical and transport characteristics of 18 CCMs using meteorological reanalyses,
aircraft and satellite observations. The main dynamical and chemical climatological characteristics of the extra-tropical UTLS are generally well represented by the models, despite the limited horizontal and vertical resolution. The seasonal cycle of lowermost stratospheric mass is realistic, however with a wide spread in its mean value. A tropopause inversion layer is
present in most models, although the maximum in static stability is too high
above the tropopause and is somewhat too weak, as expected from limited
model resolution. Similar comments apply to the extra-tropical tropopause
transition layer. The seasonality in lower stratospheric chemical tracers is
consistent with the seasonality in the Brewer-Dobson circulation. Both vertical and meridional tracer gradients are of similar strength to those found
in observations. Models that perform less well tend to use a semi-Lagrangian
transport scheme and/or exhibit a very low resolution. Two models, and the
multi-model mean, score consistently well on all diagnostics, while seven other
models score well on all diagnostics except the seasonal cycle of water vapor. Only four of the models are consistently below average. The lack of tropospheric chemistry in most models limits their evaluation in the upper troposphere. Finally, the UTLS is relatively sparsely sampled by observations, limiting our ability to quantitatively evaluate many aspects of model performance.
Lübken, F-J, John Austin, U Langematz, and J Oberheide, July 2010: Introduction to special section on Climate and Weather of the Sun Earth System. Journal of Geophysical Research: Atmospheres, 115, D00I19, DOI:10.1029/2009JD013784. Abstract
In the special section on CAWSES (Climate and Weather of the Sun Earth System) a total of 19 papers are published covering several aspects of Sun‐Earth coupling. Six papers concentrate on summer mesospheric ice clouds including detection by satellites, radar‐based derivation of particle properties, and water vapor observations in the mesosphere. Solar radiation affects ice clouds on time scales of the 11 year solar cycle and 27 days. Stratospheric shrinking contributes significantly to long‐term trends of ice clouds. The seasonal variability of smoke particles is confirmed to be impacted by global circulation. Six papers address the external forcing of the atmosphere caused by the Sun. The relevance of radionuclei and solar radiation spectral irradiance is presented. The impact of precipitating energetic solar particles on trace gas concentrations is studied. Ion chemistry and electron production can be important to destroy ozone in the mesosphere and upper stratosphere. Strong solar events can reduce ice clouds on short time scales owing to dynamical feed back mechanisms. The 27 day solar signal is identified in ozone concentration using satellite measurements. Model studies show that the dynamical response of the stratospheric polar vortex to solar cycle forcing depends on the phase of the quasi‐biennial oscillation. The year 2009 was a remarkable exception from this rule reinforcing natural variability. Regarding centennial time scales it is shown that changes in the stratosphere can influence tropospheric circulation. Tides have extensively been studied within CAWSES. As is demonstrated, nonmigrating tides originating in the troposphere can propagate into the thermosphere.
Morgenstern, Olaf, and John Austin, et al., August 2010: Review of the formulation of present-generation stratospheric chemistry-climate models and associated external forcings. Journal of Geophysical Research: Atmospheres, 115, D00M02, DOI:10.1029/2009JD013728. Abstract
The goal of the Chemistry-Climate Model Validation (CCMVal) activity is to improve understanding of chemistry-climate models (CCMs) through process-oriented evaluation and to provide reliable projections of stratospheric ozone and its impact on climate. An appreciation of the details of model formulations is essential for understanding how models respond to the changing external forcings of greenhouse gases and ozone-depleting substances, and hence for understanding the ozone and climate forecasts produced by the models participating in this activity. Here we introduce and review the models used for the second round (CCMVal-2) of this intercomparison, regarding the implementation of chemical, transport, radiative, and dynamical processes in these models. In particular, we review the advantages and problems associated with approaches used to model processes of relevance to stratospheric dynamics and chemistry. Furthermore, we state the definitions of the reference simulations performed, and describe the forcing data used in these simulations. We identify some developments in chemistry-climate modeling that make models more physically based or more comprehensive, including the introduction of an interactive ocean, online photolysis, troposphere-stratosphere chemistry, and non-orographic gravity-wave deposition as linked to tropospheric convection. The relatively new developments indicate that stratospheric CCM modeling is becoming more consistent with our physically based understanding of the atmosphere.
Oman, L D., John Austin, and Y Yamashita, et al., December 2010: Multi-model assessment of the factors driving stratospheric ozone evolution over the 21st century. Journal of Geophysical Research: Atmospheres, 115, D24306, DOI:10.1029/2010JD014362. Abstract
The evolution of stratospheric ozone from 1960 to 2100 is examined in simulations
from fourteen chemistry‐climate models, driven by prescribed levels of halogens and
greenhouse gases (GHGs). There is general agreement among the models that total column
ozone reached a minimum around year 2000 at all latitudes, projected to be followed by an
increase over the first half of the 21st century. In the second half of the 21st century, ozone
is projected to continue increasing, level off or even decrease depending on the latitude.
Separation into partial columns above and below 20 hPa reveals that these latitudinal
differences are almost completely due to differences in the model projections of ozone in
the lower stratosphere. At all latitudes, upper stratospheric ozone increases throughout
the 21st century and is projected to return to 1960 levels well before the end of the century,
although there is a spread among models in the dates that ozone returns to specific
historical values. We find decreasing halogens and declining upper atmospheric
temperatures, driven by increasing GHGs, contribute almost equally to increases in upper
stratospheric ozone. In the tropical lower stratosphere, an increase in upwelling causes a
steady decrease in ozone through the 21st century, and total column ozone does not return
to 1960 levels in most of the models. In contrast, lower stratospheric and total column
ozone in middle and high latitudes increases during the 21st century, returning to 1960
levels well before the end of the century in most models.
Scinocca, J, D B Stephenson, T Bailey, and John Austin, November 2010: Estimates of past and future ozone trends from multi-model simulations using a flexible smoothing spline methodology. Journal of Geophysical Research: Atmospheres, 115, D00M12, DOI:10.1029/2009JD013622. Abstract
A novel additive model analysis of multi-model trends is presented.
The approach is motivated by, and particularly suited to, the analysis
of multi-model time series of varying length. This Time-Series Additive
Model (TSAM) approach consists of three distinct steps: estimation of individual
model trends, baseline adjustment of the trends, and the weighted
combination of the individual model trends to produce a multi-model trend
(MMT) estimate. The baseline adjustment step is not an essential ingredient
of the TSAM, but is included to reduce model spread. The association
of the TSAM approach with a probabilistic model allows trend estimates to
be used to make formal inference (e.g. calculation of confidence and prediction
intervals). The method is applied to the analysis of multi-model ozone
time series of varying lengths as were considered for the 2006 Scientific Assessment
of Ozone Depletion. The advantages of the TSAM approach are demonstrated
to include: the production of smooth trend estimates out to the ends
of the time series, the ability to model explicitly inter-annual variability about
the trend estimate, and the ability to make rigorous probability statements.
Calculated ozone return dates are consistent with previous qualitative estimates,
but the more quantitative analysis provided by the MMT is expected
to allow such data sets to be better utilized by the community and policy
makers.
Son, S-W, Edwin P Gerber, John Austin, and Y Yamashita, et al., October 2010: The impact of stratospheric ozone on Southern Hemisphere circulation change: A multimodel assessment. Journal of Geophysical Research: Atmospheres, 115, D00M07, DOI:10.1029/2010JD014271. Abstract
The impact of stratospheric ozone on the tropospheric general circulation of the Southern Hemisphere (SH) is examined with a set of chemistry-climate models participating in the Stratospheric Processes and their Role in Climate (SPARC)/Chemistry-Climate Model Validation project phase 2 (CCMVal-2). Model integrations of both the past and future climates reveal the crucial role of stratospheric ozone in driving SH circulation change: stronger ozone depletion in late spring generally leads to greater poleward displacement and intensification of the tropospheric midlatitude jet, and greater expansion of the SH Hadley cell in the summer. These circulation changes are systematic as poleward displacement of the jet is typically accompanied by intensification of the jet and expansion of the Hadley cell. Overall results are compared with coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), and possible mechanisms are discussed. While the tropospheric circulation response appears quasi-linearly related to stratospheric ozone changes, the quantitative response to a given forcing varies considerably from one model to another. This scatter partly results from differences in model climatology. It is shown that poleward intensification of the westerly jet is generally stronger in models whose climatological jet is biased toward lower latitudes. This result is discussed in the context of quasi-geostrophic zonal mean dynamics.
Austin, John, and R John Wilson, et al., July 2009: Coupled chemistry climate model simulations of stratospheric temperatures and their trends for the recent past. Geophysical Research Letters, 36, L13809, DOI:10.1029/2009GL038462. Abstract
Temperature results from multi-decadal simulations of coupled chemistry climate models for the recent past are analyzed using multi-linear regression including a trend, solar cycle, lower stratospheric tropical wind, and volcanic aerosol terms. The climatology of the models for recent years is in good agreement with observations for the troposphere but the model results diverge from each other and from observations in the stratosphere. Overall, the models agree better with observations than in previous assessments, primarily because of corrections in the observed temperatures. The annually averaged global and polar temperature trends simulated by the models are generally in agreement with revised satellite observations and radiosonde data over much of their altitude range. In the global average, the model trends underpredict the radiosonde data slightly at the top of the observed range. Over the Antarctic some models underpredict the temperature trend in the lower stratosphere, while others overpredict the trends.
Randel, W J., and John Austin, et al., January 2009: An update of observed stratospheric temperature trends. Journal of Geophysical Research, 114, D02107, DOI:10.1029/2008JD010421. Abstract
An updated analysis of observed stratospheric temperature variability and trends is presented on the basis of satellite, radiosonde, and lidar observations. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit covering 1979–2007 and the Stratospheric Sounding Unit (SSU) covering 1979–2005. Radiosonde results are compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ~0.5 K/decade over much of the globe for 1979–2007, with some differences in detail among the different radiosonde and satellite data sets. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the lower stratosphere derived from radiosonde data are also analyzed for a longer record (back to 1958); trends for the presatellite era (1958–1978) have a large range among the different homogenized data sets, implying large trend uncertainties. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO2 increases. The results show mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995–2005. Long records of lidar temperature measurements at a few locations show reasonable agreement with SSU trends, although sampling uncertainties are large in the localized lidar measurements. Updated estimates of the solar cycle influence on stratospheric temperatures show a statistically significant signal in the tropics (~30°N–S), with an amplitude (solar maximum minus solar minimum) of ~0.5 K (lower stratosphere) to ~1.0 K (upper stratosphere).
Struthers, H, and John Austin, et al., September 2009: The simulation of the Antarctic ozone hole by chemistry-climate models. Atmospheric Chemistry and Physics, 9(17), DOI:10.5194/acp-9-6363-2009. Abstract
While chemistry-climate models are able to reproduce many characteristics of the global total column ozone field and its long-term evolution, they have fared less well in simulating the commonly used diagnostic of the area of the Antarctic ozone hole i.e. the area within the 220 Dobson Unit (DU) contour. Two possible reasons for this are: (1) the underlying Global Climate Model (GCM) does not correctly simulate the size of the polar vortex, and (2) the stratospheric chemistry scheme incorporated into the GCM, and/or the model dynamics, results in systematic biases in the total column ozone fields such that the 220 DU contour is no longer appropriate for delineating the edge of the ozone hole. Both causes are examined here with a view to developing ozone hole area diagnostics that better suit measurement-model inter-comparisons. The interplay between the shape of the meridional mixing barrier at the edge of the vortex and the meridional gradients in total column ozone across the vortex edge is investigated in measurements and in 5 chemistry-climate models (CCMs). Analysis of the simulation of the polar vortex in the CCMs shows that the first of the two possible causes does play a role in some models. This in turn affects the ability of the models to simulate the large observed meridional gradients in total column ozone. The second of the two causes also strongly affects the ability of the CCMs to track the observed size of the ozone hole. It is shown that by applying a common algorithm to the CCMs for selecting a delineating threshold unique to each model, a more appropriate diagnostic of ozone hole area can be generated that shows better agreement with that derived from observations.
Tourpali, K, and John Austin, et al., February 2009: Clear sky UV simulations for the 21st century based on ozone and temperature projections from Chemistry-Climate Models. Atmospheric Chemistry and Physics, 9(4), DOI:10.5194/acp-9-1165-2009. Abstract
We have estimated changes in surface solar ultraviolet (UV) radiation under cloud free conditions in the 21st century based on simulations of 11 coupled Chemistry-Climate Models (CCMs). The total ozone columns and vertical profiles of ozone and temperature projected from CCMs were used as input to a radiative transfer model in order to calculate the corresponding erythemal irradiance levels. Time series of monthly erythemal irradiance received at the surface during local noon are presented for the period 1960 to 2100. Starting from the first decade of the 21st century, the surface erythemal irradiance decreases globally as a result of the projected stratospheric ozone recovery at rates that are larger in the first half of the 21st century and smaller towards its end. This decreasing tendency varies with latitude, being more pronounced over areas where stratospheric ozone has been depleted the most after 1980. Between 2000 and 2100 surface erythemal irradiance is projected to decrease over midlatitudes by 5 to 15%, while at the southern high latitudes the decrease is twice as much. In this study we have not included effects from changes in cloudiness, surface reflectivity and tropospheric aerosol loading, which will likely be affected in the future due to climate change. Consequently, over some areas the actual changes in future UV radiation may be different depending on the evolution of these parameters.
Austin, John, K Tourpali, Eugene Rozanov, Hideharu Akiyoshi, S Bekki, G Bodeker, and E Manzini, et al., 2008: Coupled chemistry climate model simulations of the solar cycle in ozone and temperature. Journal of Geophysical Research, 113, D11306, DOI:10.1029/2007JD009391. Abstract
The 11-year solar cycles in ozone and temperature are examined using new simulations of coupled chemistry climate models. The results show a secondary maximum in stratospheric tropical ozone, in agreement with satellite observations and in contrast with most previously published simulations. The mean model response varies by up to about 2.5% in ozone and 0.8 K in temperature during a typical solar cycle, at the lower end of the observed ranges of peak responses. Neither the upper atmospheric effects of energetic particles nor the presence of the quasi biennial oscillation is necessary to simulate the lower stratospheric response in the observed low latitude ozone concentration. Comparisons are also made between model simulations and observed total column ozone. As in previous studies, the model simulations agree well with observations. For those models which cover the full temporal range 1960–2005, the ozone solar signal below 50 hPa changes substantially from the first two solar cycles to the last two solar cycles. Further investigation suggests that this difference is due to an aliasing between the sea surface temperatures and the solar cycle during the first part of the period. The relationship between these results and the overall structure in the tropical solar ozone response is discussed. Further understanding of solar processes requires improvement in the observations of the vertically varying and column integrated ozone.
Austin, John, and T Reichler, December 2008: Long-term evolution of the cold point tropical tropopause: Simulation results and attribution analysis. Journal of Geophysical Research, 113, D00B10, DOI:10.1029/2007JD009768. Abstract
The height, pressure, and temperature of
the cold point tropical tropopause are examined in three 140 year
simulations of a coupled chemistry climate model. Tropopause height
increases approximately steadily in the simulations at a mean rate of 63 ± 3
m/decade (2σ confidence interval). The pressure trend changes near
the year 2000 from −1.03 ± 0.30 hPa/decade in the past to −0.55 ± 0.06 hPa/decade
for the future. The trend in tropopause temperature changes even more
markedly from −0.13 ± 0.07 K/decade in the past to +0.254 ± 0.014 K/decade
in the future. The tropopause data were fit using regression by terms
representing total column ozone, tropical mean sea surface temperatures, and
tropical mass upwelling. Tropopause height and pressure closely follow the
upwelling term, whereas tropopause temperature is primarily related to sea
surface temperature and ozone. The change in tropopause temperature trend
near the year 2000 is related to the change in the sign of the ozone trend
with the sea surface temperature having an increased role after 2040. A
conceptual model is used to estimate tropopause changes. The results confirm
the regression analysis in showing the importance of upper tropospheric
warming (connected with sea surface temperature) and stratospheric cooling
(connected with CO2 and O3). In the past, global
warming and ozone depletion have opposite effects on the tropopause
temperature, which decreases slightly. For the future simulation, global
warming and ozone recovery reinforce which increases the tropopause
temperature. In particular, future tropopause change is found not to be an
indicator of climate change alone.
Charlton-Perez, Andrew J., Lorenzo M Polvani, John Austin, and F Li, 2008: The frequency and dynamics of stratospheric sudden warmings in the 21st century. Journal of Geophysical Research, 113, D16116, DOI:10.1029/2007JD009571. Abstract
Changes to stratospheric sudden warmings (SSWs) over the coming century, as predicted by the Geophysical Fluid Dynamics Laboratory (GFDL) chemistry climate model [Atmospheric Model With Transport and Chemistry (AMTRAC)], are investigated in detail. Two sets of integrations, each a three-member ensemble, are analyzed. The first set is driven with observed climate forcings between 1960 and 2004; the second is driven with climate forcings from a coupled model run, including trace gas concentrations representing a midrange estimate of future anthropogenic emissions between 1990 and 2099. A small positive trend in the frequency of SSWs is found. This trend, amounting to 1 event/decade over a century, is statistically significant at the 90% confidence level and is consistent over the two sets of model integrations. Comparison of the model SSW climatology between the late 20th and 21st centuries shows that the increase is largest toward the end of the winter season. In contrast, the dynamical properties are not significantly altered in the coming century, despite the increase in SSW frequency. Owing to the intrinsic complexity of our model, the direct cause of the predicted trend in SSW frequency remains an open question.
Li, Feng, John Austin, and R John Wilson, January 2008: The strength of the Brewer-Dobson Circulation in a changing climate: Coupled chemistry-climate model simulations. Journal of Climate, 21(1), DOI:10.1175/2007JCLI1663.1. Abstract
The strength of the Brewer–Dobson circulation (BDC) in a changing climate is studied using multidecadal simulations covering the 1960–2100 period with a coupled chemistry–climate model, to examine the seasonality of the change of the BDC. The model simulates an intensification of the BDC in both the past (1960–2004) and future (2005–2100) climate, but the seasonal cycle is different. In the past climate simulation, nearly half of the tropical upward mass flux increase occurs in December–February, whereas in the future climate simulation the enhancement of the BDC is uniformly distributed in each of the four seasons. A downward control analysis implies that this different seasonality is caused mainly by the behavior of the Southern Hemisphere planetary wave forcing, which exhibits a very different long-term trend during solstice seasons in the past and future. The Southern Hemisphere summer planetary wave activity is investigated in detail, and its evolution is found to be closely related to ozone depletion and recovery. In the model results for the past, about 60% of the lower-stratospheric mass flux increase is caused by ozone depletion, but because of model ozone trend biases, the atmospheric effect was likely smaller than this. The remaining fraction of the mass flux increase is attributed primarily to greenhouse gas increase. The downward control analysis also reveals that orographic gravity waves contribute significantly to the increase of downward mass flux in the Northern Hemisphere winter lower stratosphere.
Yang, Qiong, Qiang Fu, John Austin, Andrew Gettelman, Feng Li, and H Vömel, October 2008: Observationally derived and general circulation model simulated tropical stratospheric upward mass fluxes. Journal of Geophysical Research, 113, D00B07, DOI:10.1029/2008JD009945. Abstract
We quantify the vertical velocity and
upward mass flux in the tropical lower stratosphere on the basis of accurate
radiative heating rate calculations using 8-year Southern Hemisphere
Additional Ozonesondes balloon-borne measurements of temperature and ozone
and cryogenic frost-point hygrometer measured water vapor in the tropics
(15°S—10°N). The impact of tropospheric clouds on the stratospheric heating
rates is considered using cloud distributions from the International
Satellite Cloud Climatology Project. We find a nearly constant annual mean
upward mass flux in the tropical lower stratosphere above the top of the
tropical tropopause layer (i.e., ~70 hPa), which is 1.13 ± 0.40 kgm−2d−1
for the 40- to 30-hPa layer, and 0.89 ± 0.48 kgm−2d−1
for the 70- to 50-hPa layer. A strong seasonal cycle exists in the upward
mass flux and it is found that the mass flux below ~70 hPa is decoupled from
that above in the Northern Hemisphere summer. Simulations of the tropical
lower stratosphere from two stratospheric General Circulation Models (GCMs)
are compared with observations. The annual mean upward mass fluxes from both
GCMs for the 40- to 30-hPa layer agree well with observations, while the
simulated mass fluxes for the 70- to 50-hPa layer are twice as large. Both
GCMs also simulate seasonal variation of the mass flux reasonably well but
are incapable of simulating the observed interannual variability of the
upward mass flux, which is closely correlated with the quasi-biennial
oscillations.
Austin, John, R John Wilson, Feng Li, and H Vömel, 2007: Evolution of water vapor concentrations and stratospheric age of air in coupled chemistry-climate model simulations. Journal of the Atmospheric Sciences, 64(3), DOI:10.1175/JAS3866.1. Abstract
Stratospheric water vapor concentrations and age of air are investigated in an ensemble of coupled chemistry-climate model simulations covering the period from 1960 to 2005. Observed greenhouse gas concentrations, halogen concentrations, aerosol amounts, and sea surface temperatures are all specified in the model as time-varying fields. The results are compared with two experiments (time-slice runs) with constant forcings for the years 1960 and 2000, in which the sea surface temperatures are set to the same climatological values, aerosol concentrations are fixed at background levels, while greenhouse gas and halogen concentrations are set to the values for the relevant years.
The time-slice runs indicate an increase in stratospheric water vapor from 1960 to 2000 due primarily to methane oxidation. The age of air is found to be significantly less in the year 2000 run than the 1960 run. The transient runs from 1960 to 2005 indicate broadly similar results: an increase in water vapor and a decrease in age of air. However, the results do not change gradually. The age of air decreases significantly only after about 1975, corresponding to the period of ozone reduction. The age of air is related to tropical upwelling, which determines the transport of methane into the stratosphere. Oxidation of increased methane from enhanced tropical upwelling results in higher water vapor amounts. In the model simulations, the rate of increase of stratospheric water vapor during the period of enhanced upwelling is up to twice the long-term mean. The concentration of stratospheric water vapor also increases following volcanic eruptions during the simulations.
Austin, John, L L Hood, and B E Soukharev, 2007: Solar cycle variations of stratospheric ozone and temperature in simulations of a coupled chemistry-climate model. Atmospheric Chemistry and Physics, 7(6), DOI:10.5194/acp-7-1693-2007. Abstract
The results from three 45-year simulations of a coupled chemistry climate model are analysed for solar cycle influences on ozone and temperature. The simulations include UV forcing at the top of the atmosphere, which includes a generic 27-day solar rotation effect as well as the observed monthly values of the solar fluxes. The results are analysed for the 27-day and 11-year cycles in temperature and ozone. In accordance with previous results, the 27-day cycle results are in good qualitative agreement with observations, particularly for ozone. However, the results show significant variations, typically a factor of two or more in sensitivity to solar flux, depending on the solar cycle.
In the lower and middle stratosphere we show good agreement also between the modelled and observed 11-year cycle results for the ozone vertical profile averaged over low latitudes. In particular, the minimum in solar response near 20 hPa is well simulated. In comparison, experiments of the model with fixed solar phase (solar maximum/solar mean) and climatological sea surface temperatures lead to a poorer simulation of the solar response in the ozone vertical profile, indicating the need for variable phase simulations in solar sensitivity experiments. The role of sea surface temperatures and tropical upwelling in simulating the ozone minimum response are also discussed.
Damski, J, L Thölix, L Backman, J Kaurola, P Taalas, John Austin, Neal Butchart, and M Kulmala, May 2007: A chemistry-transport model simulation of middle atmospheric ozone from 1980 to 2019 using coupled chemistry GCM winds and temperatures. Atmospheric Chemistry and Physics, 7(9), DOI:10.5194/acp-7-2165-2007. Abstract
A global 40-year simulation from 1980 to 2019 was performed with the FinROSE chemistry-transport model based on the use of coupled chemistry GCM-data. The main focus of our analysis is on climatological-scale processes in high latitudes. The resulting trend estimates for the past period (1980–1999) agree well with observation-based trend estimates. The results for the future period (2000–2019) suggest that the extent of seasonal ozone depletion over both northern and southern high-latitudes has likely reached its maximum. Furthermore, while climate change is expected to cool the stratosphere, this cooling is unlikely to accelerate significantly high latitude ozone depletion. However, the recovery of seasonal high latitude ozone losses will not take place during the next 15 years.
Eyring, Veronika, and John Austin, et al., 2007: Multimodel projections of stratospheric ozone in the 21st century. Journal of Geophysical Research, 112, D16303, DOI:10.1029/2006JD008332. Abstract
Simulations from eleven coupled chemistry-climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model-to-model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHG-induced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lower-stratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Cly near 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease to 1980 values and before the Antarctic. None of the CCMs predict future large decreases in the Arctic column ozone. By 2100, total column ozone is projected to be substantially above 1980 values in all regions except in the tropics.
Andersen, S B., and John Austin, et al., 2006: Comparison of recent modeled and observed trends in total column ozone. Journal of Geophysical Research, 111, D02303, DOI:10.1029/2005JD006091. Abstract
We present a comparison of trends in total column ozone from 10 two-dimensional and 4 three-dimensional models and solar backscatter ultraviolet–2 (SBUV/2) satellite observations from the period 1979–2003. Trends for the past (1979–2000), the recent 7 years (1996–2003), and the future (2000–2050) are compared. We have analyzed the data using both simple linear trends and linear trends derived with a hockey stick method including a turnaround point in 1996. If the last 7 years, 1996–2003, are analyzed in isolation, the SBUV/2 observations show no increase in ozone, and most of the models predict continued depletion, although at a lesser rate. In sharp contrast to this, the recent data show positive trends for the Northern and the Southern Hemispheres if the hockey stick method with a turnaround point in 1996 is employed for the models and observations. The analysis shows that the observed positive trends in both hemispheres in the recent 7-year period are much larger than what is predicted by the models. The trends derived with the hockey stick method are very dependent on the values just before the turnaround point. The analysis of the recent data therefore depends greatly on these years being representative of the overall trend. Most models underestimate the past trends at middle and high latitudes. This is particularly pronounced in the Northern Hemisphere. Quantitatively, there is much disagreement among the models concerning future trends. However, the models agree that future trends are expected to be positive and less than half the magnitude of the past downward trends. Examination of the model projections shows that there is virtually no correlation between the past and future trends from the individual models.
Austin, John, and Feng Li, 2006: On the relationship between the strength of the Brewer-Dobson circulation and the age of stratospheric air. Geophysical Research Letters, 33, L17807, DOI:10.1029/2006GL026867. Abstract
The strength of the Brewer-Dobson circulation is computed for multi-decadal simulations of a coupled chemistry-climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations. The two different transport concepts of mass upwelling and reciprocal of the age of air are investigated empirically from the model simulations. The results indicate that the variables are linearly related in the model but with a change of gradient some time near 2005. Possible reasons for the change of gradient are discussed
Austin, John, and R John Wilson, 2006: Ensemble simulations of the decline and recovery of stratospheric ozone. Journal of Geophysical Research, 111, D16314, DOI:10.1029/2005JD006907. Abstract
An ensemble of simulations of a coupled chemistry-climate model is completed for 1960–2100. The simulations are divided into two periods, 1960–2005 and 1990–2100. The modeled total ozone amount decrease throughout the atmosphere from the 1960s until about 2000–2005, depending on latitude. The Antarctic ozone hole develops rapidly in the model from about the late 1970s, in agreement with observations, but it does not disappear until about 2065, about 15 years later than previous estimates. Spring averaged ozone takes even longer to recover to 1980 values. Ozone amounts in the Antarctic are determined largely by halogen amounts. In contrast, in the Arctic, ozone recovers to 1980 values about 25–35 years earlier, depending on the recovery criterion adopted. By the end of the 21st century, the climate change associated with greenhouse gas changes gives rise to a significant superrecovery of ozone in the Arctic but a less marked recovery in the Antarctic. For both polar regions, ensemble and interannual variability is greater in the future than in the past, and hence the timing of the full recovery of polar ozone is very sensitive to the definition of recovery. It is suggested that the range of recovery rates between the hemispheres simulated in the model is related to the overall increase in the strength of the Brewer-Dobson circulation, driven by increases in greenhouse gas concentrations
Eyring, Veronika, and John Austin, et al., November 2006: Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past. Journal of Geophysical Research, 111, D22308, DOI:10.1029/2006JD007327. Abstract
Simulations of the stratosphere from thirteen coupled chemistry-climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long-term trends are compared for an extended period (1960–2004). Comparisons of polar high-latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long-term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.
Austin, John, 2005: Comment on the paper: On the design of practicable numerical experiments to investigate stratospheric temperature change, by S. Hare et al. (2005). Atmospheric Science Letters, 6(3), DOI:10.1002/asl.107. Abstract
If stratospheric temperature trends are to be understood, coupled chemistry climate models will need to be run. Simulations with fixed ozone trends might provide a misleading indication of future temperature trends.
Eyring, Veronika, N R P Harris, M Rex, T G Shepherd, D W Fahey, G T Amanatidis, John Austin, M P Chipperfield, M Dameris, Piers M Forster, Andrew Gettelman, Hans-F Graf, T Nagashima, P A Newman, S Pawson, Michael J Prather, J A Pyle, R J Salawitch, B D Santer, and D W Waugh, 2005: A strategy for process-oriented validation of coupled chemistry-climate models. Bulletin of the American Meteorological Society, 86(8), DOI:10.1175/BAMS-86-8-1117. Abstract
Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.
Struthers, H, K Kreher, John Austin, J T Schofield, G Bodeker, P Johnston, H Shiona, and A Thomas, November 2004: Past and future simulations of NO2 from a coupled chemistry-climate model in comparison with observations. Atmospheric Chemistry and Physics, 4, 2227-2239. Abstract PDF
Trends in derived from a 45 year integration of a chemistry-climate model (CCM) run have been compared with ground-based measurements at Lauder (45° S) and Arrival Heights (78° S). Observed trends in at both sites exceed the modelled trends in N2O, the primary source gas for stratospheric NO2. This suggests that the processes driving the trend are not solely dictated by changes in but are coupled to global atmospheric change, either chemically or dynamically or both. If CCMs are to accurately estimate future changes in ozone, it is important that they comprehensively include all processes affecting NOx (NO+NO2) because NOx concentrations are an important factor affecting ozone concentrations. Comparison of measured and modelled NO2 trends is a sensitive test of the degree to which these processes are incorporated in the CCM used here. At Lauder the 1980-2000 CCM NO2 trends (4.2% per decade at sunrise, 3.8% per decade at sunset) are lower than the observed trends (6.5% per decade at sunrise, 6.0% per decade at sunset) but not significantly different at the 2σ level. Large variability in both the model and measurement data from Arrival Heights makes trend analysis of the data difficult. CCM predictions (2001-2019) of NO2 at Lauder and Arrival Heights show significant reductions in the rate of increase of NO2 compared with the previous 20 years (1980-2000). The model results indicate that the partitioning of oxides of nitrogen changes with time and is influenced by both chemical forcing and circulation changes.
Austin, John, and Neal Butchart, October 2003: Coupled chemisty-climate model simulations for the period 1980 to 2020: Ozone depletion and the start of ozone recovery. Quarterly Journal of the Royal Meteorological Society, 129(595), Part B, DOI:10.1256/qj.02.203. Abstract PDF
Two simulations of a coupled chemistry-climate model are completed for the period 1980 to 2020, covering the recent past during which extensive satellite ozone and temperature data exist, and covering the near future when ozone levels are expected to begin to recover. In the first simulation, Rayleigh friction is used to decelerate the polar night jet. In the second simulation, a parametrized spectral gravity-wave forcing scheme is included. This has the effect of considerably reducing the model temperature bias in the polar regions and weakening the polar night jet. In the simulations the concentrations of chlorine, bromine and the well-mixed greenhouse gas concentrations are specified in accordance with past observations and future projected values. The calculated trends in temperature and ozone in the two runs are similar, indicating that model internal variability does not have a significant impact and suggesting that the trends arise largely from changes in external parameters. Typically, after about the year 2000, the trend in the modelled annually averaged ozone changed from a decrease to a small increase. The change was found to be statistically significant in the upper stratosphere and in the lower stratosphere over Antarctica, which are the regions most affected by halogen chemistry. Globally averaged temperature results suggest that the best place to look for future atmospheric change is in the upper stratosphere. Decadally averaged statistics are used to estimate the timing of the start of recovery of total ozone. The simulations indicate no significant further ozone loss from the current atmosphere with minima typically occurring in the years from 2000 to 2005, except in the spring Arctic where ozone values continued to decrease slowly until the end of the integrations. One major problem with the detection of the start of ozone recovery, is that the concentrations of halogens are expected to reduce only slowly from their peak value. Hence, no substantial recovery is simulated before the year 2020. The difficulty in detecting the start of ozone recovery suggests the need to continue the model simulations until the second half of this century which would also help to establish the timing of complete ozone recovery.
Shine, K P., M S Bourqui, Piers M Forster, S H E Hare, U Langematz, P Braesicke, V Grewe, M Ponater, C Schnadt, C A Smith, J D Haigh, John Austin, Neal Butchart, Drew Shindell, W J Randel, T Nagashima, R W Portmann, S Solomon, D J Seidel, John R Lanzante, Stephen A Klein, V Ramaswamy, and M Daniel Schwarzkopf, 2003: A comparison of model-simulated trends in stratospheric temperatures. Quarterly Journal of the Royal Meteorological Society, 129(590), DOI:10.1256/qj.02.186. Abstract
Estimates of annual-mean stratospheric temperature trends over the past twenty years, from a wide variety of models, are compared both with each other and with the observed cooling seen in trend analyses using radiosonde and satellite observations. The modelled temperature trends are driven by changes in ozone (either imposed from observations or calculated by the model), carbon dioxide and other relatively well-mixed greenhouse gases, and stratospheric water vapour.
The comparison shows that whilst models generally simulate similar patterns in the vertical profile of annual-and global-mean temperature trends, there is a significant divergence in the size of the modelled trends, even when similar trace gas perturbations are imposed. Coupled-chemistry models are in as good agreement as models using imposed observed ozone trends, despite the extra degree of freedom that the coupled models possess.
The modelled annual- and global-mean cooling of the upper stratosphere (near 1 hPa) is dominated by ozone and carbon dioxide changes, and is in reasonable agreement with observations. At about 5 hPa, the mean cooling from the models is systematically greater than that seen in the satellite data; however, for some models, depending on the size of the temperature trend due to stratospheric water vapour changes, the uncertainty estimates of the model and observations just overlap. Near 10 hPa there is good agreement with observations. In the lower stratosphere (20-70 hPa), ozone appears to be the dominant contributor to the observed cooling, although it does not, on its own, seem to explain the entire cooling.
Annual- and zonal-mean temperature trends at 100 hPa and 50 hPa are also examined. At 100 hPa, the modelled cooling due to ozone depletion alone is in reasonable agreement with the observed cooling at all latitudes. At 50 hPa, however, the observed cooling at midlatitudes of the northern hemisphere significantly exceeds the modelled cooling due to ozone depletion alone. There is an indication of a similar effect in high northern latitudes, but the greater variability in both models and observations precludes a firm conclusion.
The discrepancies between modelled and observed temperature trends in the lower stratosphere are reduced if the cooling effects of increased stratospheric water vapour concentration are included, and could be largely removed if certain assumptions were made regarding the size and distribution of the water vapour increase. However, given the uncertainties in the geographical extent of water vapour changes in the lower stratosphere, and the time period over which such changes have been sustained, other reasons for the discrepancy between modelled and observed temperature trends cannot be ruled out.
