Liu, Ping, Kevin A Reed, Ming Zhao, Stephen T Garner, Ngar-Cheung Lau, Levi G Silvers, and Brian A Colle, in press: Record-breaking persistent high-pressure systems fueled unprecedented Canadian wildfire disasters in 2023. Environmental Research Communications. DOI:10.1088/2515-7620/adc6de. March 2025. Abstract
Canada experienced its most severe wildfire season on record in 2023, with nearly 5% of its forested land burned—almost four times the previous record set in 1995. Our analysis indicated that fire severity, strongly correlated with the monthly Fire Weather Index (FWI), was most intense in the western provinces and territories during May and July, whereas in the eastern provinces, it peaked in June, leading to a seasonal and areal average of more than 3.5 standard deviations (STD). This unprecedented fire activity was fueled by record-breaking, persistent high-pressure systems, with both their frequency and intensities surpassing 3 STD, along with variable winds. These abnormal atmospheric patterns exacerbated dry conditions, reduced cloud cover, and increased surface solar radiation, driving record-high temperatures and FWI values, all exceeding ±3 STD. The extreme high-pressure events were primarily linked to a combination of climatological standing waves and exceptionally strong, transient quasi-stationary waves. The dominant patterns in the mid-troposphere were characterized by large-scale planetary waves at low zonal wavenumbers (1–4). Long-term warming trends also contributed, though they played a lesser role, accounting for roughly 10–20% of the overall anomalies. These findings provide critical insights into the atmospheric dynamics driving Canada's unprecedented wildfire season.
Chen, Jilong, Chi-Yung Tam, Kevin Cheung, Ziqian Wang, Hiroyuki Murakami, Ngar-Cheung Lau, Stephen T Garner, Ziniu Xiao, Chun-Wing Choy, and Peng Wang, November 2021: Changing impacts of tropical cyclones on east and southeast Asian inland regions in the past and a globally warmed future climate. Frontiers in Earth Science, 9:769005, DOI:10.3389/feart.2021.769005. Abstract
The impacts of the western North Pacific (WNP) tropical cyclone (TC) on East and Southeast Asian inland regions are analyzed. Here, based on a stringent TC selecting criterion, robust increase of TC-related inland impacts between 1979 and 2016 over East and Southeast Asian regions have been detected. The storms sustained for 2–9 h longer and penetrated 30–190 km further inland, as revealed from different best track datasets. The most significant increase of the TC inland impacts occurred over Hanoi and South China. The physical mechanism that affects TC-related inland impacts is shortly discussed. First, the increasing TC inland impacts just occur in the WNP region, but it is not a global effect. Second, besides the significant WNP warming effects on the enhanced TC landfall intensity and TC inland impacts, it is suggested that the weakening of the upper-level Asian Pacific teleconnection pattern since 1970s may also play an important role, which may reduce the climatic 200 hPa anti-cyclonic wind flows over the Asian region, weakening the wind shear near the Philippine Sea, and may eventually intensify the TC intensity when the TCs across the basin. Moreover, the TC inland impacts in the warming future are projected based on a high-resolution (20 km) global model according to the Representative Concentration Pathway 8.5 scenario. By the end of the 21st century, TC mean landfall intensity will increase by 2 m/s (6%). The stronger storms will sustain 4.9 h (56%) longer and penetrate 92.4 km (50%) farther inland, thereby almost doubling the destructive power delivered to Asian inland regions. More inland locations will therefore be exposed to severe storm–related hazards in the future due to warmer climate. Long-term planning to enhance disaster preparedness and resilience in these regions is called for.
Liu, Zhen, Yi Ming, Chun Zhao, and Ngar-Cheung Lau, et al., January 2020: Contribution of local and remote anthropogenic aerosols to a record-breaking torrential rainfall event in Guangdong Province, China. Atmospheric Chemistry and Physics, 20(1), DOI:10.5194/acp-20-223-2020. Abstract
A torrential rainfall case, which happened in Guangdong Province during 14–16 December 2013, broke the historical rainfall record in the province in terms of duration, affected area, and accumulative precipitation. The influence of anthropogenic aerosols on this extreme rainfall event is examined using a coupled meteorology–chemistry–aerosol model. Up to 33.7 mm precipitation enhancement in the estuary and near the coast is mainly attributed to aerosol–cloud interactions (ACI), whereas aerosol–radiation interaction partially offsets 14 % of the precipitation increase. Our further analysis of changes in hydrometeors and latent heat sources suggests that the ACI effects on the intensification of precipitation can be divided into two stages: cold rain enhancement in the former stage followed by warm rain enhancement in the latter. Responses of precipitation to the changes in anthropogenic aerosol concentration from local (i.e., Guangdong Province) and remote (i.e., outside Guangdong Province) sources are also investigated through simulations with reduced aerosol emissions from either local or remote sources. Accumulated aerosol concentration from local sources aggregates mainly near the ground surface and dilutes quickly after the precipitation initiated. By contrast, the aerosols from remote emissions extend up to 8 km above ground and last much longer before decreasing until peak rainfall begins, because aerosols are continuously transported by the strong northerly winds. The patterns of precipitation response to remote and local aerosol concentrations resemble each other. However, compared with local aerosols through warm rain enhancement, remote aerosols contribute more than twice the precipitation increase by intensifying both cold and warm rain, occupying a predominant role. A 10-time emission sensitivity test shows about 10 times the PM2.5 concentration compared with the control run. Cold (warm) rain is drastically enhanced (suppressed) in the 10× run. In response to 10× aerosol emissions, the pattern of precipitation and cloud property changes resembles the differences between CTL and CLEAN, but with a much greater magnitude. The precipitation average over Guangdong decreases by 1.0 mm in the 10× run but increases by 1.4 mm in the control run compared with the CLEAN run. We note that the precipitation increase is concentrated within a more narrowed downstream region of the aerosol source, whereas the precipitation decrease is more dispersed across the upstream region. This indicates that the excessive aerosols not only suppress rainfall, but also change the spatial distribution of precipitation, increasing the rainfall range, thereby potentially exacerbating flood and drought elsewhere. This study highlights the importance of considering aerosols in meteorology to improve extreme weather forecasting. Furthermore, aerosols from remote emissions may outweigh those from local emissions in the convective invigoration effect.
Liu, Zhen, Yi Ming, Lin Wang, Massimo Bollasina, M Luo, and Ngar-Cheung Lau, et al., August 2019: A Model Investigation of Aerosol‐Induced Changes in the East Asian Winter Monsoon. Geophysical Research Letters, 46(16), DOI:10.1029/2019GL084228. Abstract
The response of the East Asian winter monsoon (EAWM) circulation to aerosols is studied using a coupled atmosphere‐slab ocean general circulation model. In the extratropics, the aerosol‐induced cooling in the mid‐latitudes leads to an intensified subtropical jet stream, a deepened East Asian trough, and thus an enhanced EAWM. In the tropics, the local Hadley circulation shifts southward to compensate for the interhemispheric asymmetry in aerosol radiative cooling. Anomalous subsidence at around 10°N leads to a salient anticyclone to the southwest of the Philippines. The associated southwesterlies advect abundant moisture to South China, resulting in local precipitation increase and suggesting a weaker EAWM. The EAWM response to aerosol forcing is thus driven by a competition between tropical and extratropical mechanisms, which has important implications for the future monsoon evolution as aerosol changes may follow different regional‐dependent trajectories.
Yang, Y, Shang-Ping Xie, L Wu, Yu Kosaka, Ngar-Cheung Lau, and Gabriel A Vecchi, October 2015: Seasonality and Predictability of the Indian Ocean Dipole Mode: ENSO Forcing and Internal Variability. Journal of Climate, 28(20), DOI:10.1175/JCLI-D-15-0078.1. Abstract
This study evaluates the relative contributions to the Indian Ocean Dipole (IOD) mode of interannual variability from the El Niño-Southern Oscillation (ENSO) forcing and ocean-atmosphere feedbacks internal to the Indian Ocean. The ENSO forcing and internal variability is extracted by conducting a 10-member coupled simulation for 1950-2012 where sea surface temperature (SST) is restored to the observed anomalies over the tropical Pacific but interactive with the atmosphere over the rest of the world ocean. In these experiments, the ensemble mean is due to ENSO forcing and the inter-member difference arises from internal variability of the climate system independent of ENSO. These elements contribute one third and two thirds of the total IOD variance, respectively. Both types of IOD variability develop into an east-west dipole pattern due to Bjerknes feedback and peak in September-November. The ENSO forced and internal IOD modes differ in several important ways. The forced IOD mode develops in August with a broad meridional pattern, and eventually evolves into the Indian Ocean Basin mode; while the internal IOD mode grows earlier in June, is more confined to the equator and decays rapidly after October. The internal IOD mode is more skewed than the ENSO forced response. The destructive interference of ENSO forcing and internal variability can explain early-terminating IOD events, referred to IOD-like perturbations that fail to grow during boreal summer.
Our results have implications for predictability. Internal variability, as represented by pre-season sea surface height anomalies off Sumatra, contributes to predictability considerably. Including this indicator of internal variability, together with ENSO, improves the predictability of IOD.
Christensen, J H., and Ngar-Cheung Lau, et al., March 2014: Climate phenomena and their relevance for future regional climate change In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 1217-1308.
The synoptic behavior of present-day heat waves (HW) over Europe is studied using a global high-resolution atmospheric model (HiRAM) with 50-km grid-spacing. Three regions of enhanced and coherent temperature variability are identified over western Russia, eastern Europe and western Europe. The simulated HW characteristics are compared with those derived from Climate Forecast System Reanalysis products. Composite charts for outstanding HW episodes resemble well-known recurrent circulation types. The HW region is overlain by a prominent upper-level anticyclone, which blocks the passage of synoptic-scale transients. The altered eddy vorticity transports in turn reinforce the anticyclone. The anticyclone is part of a planetary-scale wave train. The successive downstream development of this wave train is indicative of Rossby wave dispersion.
Additional runs of HiRAM are conducted for the ‘time slices’ of 2026-2035 and 2086-2095 in the climate scenario corresponding to Representative Concentration Pathway 4.5. By the end of the 21st century, the averaged duration and frequency of HW in the three European sites are projected to increase by a factor of 1.4-2.0 and 2.2-4.5, respectively, from the present-day values. These changes can be reproduced by adding the mean shift between the present and future climatological temperatures to the daily fluctuations in the present-day simulation. The output from a continuous integration of a coupled general circulation model through the 1901-2100 period indicates a monotonic increase in severity, duration and HW days during the 21st century.
Kosaka, Yu, Shang-Ping Xie, Ngar-Cheung Lau, and Gabriel A Vecchi, May 2013: Origin of seasonal predictability for summer climate over the Northwestern Pacific. Proceedings of the National Academy of Sciences, 110(19), DOI:10.1073/pnas.1215582110. Abstract
Summer climate in the Northwestern Pacific (NWP) displays large year-to-year variability, affecting densely populated Southeast and East Asia by impacting precipitation, temperature, and tropical cyclones. The Pacific–Japan (PJ) teleconnection pattern provides a crucial link of high predictability from the tropics to East Asia. Using coupled climate model experiments, we show that the PJ pattern is the atmospheric manifestation of an air–sea coupled mode spanning the Indo-NWP warm pool. The PJ pattern forces the Indian Ocean (IO) via a westward propagating atmospheric Rossby wave. In response, IO sea surface temperature feeds back and reinforces the PJ pattern via a tropospheric Kelvin wave. Ocean coupling increases both the amplitude and temporal persistence of the PJ pattern. Cross-correlation of ocean–atmospheric anomalies confirms the coupled nature of this PJIO mode. The ocean–atmosphere feedback explains why the last echoes of El Niño–Southern Oscillation are found in the IO-NWP in the form of the PJIO mode. We demonstrate that the PJIO mode is indeed highly predictable; a characteristic that can enable benefits to society.
Lau, Ngar-Cheung, and Jeff J Ploshay, December 2013: Model projections of the changes in atmospheric circulation and surface climate over North America, North Atlantic and Europe in the 21st century. Journal of Climate, 26(23), DOI:10.1175/JCLI-D-13-00151.1. Abstract
The impacts of climate change on the North America-North Atlantic-Europe sector are studied using a coupled general circulation model (CM3) and a high-resolution atmosphere-only model (HiRAM), both developed at the Geophysical Fluid Dynamics Laboratory. The CM3 experiment is conducted under two climate change scenarios for the 1860-2100 period. The sea surface temperature (SST) forcing prescribed in the ‘time-slice’ integrations with HiRAM is derived from observations for the 1979-2008 period, and projection by CM3 for the 2086-2095 period.
The wintertime response in the late 21st century is characterized by an enhancement of the positive phase of the North Atlantic Oscillation in sea level pressure (SLP), and poleward and eastward displacements of the Atlantic jetstream and storm track. The forcing pattern due to eddy vorticity fluxes in the perturbed storm track matches well with the response pattern of the SLP field in the late 21st century. The model results suggest that the above circulation changes are linked to the gradient of the altered SST forcing in the North Atlantic.
In summer, the projected enhancement of convection over the eastern tropical Pacific is accompanied by a wavetrain spanning the North America-North Atlantic-Europe sector. This quasi-stationary circulation pattern is associated with diminished storm track activity at 40°-50°N, and an eddy forcing pattern that is similar to the summertime SLP response in the late 21st century.
Li, Y, and Ngar-Cheung Lau, July 2013: Influences of ENSO on Stratospheric Variability, and the Descent of Stratospheric Perturbations into the Lower Troposphere. Journal of Climate, 26(13), DOI:10.1175/JCLI-D-12-00581.1. Abstract
The linkage between El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) through the stratospheric pathway is examined using a global coupled climate model (CM3), with increased vertical resolution and extent in the stratosphere as compared to an earlier model (CM2). It is demonstrated that the relationship between ENSO and NAO is stronger in CM3 than in CM2.
It is found that ENSO plays an important role in modulating the frequency of occurrence of the stratospheric polar vortex anomalies through enhancing/attenuation the amplitudes of zonal wavenumbers 1 and 2, especially in late winter. A higher frequency of weak (strong) stratospheric vortex events is simulated in CM3 during El Niño (La Niña) episodes.
The weak vortex events during El Niño winters are preceded by enhancement of the zonal wave-1 pattern, and weakening of zonal wave-2 pattern. These modified tropospheric planetary waves propagate upward, and then weaken the stratospheric polar vortex through eddy-mean flow interaction. The zonal-mean geopential response in the stratosphere propagate downward and weaken the polar vortex throughout the troposphere.
The effects of planetary wave refraction in the upper troposphere on the zonally averaged circulation cells in the tropospheric meridional plane, and the linkage between the lower branches of these cells and the near surface wind patterns, play an important role in the flow pattern over the region corresponding to the southern lobe of the NAO. Specifically, a negative annular mode and NAO response is discernible in weak stratospheric vortex events during El Niño. Conversely, the positive annular mode and NAO is evident in strong vortex events during La Niña.
Lau, Ngar-Cheung, and Mary Jo Nath, February 2012: A model study of the air-sea interaction associated with the climatological aspects and interannual variability of the South Asian summer monsoon development. Journal of Climate, 25(3), DOI:10.1175/JCLI-D-11-00035.1. Abstract
The climatological characteristics and interannual variations of the development of the South Asian summer monsoon (SASM) in early summer are studied using output from a 200-yr simulation of a coupled atmosphere-ocean general circulation model (CM2.1). Some of the model results are compared with corresponding observations. Climatological charts of the model and observational data at pentadal intervals indicate that both the precipitation and SST signals exhibit a tendency to migrate northward. Enhanced monsoonal precipitation at a given site is accompanied by reduction in incoming shortwave radiation and intensification of upward latent heat flux, and by oceanic cooling.
An extended empirical orthogonal function analysis is used to identify the dates for initiation of the northward march of SASM in individual summers. It is noted that early monsoon development prevails after the mature phase of La Niña events; whereas delayed development occurs after El Niño.
Sensitivity experiments based on the atmospheric component of CM2.1 indicate that the effects of SST forcings in the tropical Pacific (TPAC) and Indian Ocean (IO) on monsoon development are opposite to each other. During El Niño events, the atmospheric response to remote TPAC forcing tends to suppress or postpone monsoon development over South Asia. Conversely, the warm SST anomalies in IO, which are generated by the ‘atmospheric bridge’ mechanism in El Niño episodes, lead to accelerated monsoon development. The net result of these two competing effects is an evolution scenario with a timing which is intermediate between the response to TPAC forcing only and the response to IO forcing only.
The characteristics of summertime heat waves in North America are examined using reanalysis data and simulations by two general circulation models with horizontal resolution of 50 and 200 km. Several ‘key regions’ with spatially coherent and high-amplitude fluctuations in daily surface air temperature are identified. The typical synoptic features accompanying warm episodes in these regions are described. The averaged intensity, duration and frequency of occurrence of the heat waves in various key regions, as simulated in the two models for 20th-century climate, are in general agreement with the results based on reanalysis data.
The impact of climate change on the heat wave characteristics in various key regions is assessed by contrasting model runs based on a scenario for the 21st century with those for the 20th century. Both models indicate considerable increases in the duration and frequency of heat wave episodes, and in number of heat wave days per year, during the 21st century. The duration and frequency statistics of the heat waves in the mid-21st century, as generated by the model with 50-km resolution, can be reproduced by adding the projected warming trend to the daily temperature data for the late-20th century, and then recomputing these statistics. The detailed evolution of the averaged intensity, duration and frequency of the heat waves through individual decades of the 20th and 21st centuries, as simulated and projected by the model with 200-km resolution, indicates that the upward trend in these heat wave measures should become apparent in the early decades of the 21st century.
Li, Y, and Ngar-Cheung Lau, January 2012: Impact of ENSO on the atmospheric variability over the North Atlantic in late winter – Role of transient eddies. Journal of Climate, 25(1), DOI:10.1175/JCLI-D-11-00037.1. Abstract
The dynamical mechanism for the late-winter teleconnection between El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) is examined using the output from a 2000-yr integration of a coupled general circulation model (GCM). The coupled model captures many salient features of the observed behavior of both ENSO and NAO, as well as their impact on the surface climate in late winter. Both the observational and model data indicate more occurrences of negative phase of NAO in late winter during El Niño events, and positive NAO in La Niña episodes.
The potential role of high-frequency transient eddies in the above teleconnection is diagnosed. During El Niño winters, the intensified transient disturbances along the equatorward-shifted North Pacific storm track extend their influences farther downstream. The eddy-induced negative height tendencies are found to be more coherent and stronger over North Atlantic than that over North Pacific. These negative height tendencies over North Atlantic are coincident with the southern lobe of NAO, and thus favors more occurrences of negative NAO events.
During those El Niño winters with relatively strong SST warming in eastern equatorial Pacific, the eastward extension of eddy activity is reinforced by the enhanced near-surface baroclinicity over the subtropical eastern Pacific. This flow environment supports a stronger linkage between the Pacific and Atlantic storm tracks, and is more conductive to a negative NAO phase.
Our model results are supported by a parallel analysis of various observational datasets. It is further demonstrated that these transient eddy effects can be reproduced in atmospheric GCM integrations subjected to ENSO-related SST forcing in the tropical Pacific.
Li, Y, and Ngar-Cheung Lau, July 2012: Contributions of Downstream Eddy Development to the Teleconnection between ENSO and the Atmospheric Circulation over the North Atlantic. Journal of Climate, 25(14), DOI:10.1175/JCLI-D-11-00377.1. Abstract
The spatio-temporal evolution of various meteorological phenomena associated with El Niño-Southern Oscillation (ENSO) in the North Pacific-North American-North Atlantic sector is examined using both NCEP/NCAR reanalyses and output from a 2000-y integration of a global coupled climate model. Particular attention is devoted to the implications of downstream eddy developments on the relationship between ENSO and the atmospheric circulation over the North Atlantic.
The El Niño-related persistent events are characterized by a strengthened Pacific subtropical jet stream and an equatorward-shifted storm track over the North Pacific. The wave packets that populate the storm tracks travel eastward through downstream development. In such downstream development, the barotropic forcing of the embedded synoptic-scale eddies is conductive to the formation of a flow that resembles the negative phase of the North Atlantic Oscillation (NAO). The more frequent and higher persistence of those episodes during El Niño winters contribute to the prevalence of negative NAO conditions.
The above processes are further delineated by conducting a case study for the 2009/2010 winter season, in which both El Niño and negative NAO conditions prevailed. It is illustrated that the frequent and intense surface cyclone development over North America and the western Atlantic throughout that winter are associated with upper-level troughs propagating across North America, which in turn are linked to downstream evolution of wave packets originating from the Pacific storm track.
Chang, C-P, Ngar-Cheung Lau, R H Johnson, and M Jiao, March 2011: Bridging Weather and Climate in Research and Forecasts of the Global Monsoon System. Bulletin of the American Meteorological Society, 92(3), DOI:10.1175/2010BAMS2984.1.
Chang, C-P, Y Ding, Ngar-Cheung Lau, R H Johnson, Bin Wang, and T Yasunari, April 2011: The Global Monsoon System: Research and Forecast, 2nd Edition, Singapore: World Scientific, 594pp.
Lau, Ngar-Cheung, February 2011: Contributions of observational studies to the evaluation and diagnosis of atmospheric GCM simulations In The Development of Atmospheric General Circulation Models: Complexity, Synthesis and Computation, New York, NY, Cambridge University Press, 117-147.
Lau, Ngar-Cheung, April 2011: Simulation of synoptic and sub-synoptic scale phenomena associated with the East Asian Monsoon using a high-resolution GCM In The Global Monsoon System: Research and Forecast, 2nd Edition, Singapore, World Scientific, 493-508.
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.
The simulation of the diurnal cycle (DC) of precipitation and surface wind
circulation by a global general circulation model (GCM) with a horizontal resolution of
50 km is evaluated. The model output is compared with observational counterparts based
on datasets produced by the Tropical Rainfall Measurement Mission and the European
Centre for Medium-Range Weather Forecasts. The summertime diurnal characteristics
over tropical regions in Asia, the Americas and Africa are portrayed using the amplitude
and phase of the first harmonic of the 24-h cycle, departures of data fields during selected
hours from the daily mean, and differences between extreme phases of the DC.
There is general agreement between model and observations with respect to the
large-scale land-sea contrasts in the DC. Maximum land precipitation, onshore flows and
landward migration of rainfall signals from the coasts occur in afternoon, whereas peak
maritime rainfall and offshore flows prevail in morning. Seaward migration of
precipitation is discernible over western Bay of Bengal and South China Sea during
nocturnal and morning hours. The evolution from low-intensity rainfall in morning/early
afternoon to heavier precipitation several hours later is also evident over selected
continental sites.
Although the model provides an adequate simulation of the daytime upslope and
nighttime downslope winds in the vicinity of mountain ranges, valleys and basins, there
are notable discrepancies between model and observations in the DC of precipitation near
some of these orographic features. The model does not reproduce the observed seaward
migration of precipitation from the western coasts of Myanmar and India, and from
individual islands of the Indonesian Archipelago at nighttime.
Kucharski, Fred, Ngar-Cheung Lau, and Mary Jo Nath, et al., October 2009: The CLIVAR C20C project: skill of simulating Indian monsoon rainfall on interannual to decadal timescales. Does GHG forcing play a role?Climate Dynamics, 33(5), DOI:10.1007/s00382-008-0462-y. Abstract
The ability of atmospheric general circulation models (AGCMs), that are forced with observed sea surface temperatures (SSTs), to simulate the Indian monsoon rainfall (IMR) variability on interannual to decadal timescales is analyzed in a multimodel intercomparison. The multimodel ensemble has been performed within the CLIVAR International “Climate of the 20th Century” (C20C) Project. This paper is part of a C20C intercomparison of key climate time series. Whereas on the interannual timescale there is modest skill in reproducing the observed IMR variability, on decadal timescale the skill is much larger. It is shown that the decadal IMR variability is largely forced, most likely by tropical sea surface temperatures (SSTs), but as well by extratropical and especially Atlantic Multidecadal Oscillation (AMO) related SSTs. In particular there has been a decrease from the late 1950s to the 1990s that corresponds to a general warming of tropical SSTs. Using a selection of control integrations from the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3), it is shown that the increase of greenhouse gases (GHG) in the twentieth century has not significantly contributed to the observed decadal IMR variability.
Lau, Ngar-Cheung, and Mary Jo Nath, September 2009: A model investigation of the role of air-sea interaction in the climatological evolution and ENSO-related variability of the summer monsoon over the South China Sea and western North Pacific. Journal of Climate, 22(18), DOI:10.1175/2009JCLI2758.1. Abstract
The summertime northeastward march of the climatological maritime monsoon over the South China Sea (SCS) and subtropical western North Pacific (WNP) is examined using the output from a 200-yr integration of a coupled atmosphere–ocean general circulation model (GCM). Increased cloud cover and surface wind speed during monsoon onset over the SCS in May–June reduce the incoming shortwave flux and enhance the upward latent heat flux at the ocean surface, thereby cooling the local sea surface temperature (SST). The resulting east–west gradient in the SST pattern, with lower temperature in the SCS and higher temperature in the WNP, is conducive to eastward migration of the monsoon precipitation over this region. Upon arrival of the precipitation center in the WNP in July–August, the local circulation changes lead to weakening of the mei-yu–baiu rainband near 30°N. The subsequent increases in local shortwave flux and SST impart a northward tendency to the evolution of the WNP monsoon. Many of these model inferences are supported by a parallel analysis of various observational datasets.
The modulation of the above climatological scenario by El Niño–Southern Oscillation (ENSO) events is investigated by diagnosing the output from the coupled GCM and from experiments based on the atmospheric component of this GCM with SST forcings being prescribed separately in the equatorial Pacific, Indian Ocean, and SCS/WNP domains. During the May period after the peak phase of ENSO, the simulated monsoon onset over the SCS occurs later (earlier) than normal in El Niño (La Niña) events. These changes are primarily remote responses to the anomalous SST forcing in the equatorial Pacific and Indian Ocean. The ENSO-related changes in the SCS/WNP are associated with above-normal (below normal) mei-yu–baiu activity during warm (cold) events. In the ensuing July period of the warm events, the simulated precipitation response over the SCS to the local warm SST anomaly tends to oppose the remote response to SST forcing in the northern Indian Ocean. In the July period of cold events, the equatorial Pacific SST anomaly retains its strength and moves still farther westward. This forcing cooperates with the cold SST anomaly in the SCS in influencing the precipitation pattern in the SCS/WNP sector.
Lau, Ngar-Cheung, and Jeff J Ploshay, February 2009: Simulation of synoptic- and subsynoptic-scale phenomena associated with the East Asian summer monsoon using a high-resolution GCM. Monthly Weather Review, 137(1), DOI:10.1175/2008MWR2511.1. Abstract
A 20-yr simulation using a global
atmospheric general circulation model with a resolution of 0.5° latitude ×
0.625° longitude is compared with observational findings. The primary goal
of this survey is to assess the model performance in reproducing various
summertime phenomena related to the continental-scale Asian monsoon in
general, and the regional-scale East Asian monsoon in particular. In both
model and observed atmospheres, the seasonal march of the precipitation
centers associated with the Asian summer monsoon is characterized by onsets
occurring earliest over the southeastern Bay of Bengal, followed by rapid
northeastward advances over Indochina, the South China Sea–Philippine Sea
and the western Pacific, northward evolution in the East Asian sector, as
well as northwestward development over the Bay of Bengal, the Indian
subcontinent, and the Arabian Sea. This onset sequence is accompanied by
southwesterly low-level flows over the rainy regions, as well as
northwestward migration of the 200-mb Tibetan anticyclone. Analysis of the
heat sources and sinks in various regions illustrates the prominent role of
condensational heating in the local energy budget during the mature phases
of monsoon development. In accord with observations, the simulated monsoon
rains in the East Asian sector are organized about zonally elongated
“mei-yu–baiu” (plum rain) systems. These precipitation features advance to
higher latitudes during the June–July period, in conjunction with
displacements of the axis of the low-level anticyclone over the subtropical
western Pacific. A detailed case study is performed on a prominent rainy
episode in the simulation. The model is capable of reproducing the observed
intense gradients in temperature, humidity, and moist static stability in
the vicinity of the mei-yu–baiu front, as well as the spatial relationships
between the rainband and the three-dimensional flow field. The axis of the
mei-yu–baiu rainband in this event is aligned with the trajectory of a
succession of mesoscale cyclonic vortices, which originate from southwestern
China and travel northeastward over the Yangtze River basin.
We use a simple methodology to test whether a set of atmospheric climate models with prescribed radiative forcings and ocean surface conditions can reproduce twentieth century climate variability. Globally, rapid land surface warming since the 1970s is reproduced by some models but others warm too slowly. In the tropics, air-sea coupling allows models to reproduce the Southern Oscillation but its strength varies between models. We find a strong relationship between the Southern Oscillation in global temperature and the rate of global warming, which could in principle be used to identify models with realistic climate sensitivity. This relationship and a weak response to ENSO suggests weak sensitivity to changes in sea surface temperature in some of the models used here. In the tropics, most models reproduce part of the observed Sahel drought. In the extratropics, models do not reproduce the observed increase in the North Atlantic Oscillation in response to forcings, through internal variability, or as a combination of both.
We assessed current status of multi-model ensemble (MME) deterministic and probabilistic seasonal prediction based on 25-year (1980–2004) retrospective forecasts performed by 14 climate model systems (7 onetier and 7 two-tier systems) that participate in the Climate Prediction and its Application to Society (CliPAS) project sponsored by the Asian-Pacific Economic Cooperation Climate Center (APCC). We also evaluated seven DEMETER models’ MME for the period of 1981–2001 for comparison. Based on the assessment, future direction for improvement of seasonal prediction is discussed. We found that two measures of probabilistic forecast skill, the Brier Skill Score (BSS) and Area under the Relative Operating Characteristic curve (AROC), display similar spatial patterns as those represented by temporal correlation coefficient (TCC) score of deterministic MME forecast. A TCC score of 0.6 corresponds approximately to a BSS of 0.1 and an AROC of 0.7 and beyond these critical threshold values, they are almost linearly correlated. The MME method is demonstrated to be a valuable approach for reducing errors and quantifying forecast uncertainty due to model formulation. The MME prediction skill is substantially better than the averaged skill of all individual models. For instance, the TCC score of CliPAS one-tier MME forecast of Ni ńo 3.4 index at a 6-month lead initiated from 1 May is 0.77, which is significantly higher than the corresponding averaged skill of seven individual coupled models (0.63). The MME made by using 14 coupled models from both DEMETER and CliPAS shows an even higher TCC score of 0.87. Effectiveness of MME depends on the averaged skill of individual models and their mutual independency. For probabilistic forecast the CliPAS MME gains considerable skill from increased forecast reliability as the number of model being used increases; the forecast resolution also increases for 2 m temperature but slightly decreases for precipitation. Equatorial Sea Surface Temperature (SST) anomalies are primary sources of atmospheric climate variability worldwide. The MME 1-month lead hindcast can predict, with high fidelity, the spatial–temporal structures of the first two leading empirical orthogonal modes of the equatorial SST anomalies for both boreal summer (JJA) and winter (DJF), which account for about 80–90% of the total variance. The major bias is a westward shift of SST anomaly between the dateline and 120E, which may potentially degrade global teleconnection associated with it. The TCC score for SST predictions over the equatorial eastern Indian Ocean reaches about 0.68 with a 6-month lead forecast. However, the TCC score for Indian Ocean Dipole (IOD) index drops below 0.40 at a 3-month lead for both the May and November initial conditions due to the prediction barriers across July, and January, respectively. The MME prediction skills are well correlated with the amplitude of Nińo 3.4 SST variation. The forecasts for 2 m air temperature are better in El Nińo years than in La Nińa years. The precipitation and circulation are predicted better in ENSO-decaying JJA than in ENSO-developing JJA. There is virtually no skill in ENSO-neutral years. Continuing improvement of the onetier climate model’s slow coupled dynamics in reproducing realistic amplitude, spatial patterns, and temporal evolution of ENSO cycle is a key for long-lead seasonal forecast. Forecast of monsoon precipitation remains a major challenge. The seasonal rainfall predictions over land and during local summer have little skill, especially over tropical Africa. The differences in forecast skills over land areas between the CliPAS and DEMETER MMEs indicate potentials for further improvement of prediction over land. There is an urgent need to assess impacts of land surface initialization on the skill of seasonal and monthly forecast using a multi-model framework.
Zhou, Tianjun, Ngar-Cheung Lau, and Mary Jo Nath, et al., December 2009: The CLIVAR C20C project: which components of the Asian–Australian monsoon circulation variations are forced and reproducible?Climate Dynamics, 33(7-8), DOI:10.1007/s00382-008-0501-8. Abstract
A multi-model set of atmospheric simulations forced by historical sea surface temperature (SST) or SSTs plus Greenhouse gases and aerosol forcing agents for the period of 1950–1999 is studied to identify and understand which components of the Asian–Australian monsoon (A–AM) variability are forced and reproducible. The analysis focuses on the summertime monsoon circulations, comparing model results against the observations. The priority of different components of the A–AM circulations in terms of reproducibility is evaluated. Among the subsystems of the wide A–AM, the South Asian monsoon and the Australian monsoon circulations are better reproduced than the others, indicating they are forced and well modeled. The primary driving mechanism comes from the tropical Pacific. The western North Pacific monsoon circulation is also forced and well modeled except with a slightly lower reproducibility due to its delayed response to the eastern tropical Pacific forcing. The simultaneous driving comes from the western Pacific surrounding the maritime continent region. The Indian monsoon circulation has a moderate reproducibility, partly due to its weakened connection to June–July–August SSTs in the equatorial eastern Pacific in recent decades. Among the A–AM subsystems, the East Asian summer monsoon has the lowest reproducibility and is poorly modeled. This is mainly due to the failure of specifying historical SST in capturing the zonal land-sea thermal contrast change across the East Asia. The prescribed tropical Indian Ocean SST changes partly reproduce the meridional wind change over East Asia in several models. For all the A–AM subsystem circulation indices, generally the MME is always the best except for the Indian monsoon and East Asian monsoon circulation indices.
Jiang, Xianan, and Ngar-Cheung Lau, 2008: Intraseasonal teleconnection between North American and western North Pacific monsoons with 20-day time scale. Journal of Climate, 21(11), DOI:10.1175/2007JCLI2024.1. Abstract
Based on a recently released, high-resolution reanalysis dataset for the North American region, the intraseasonal variability (ISV; with a time scale of about 20 days) of the North American monsoon (NAM) is examined. The rainfall signals associated with this phenomenon first emerge near the Gulf of Mexico and eastern Pacific at about 20°N. They subsequently migrate to the southwestern United States along the slope of the Sierra Madre Occidental. The rainfall quickly dissipates upon arrival at the desert region of Arizona and New Mexico (AZNM). The enhanced rainfall over AZNM is accompanied by strong southeasterly low-level flow along the Gulf of California. This pattern bears strong resemblance to the circulation related to “gulf surge” events, as documented by many studies. The southeasterly flow is associated with an anomalous low vortex over the subtropical eastern Pacific Ocean off California, and a midlatitude anticyclone over the central United States in the lower troposphere. This flow pattern is in broad agreement with that favoring the “wet surges” over the southwestern United States.
It is further demonstrated that the aforementioned low-level circulations associated with ISV of the NAM are part of a prominent trans-Pacific wave train extending from the western North Pacific (WNP) to the Eastern Pacific/North America along a “great circle” path. The circulation anomalies along the axis of this wave train exhibit a barotropic vertical structure over most regions outside of the WNP, and a baroclinic structure over the WNP, thus suggesting the important role of convective activities over the WNP in sustaining this wave train. This inference is further substantiated by an analysis of the pattern of wave-activity–flux vectors. Variations in the WNP convection are correlated with the ISV of the monsoons in both North American and East Asian (EA)/WNP sectors. These relationships lead to notable teleconnections between NAM and the EA/WNP monsoon on 20-day time scales.
Lau, Ngar-Cheung, Ants Leetmaa, and Mary Jo Nath, February 2008: Interactions between the responses of North American climate to El Niño–La Niña and to the secular warming trend in the Indian–Western Pacific Oceans. Journal of Climate, 21(3), 476-494. Abstract PDF
The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector.
The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño).
The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.
LinHo, L H., X Huang, and Ngar-Cheung Lau, 2008: Winter-to-spring transition in East Asia: A planetary-scale perspective of the South China spring rain onset. Journal of Climate, 21(13), DOI:10.1175/2007JCLI1611.1. Abstract
Analysis of observations from 1979 to 2002 shows that the seasonal transition from winter to spring in East Asia is marked with a distinctive event—the onset of the south China spring rain (SCSR). In late February, the reduced thermal contrast between ocean and land leads to weakening of the Asian winter monsoon as well as the Siberian high and the Aleutian low. Meanwhile, convection over Australia and the western Pacific Maritime Continent is suppressed on the passage of the dry phase of a Madden–Julian oscillation (MJO). In conjunction with the seasonal march of monsoon circulation in the Indonesian–Australian sector, this MJO passage weakens the local thermally direct cell in the East Asia–Australia sector. This development is further accompanied by a series of adjustments in both the tropics and midlatitudes. These changes include attenuation of the planetary stationary wave, considerable weakening of the westerly jet stream over much of the central Pacific adjacent to Japan, and reduction of baroclinicity near the East Asian trough. The influence of concurrent local processes in midlatitudes on the SCSR onset is also important. The weakened jet stream is associated with confinement of frontal activities to the coastal regions of East Asia as well as with rapid expansion of the subtropical Pacific high from the eastern Pacific to the western Pacific. A parallel analysis using output from an experiment with a GFDL-coupled GCM shows that the above sequence of circulation changes is well simulated in that model.
A model diagnosis has been performed on the nocturnal Great Plains low-level jet (LLJ), which is one of the key elements of the warm season regional climate over North America. The horizontal–vertical structure, diurnal phase, and amplitude of the LLJ are well simulated by an atmospheric general circulation model (AGCM), thus justifying a reevaluation of the physical mechanisms for the formation of the LLJ based on output from this model. A diagnosis of the AGCM data confirms that two planetary boundary layer (PBL) processes, the diurnal oscillation of the pressure gradient force and of vertical diffusion, are of comparable importance in regulating the inertial oscillation of the winds, which leads to the occurrence of maximum LLJ strength during nighttime. These two processes are highlighted in the theories for the LLJ proposed by Holton (1967) and Blackadar (1957). A simple model is constructed in order to study the relative roles of these two mechanisms. This model incorporates the diurnal variation of the pressure gradient force and vertical diffusion coefficients as obtained from the AGCM simulation. The results reveal that the observed diurnal phase and amplitude of the LLJ can be attributed to the combination of these two mechanisms. The LLJ generated by either Holton’s or Blackadar’s mechanism alone is characterized by an unrealistic meridional phase shift and weaker amplitude.
It is also shown that the diurnal phase of the LLJ exhibits vertical variations in the PBL, more clearly at higher latitudes, with the upper PBL wind attaining a southerly peak several hours earlier than the lower PBL. The simple model demonstrates that this phase tilt is due mainly to sequential triggering of the inertial oscillation from upper to lower PBL when surface cooling commences after sunset. At lower latitudes, due to the change of orientation of prevailing mean wind vectors and the longer inertial period, the inertial oscillation in the lower PBL tends to be interrupted by strong vertical mixing in the following day, whereas in the upper PBL, the inertial oscillation can proceed in a low-friction environment for a relatively longer duration. Thus, the vertical phase tilt initiated at sunset is less evident at lower latitudes.
Lau, Ngar-Cheung, 2007: Scientific basis of climate change. Bulletin of the Hong Kong Meteorological Society, 17, 1-27 pp. Abstract
Our awareness of possible human impacts on the earth’s climate system dates
back to the early 19th century. In 1827, the renowned French
mathematician Jean-Baptiste Fourier recognized that the atmosphere acts like
a ‘glass vessel’ by trapping heat energy derived from sunlight. This idea
later evolved into the concept of ‘greenhouse effect’ as the principal cause
for global warming. The marked increase in the burning of fossil fuels
(mainly coal and petroleum) since the Industrial Revolution has led to
emissions of large amounts of carbon dioxide (CO2) to the atmosphere. In
1896, the Swedish chemist Svante Arrhenius first noted that the rising level
of this gas could warm our climate. Systematic efforts to monitor the
atmospheric concentration of CO2 and various indicators of the global
environment (such as temperature, precipitation, ice cover, etc.) were
launched in the 20th century. The emerging scientific evidence
has caused sufficient concern that the Massachusetts Institute of Technology
(MIT) sponsored a month-long study in the summer of 1970. More than forty
eminent scientists and professionals contributed their knowledge on the
emission rates, pathways and global impacts of various pollutants.
Lau, Ngar-Cheung, 2007: Scientific basis of climate change (in Chinese). Science Monthly, 102, 15-24. PDF
Lau, Ngar-Cheung, 2007: Scientific basis of climate change (in Chinese). Twenty-First Century, 38(12), 924-932. PDF
Lee, M-I, S D Schubert, M J Suarez, Isaac M Held, Arun Kumar, T L Bell, J-K E Schemm, Ngar-Cheung Lau, Jeff J Ploshay, H-K Kim, and S-H Yoo, May 2007: Sensitivity to horizontal resolution in the AGCM simulations of warm season diurnal cycle of precipitation over the United States and Northern Mexico. Journal of Climate, 20(9), DOI:10.1175/JCLI4090.1. Abstract
This study examines the sensitivity of the North American warm season diurnal cycle of precipitation to changes in horizontal resolution in three atmospheric general circulation models, with a primary focus on how the parameterized moist processes respond to improved resolution of topography and associated local/regional circulations on the diurnal time scale. It is found that increasing resolution (from approximately 2° to ½° in latitude–longitude) has a mixed impact on the simulated diurnal cycle of precipitation. Higher resolution generally improves the initiation and downslope propagation of moist convection over the Rockies and the adjacent Great Plains. The propagating signals, however, do not extend beyond the slope region, thereby likely contributing to a dry bias in the Great Plains. Similar improvements in the propagating signals are also found in the diurnal cycle over the North American monsoon region as the models begin to resolve the Gulf of California and the surrounding steep terrain. In general, the phase of the diurnal cycle of precipitation improves with increasing resolution, though not always monotonically. Nevertheless, large errors in both the phase and amplitude of the diurnal cycle in precipitation remain even at the highest resolution considered here. These errors tend to be associated with unrealistically strong coupling of the convection to the surface heating and suggest that improved simulations of the diurnal cycle of precipitation require further improvements in the parameterizations of moist convection processes.
Lee, M-I, S D Schubert, M J Suarez, Isaac M Held, Ngar-Cheung Lau, Jeff J Ploshay, Arun Kumar, H-K Kim, and J-K E Schemm, June 2007: An analysis of the warm-season diurnal cycle over the continental United States and Northern Mexico in general circulation models. Journal of Hydrometeorology, 8(3), DOI:10.1175/JHM581.1. Abstract
The diurnal cycle of warm-season rainfall over the continental United States and northern Mexico is analyzed in three global atmospheric general circulation models (AGCMs) from NCEP, GFDL, and the NASA Global Modeling Assimilation Office (GMAO). The results for each model are based on an ensemble of five summer simulations forced with climatological sea surface temperatures.
Although the overall patterns of time-mean (summer) rainfall and low-level winds are reasonably well simulated, all three models exhibit substantial regional deficiencies that appear to be related to problems with the diurnal cycle. Especially prominent are the discrepancies in the diurnal cycle of precipitation over the eastern slopes of the Rocky Mountains and adjacent Great Plains, including the failure to adequately capture the observed nocturnal peak. Moreover, the observed late afternoon–early evening eastward propagation of convection from the mountains into the Great Plains is not adequately simulated, contributing to the deficiencies in the diurnal cycle in the Great Plains. In the southeast United States, the models show a general tendency to rain in the early afternoon—several hours earlier than observed. Over the North American monsoon region in the southwest United States and northern Mexico, the phase of the broad-scale diurnal convection appears to be reasonably well simulated, though the coarse resolution of the runs precludes the simulation of key regional phenomena.
All three models employ deep convection schemes that assume fundamentally the same buoyancy closure based on simplified versions of the Arakawa–Schubert scheme. Nevertheless, substantial differences between the models in the diurnal cycle of convection highlight the important differences in their implementations and interactions with the boundary layer scheme. An analysis of local diurnal variations of convective available potential energy (CAPE) shows an overall tendency for an afternoon peak—a feature well simulated by the models. The simulated diurnal cycle of rainfall is in phase with the local CAPE variation over the southeast United States and the Rocky Mountains where the local surface boundary forcing is important in regulating the diurnal cycle of convection. On the other hand, the simulated diurnal cycle of rainfall tends to be too strongly tied to CAPE over the Great Plains, where the observed precipitation and CAPE are out of phase, implying that free atmospheric large-scale forcing plays a more important role than surface heat fluxes in initiating or inhibiting convection.
How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.
Jiang, Xianan, Ngar-Cheung Lau, and Stephen A Klein, October 2006: Role of eastward propagating convection systems in the diurnal cycle and seasonal mean of summertime rainfall over the U.S. Great Plains. Geophysical Research Letters, 33, L19809, DOI:10.1029/2006GL027022. Abstract
By diagnosing the 3-hourly North American Regional Reanalysis rainfall data set for the 1979–2003 period, it is illustrated that the eastward propagation of convection systems from the Rockies to the Great Plains plays an essential role for the warm season climate over the central U.S. This eastward propagating mode could be the deciding factor for the observed nocturnal rainfall peak over the Great Plains. The results also suggest that nearly half of the total summer mean rainfall over this region is associated with these propagating convection systems. For instance, the extreme wet condition of the 1993 summer may be attributed to the frequent occurrence of propagating convection events and enhanced diurnal rainfall amplitude over the Great Plains. Thus, proper representation of this important propagating component in GCMs is essential for simulating the diurnal and seasonal mean characteristics of summertime rainfall over the central US.
Lau, Ngar-Cheung, Ants Leetmaa, and Mary Jo Nath, 2006: Attribution of Atmospheric Variations in the 1997–2003 Period to SST Anomalies in the Pacific and Indian Ocean Basins. Journal of Climate, 19(15), 3507-3628. Abstract PDF
The individual impacts of sea surface temperature (SST) anomalies in the deep tropical eastern–central Pacific (DTEP) and Indo-western–central Pacific (IWP) on the evolution of the observed global atmospheric circulation during the 1997–2003 period have been investigated using a new general circulation model. Ensemble integrations were conducted with monthly varying SST conditions being prescribed separately in the DTEP sector, the IWP sector, and throughout the World Ocean. During the 1998–2002 subperiod, when prolonged La Niña conditions occurred in DTEP and the SST in IWP was above normal, the simulated midlatitude atmospheric responses to SST forcing in the DTEP and IWP sectors reinforced each other. The anomalous geopotential height ridges at 200 mb in the extratropics of both hemispheres exhibited a distinct zonal symmetry. This circulation change was accompanied by extensive dry and warm anomalies in many regions, including North America. During the 1997–98 and 2002–03 El Niño events, the SST conditions in both DTEP and IWP were above normal, and considerable cancellations were simulated between the midlatitude responses to the oceanic forcing from these two sectors. The above findings are contrasted with those for the 1953–58 and 1972–77 periods, which were characterized by analogous SST developments in DTEP, but by cold conditions in IWP. It is concluded that a warm anomaly in IWP and a cold anomaly in DTEP constitute the optimal SST configuration for generating zonally elongated ridges in the midlatitudes.
Local diagnoses indicate that the imposed SST anomaly alters the strength of the zonal flow in certain longitudinal sectors, which influences the behavior of synoptic-scale transient eddies farther downstream. The modified eddy momentum transports in the regions of eddy activity in turn feed back on the local mean flow, thus contributing to its zonal elongation. These results are consistent with the inferences drawn from zonal mean analyses, which accentuate the role of the eddy-induced circulation on the meridional plane.
Lau, Ngar-Cheung, and Mary Jo Nath, 2006: ENSO Modulation of the Interannual and Intraseasonal Variability of the East Asian Monsoon—A Model Study. Journal of Climate, 19(18), 4508-4530. Abstract PDF
The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment. Observed monthly variations of the sea surface temperature (SST) field have been prescribed in the tropical eastern and central Pacific, whereas the atmosphere has been coupled to an oceanic mixed layer model beyond this forcing region. During the boreal summer of typical El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific in response to enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to temperature tendencies that set the stage for the abrupt establishment of a strong Philippine Sea anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold-air surges over East Asia.
The air–sea interactions accompanying the intraseasonal variations (ISV) in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks. The onset of the PSAC anomaly is seen to be coincident with a prominent episode of the leading ISV mode. The ENSO events could influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air–sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. A weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold-air surges associated with the leading ISV mode in that season.
Many aspects of the model simulation of the relationships between ENSO and the East Asian monsoon are in agreement with observational findings.
Lau, Ngar-Cheung, and Bin Wang, 2006: Interactions between the Asian monsoon and the El Nino/Southern Oscillation In The Asian Monsoon, Berlin, Praxis, 479-512.
Multicentury integrations from two global coupled ocean–atmosphere–land–ice models [Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), developed at the Geophysical Fluid Dynamics Laboratory] are described in terms of their tropical Pacific climate and El Niño–Southern Oscillation (ENSO). The integrations are run without flux adjustments and provide generally realistic simulations of tropical Pacific climate. The observed annual-mean trade winds and precipitation, sea surface temperature, surface heat fluxes, surface currents, Equatorial Undercurrent, and subsurface thermal structure are well captured by the models. Some biases are evident, including a cold SST bias along the equator, a warm bias along the coast of South America, and a westward extension of the trade winds relative to observations. Along the equator, the models exhibit a robust, westward-propagating annual cycle of SST and zonal winds. During boreal spring, excessive rainfall south of the equator is linked to an unrealistic reversal of the simulated meridional winds in the east, and a stronger-than-observed semiannual signal is evident in the zonal winds and Equatorial Undercurrent.
Both CM2.0 and CM2.1 have a robust ENSO with multidecadal fluctuations in amplitude, an irregular period between 2 and 5 yr, and a distribution of SST anomalies that is skewed toward warm events as observed. The evolution of subsurface temperature and current anomalies is also quite realistic. However, the simulated SST anomalies are too strong, too weakly damped by surface heat fluxes, and not as clearly phase locked to the end of the calendar year as in observations. The simulated patterns of tropical Pacific SST, wind stress, and precipitation variability are displaced 20°–30° west of the observed patterns, as are the simulated ENSO teleconnections to wintertime 200-hPa heights over Canada and the northeastern Pacific Ocean. Despite this, the impacts of ENSO on summertime and wintertime precipitation outside the tropical Pacific appear to be well simulated. Impacts of the annual-mean biases on the simulated variability are discussed.
Lau, K M., Ngar-Cheung Lau, and S Yang, 2005: Current topics on interannual variability of the Asian Monsoon In The Global Monsoon System: Research and Forecast, Geneva, Switzerland, WMO, WMO/TD No. 1266, TMRP Report No. 70, 440-454.
Lau, Ngar-Cheung, and F-C Chen, 2005: Between the sky and the sea. Twenty-First Century, 85-97. Abstract
A review is presented on recent advances in our scientific understanding of the roles of the world's oceans in climate change. Recent studies on this subject based on observational evidence, numerical modeling and theoretical analyses are summarized. The oceans serve as the principal repository of anthropogenic carbon dioxide. The intensity and frequency of El Nino-Southern Oscillation events, in which the tropical Pacific is a key participant, may be altered by global warming. Variations in the strength of the thermohaline circulation in the Atlantic basin are known to be linked to abrupt climate changes during the Younger-Dryas period at 12,800-11,600 years before present. The potential for analogous climate shifts associated with perturbations of this current system due to global warming is evaluated.
Lau, Ngar-Cheung, and F-C Chen, 2005: Between the sky and the sea. Science and Technology in China, 42-49.
The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño-Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo-western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the "atmospheric bridge" mechanism.
Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes.
The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.
Lau, Ngar-Cheung, and Bin Wang, 2005: Monsoon-ENSO interactions In The Global Monsoon System: Research and Forecast, Geneva, Switzerland, WMO, WMO/TD No. 1266, TMRP Report No. 70, 299-309.
Sumi, A, Ngar-Cheung Lau, and W-C Wang, 2005: Present status of Asian Monsoon simulation In The Global Monsoon System: Research and Forecast, WMO/TD No. 1266 and TMRP Report No. 70, Geneva, Switzerland, WMO, 376-385.
Tam, Chi-Yung, and Ngar-Cheung Lau, June 2005: The impact of ENSO on atmospheric intraseasonal variability as inferred from observations and GCM simulations. Journal of Climate, 18(12), 1902-1924. Abstract PDF
The impact of the El Niño–Southern Oscillation (ENSO) on the atmospheric intraseasonal variability in the North Pacific is assessed, with emphasis on how ENSO modulates midlatitude circulation anomalies associated with the Madden–Julian oscillation (MJO) in the Tropics and the westward-traveling patterns (WTP) in high latitudes. The database for this study consists of the output of a general circulation model (GCM) experiment subjected to temporally varying sea surface temperature (SST) forcing in the tropical Pacific, and observational reanalysis products.
Diagnosis of the GCM experiment indicates a key region in the North Pacific over which the year-to-year variation of intraseasonal activity is sensitive to the SST conditions in the Tropics. In both the simulated and observed atmospheres, the development phase of the dominant circulation anomaly in this region is characterized by incoming wave activity from northeast Asia and the subtropical western Pacific. Southeastward dispersion from the North Pacific to North America can be found in later phases of the life cycle of the anomaly. The spatial pattern of this recurrent extratropical anomaly contains regional features that are similar to those appearing in composite charts for prominent episodes of the MJO and the WTP.
Both the GCM and reanalysis data indicate that the amplitude of intraseasonal variability near the key region, as well as incoming wave activity in the western Pacific and dispersion to the western United States, are enhanced in cold ENSO events as compared to warm events. Similar modulations of the MJO-related circulation patterns in the extratropics by ENSO forcing are discernible in the model simulation. It is inferred from these findings that ENSO can influence the North Pacific intraseasonal activity through its effects on the evolution of convective anomalies in the tropical western Pacific. On the other hand, there is little modification by ENSO of the circulation features associated with the WTP.
The combined effect of the MJO and WTP on the intraseasonal circulation in the North Pacific is studied. Based on multiple regression analysis, it is found that the MJO and WTP make comparable contributions to the variability in the midlatitude North Pacific. These contributions may be treated as a linear combination of the anomalies attributed to the MJO and WTP separately.
Tam, Chi-Yung, and Ngar-Cheung Lau, November 2005: Modulation of the Madden-Julian Oscillation by ENSO: Inferences from observations and GCM simulations. Journal of the Meteorological Society of Japan, 83(5), 727-743. Abstract PDF
The impact of the El Niño-Southern Oscillation (ENSO) on the Madden-Julian Oscillation (MJO) is studied, based on reanalysis data and output from an ensemble general circulation model (GCM) experiment. Observed monthly sea surface temperature variations over the period of 1950-99 are imposed in the deep tropical eastern/central Pacific in the course of the SST experiment. Both GCM, and reanalysis data, indicate that intraseasonal activity of the low-level zonal wind is enhanced (reduced) over the central (western) Pacific during El Niño events. The propagation and growth/decay characteristics of the MJO in different phases of ENSO is also examined, based on a lag correlation technique. During warm events there is an eastward shift in the locations of strong growth and decay, and the propagation of the MJO becomes slower in the warm ENSO phase. These changes are reversed during La Niña epsiodes.
Using output from the GCM experiment, the effects of ENSO on the circulation and convection during the MJO lifecycle are studied in detail. Further eastward penetration of MJO-related convection is simulated during warm events over the central Pacific. An instability index related to the vertical gradient of the moist static energy is found to be useful for depicting the onset of MJO convection along the equator. During warm events, the stronger magnitudes of this index over the central Pacific are conducive to more eastward penetration of convective anomalies in the region. These changes are mainly due to the intensified moisture accumulation at low levels. Analysis of the moisture budget suggests that the stronger moisture accumulation can be related to the increased low-level humidity over the central Pacific during warm events.
High-resolution (0.1‹ ~ 0.1‹) geostationary satellite infrared radiances at 11 ƒÊm in combination with gridded (2.5‹ ~ 2.0‹) hourly surface precipitation observations are employed to document the spatial structure of the diurnal cycle of summertime deep convection and associated precipitation over North America. Comparison of the diurnal cycle pattern between the satellite retrieval and surface observations demonstrates the reliability of satellite radiances for inferring the diurnal cycle of precipitation, especially the diurnal phase. On the basis of the satellite radiances, we find that over most land regions, deep convection peaks in the late afternoon and early evening, a few hours later than the peak of land surface temperature. However, strong regional variations exist in both the diurnal phase and amplitude, implying that topography, land-sea contrast, and coastline curvature play an important role in modulating the diurnal cycle. Examples of such effects are highlighted over Florida, the Great Plains, and the North American monsoon region.
Alexander, Michael A., Ngar-Cheung Lau, and J D Scott, 2004: Broadening the atmospheric bridge paradigm: ENSO teleconnections to the tropical West Pacific-Indian Oceans over the seasonal cycle and to the North Pacific in Summer In Earth's Climate: The Ocean-Atmosphere Interaction, Geophysical Monograph 147, Washington, DC, American Geophysical Union, 85-103. Abstract
During El Niño-Southern Oscillation (ENSO) events, atmospheric teleconnections associated with sea surface temperature (SST) anomalies in the equatorial Pacific can influence the ocean thousands of kilometers away. We use several data sets to delineate this "atmospheric bridge" between ocean basins, focusing on two emerging research areas: 1) the evolution of atmosphere-ocean interactions in the tropical Indian-Western Pacific Oceans over the full ENSO cycle and 2) the formation of large amplitude SST anomalies in North Pacific in the summer before ENSO peaks. In ENSO composites [where events peak near the end of Yr(0)], an east-west SST dipole develops in the Indian Ocean during the summer-fall of Yr(0), followed by basin-wide warming through spring of Yr(1). The SST anomalies over most of the tropical west Pacific also reverse sign, from negative in summer of Yr(0) to positive in the following summer. Local air-sea interactions influence the evolution of these ENSO-induced SST anomalies and related sea level pressure (SLP) and precipitation anomalies. Over the western North Pacific, the southward displacement of the jet stream and storm track in the summer of Yr(0) changes the solar radiation and latent heat flux at the surface, which results in anomalous cooling (and deepening) of the oceanic mixed layer at ~40°N. The potential impact of both the tropical and North Pacific SST anomalies on the broader climate is discussed.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Lau, Ngar-Cheung, and F-C Chen, 2004: Role of ocean in climate change. Hong Kong Meteorological Society Bulletin, 14(1-2), 3-20.
Lau, Ngar-Cheung, and Mary Jo Nath, 2004: Coupled GCM simulation of atmosphere-ocean variability associated with zonally asymmetric SST changes in the tropical Indian Ocean. Journal of Climate, 17(2), 245-265. Abstract PDF
The nature of a recurrent pattern of variability in the tropical Indian Ocean (IO) during the boreal autumn has been investigated using a 900-yr experiment with a coupled atmosphere-ocean general circulation model. This Indian Ocean Pattern (IOP) is characterized by zonal surface wind perturbations along the equator, as well as east-west contrasts in the anomalous sea surface temperature (SST), surface pressure, and precipitation fields. The IOP is seen to be linked to the El Niño-Southern Oscillation (ENSO) phenomenon in the tropical Pacific. By constructing composite charts and analyzing the heat budget for the top ocean layer, it is illustrated that the ENSO-related changes in the surface wind modify the intensity of oceanic upwelling, horizontal temperature advection, and surface heat fluxes in various parts of the IO basin. These processes lead to SST perturbations with opposite signs in the eastern and western equatorial IO. Further diagnosis of the model output reveals that some strong IOP episodes occur even in the near absence of ENSO influences. In such IOP events that do not coincide with prominent ENSO development, the most noteworthy signal is a zonally elongated sea level pressure anomaly situated south of Australia during the southern winter. The anomalous atmospheric circulation on the equatorward flank of this feature contributes to the initiation of IOP-like events when the ENSO forcing is weak. Both simulated and observational data show that the pressure anomaly south of Australia is part of a hemisphere-wide pattern bearing a considerable resemblance to the Antarctic Oscillation. This annular mode of variability is characterized by opposite pressure changes in the midlatitude and polar zones, and is only weakly correlated with ENSO. The findings reported here indicate that the IOP is attributable to multiple factors, including remote influences due to ENSO and extratropical changes, as well as internal air-sea feedbacks occurring within the IO basin.
Lau, Ngar-Cheung, Mary Jo Nath, and H Wang, 2004: Simulations by a GFDL GCM of ENSO-related variability of the coupled atmosphere-ocean system in the East Asian monsoon region In East Asian Monsoon, C.-P. Chang, Ed., World Scientific Series on Meteorology of East Asia, 271-300. Abstract
The impact of El Niño-Southern Oscillation (ENSO) on the East Asian Monsoon (EAM) has been examined using a general circulation model (GCM). The observed monthly changes in sea surface temperature (SST) in the equatorial Pacific east of 172°E during 1950-99 were inserted as the lower boundary condition of the model. For all oceanic grid points lying outside of the region of SST prescription, the atmosphere was coupled to an oceanic mixed layer model.
The typical evolution of the atmosphere-ocean system during ENSO was analyzed using composite charts. These patterns show that the key changes in the EAM sector are related to a prominent sea level pressure anomaly simulated over the South China Sea and subtropical northwestern Pacific. During warm ENSO events, the circulation anomalies associated with this anomalous anticyclone correspond to weaker winter monsoon flows along the East Asian coast, as well as above-normal precipitation over southern China. These signals move systematically eastward during the following spring and summer. Stationary wave modeling indicates that the atmospheric anomaly in the EAM region is essentially a Rossby-wave response to the ENSO-related diabatic heating pattern over the equatorial western Pacific.
The wintertime atmospheric circulation anomalies over the western Pacific generate strong SST anomalies in the following spring. Further model diagnoses indicate that the feedback of these SST changes on the atmosphere leads to eastward propagation of the pressure anomaly in the EAM region, and to amplification of rainfall anomalies along the Meiyu-Baiu front.
The atmospheric and oceanic changes in the EAM sector described in this chapter are discussed in the broader context of ENSO influences on the entire Asian-Australian monsoon system.
Lau, Ngar-Cheung, and Mary Jo Nath, 2003: Atmosphere-Ocean variations in the Indo-Pacific sector during ENSO episodes. Journal of Climate, 16(1), 3-20. Abstract PDF
The influences of El Niño–Southern Oscillation (ENSO) events on air–sea interaction in the Indian–western Pacific (IWP) Oceans have been investigated using a general circulation model. Observed monthly sea surface temperature (SST) variations in the deep tropical eastern/central Pacific (DTEP) have been inserted in the lower boundary of this model through the 1950–99 period. At all maritime grid points outside of DTEP, the model atmosphere has been coupled with an oceanic mixed layer model with variable depth. Altogether 16 independent model runs have been conducted. #Composite analysis of selected ENSO episodes illustrates that the prescribed SST anomalies in DTEP affect the surface atmospheric circulation and precipitation patterns in IWP through displacements of the near-equatorial Walker circulation and generation of Rossby wave modes in the subtropics. Such atmospheric responses modulate the surface fluxes as well as the oceanic mixed layer depth, and thereby establish a well-defined SST anomaly pattern in the IWP sector several months after the peak in ENSO forcing in DTEP. In most parts of the IWP region, the net SST tendency induced by atmospheric changes has the same polarity as the local composite SST anomaly, thus indicating that the atmospheric forcing acts to reinforce the underlying SST signal. #By analyzing the output from a suite of auxiliary experiments, it is demonstrated that the SST perturbations in IWP (which are primarily generated by ENSO-related atmospheric changes) can, in turn, exert notable influences on the atmospheric conditions over that region. This feedback mechanism also plays an important role in the eastward migration of the subtropical anticyclones over the western Pacific in both hemispheres.
Alexander, Michael A., I Bladé, Matthew Newman, John R Lanzante, Ngar-Cheung Lau, and J D Scott, 2002: The atmospheric bridge: the influence of ENSO teleconnections on air-sea interaction over the global oceans. Journal of Climate, 15(16), 2205-2231. Abstract PDF
During El Niño-Southern Oscillation (ENSO) events, the atmospheric response to sea surface temperature (SST) anomalies in the equatorial Pacific influences ocean conditions over the remainder of the globe. This connection between ocean basins via the "atmospheric bridge" is reviewed through an examination of previous work augmented by analyses of 50 years of data from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis project and coupled atmospheric general circulation (AGCM)-mixed layer ocean model experiments. Observational and modeling studies have now established a clear link between SST anomalies in the equatorial Pacific with those in the North Pacific, north tropical Atlantic, and Indian Oceans in boreal winter and spring. ENSO-related SST anomalies also appear to be robust in the western North Pacific during summer and in the Indian Ocean during fall. While surface heat fluxes are the key component of the atmospheric bridge driving SST anomalies, Ekman transport also creates SST anomalies in the central North Pacific although the full extent of its impact requires further study. The atmospheric bridge not only influences SSTs on interannual timescales but also affects mixed layer depth (MLD), aalinity, the seasonal evolution of upper-ocean temperatures, and North Pacific SST variability at lower frequencies. The model results indicate that a significant fractionof the dominant pattern of low-frequency (>10 yr) SST variability in the North Pacific is associated with tropical forcing. AGCM experiments suggest that the oceanic feedback on the extratropical response to ENSO is complex, but of modest amplitude. Atmosphere-ocean coupling outside of the tropical Pacific slightly modifies the atmospheric circulation anomalies in the Pacific-North America (PNA) region but these modifications appear to depend on the seasonal cycle and air-sea interactions both within and beyond the North Pacific Ocean.
Lau, Ngar-Cheung, 2002: Impact of ENSO on the variability of the Asian-Australian monsoons In Selected Papers of the Fourth Conference on East Asia and Western Pacific Meteorology and Climate., Vol. 1, Singapore, World Scientific Series on Meteorology of East Asia, 77-85.
The effect of changes in observational coverage on the association between the Arctic oscillation (AO) and extratropical Northern Hemisphere surface temperature is examined. A coupled atmosphere-ocean model, which produces a realistic simulation of the circulation and temperature patterns associated with the AO, is used as a surrogate for the real-climate system. The association between the AO and spatial mean-temperature, as quantified by regressing the latter on the AO index, is subject to a positive bias due to the incomplete spatial coverage of the observational network. The bias is largest during the early part of the twentieth century and decreases, but does not vanish, thereafter.
Lau, Ngar-Cheung, and Mary Jo Nath, 2000: Impact of ENSO on the variability of the Asian-Australian monsoons as simulated in GCM experiments. Journal of Climate, 13(24), 4287-4309. Abstract PDF
The influences of El NiñoSouthern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 195095 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios.
The essential characteristics of the model climatology for the AsianAustralian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding warm and cold ENSO events reveal that a majority of the warm episodes are accompanied by below-normal summer rainfall in India and northern Australia, and above-normal winter rainfall in southeast Asia. The polarity of these anomalies is reversed in the cold events. These relationships are particularly evident in the TOGA experiment.
Composite charts of the simulated flow patterns at 850 and 200 mb indicate that the above-mentioned precipitation changes are associated with well-defined circulation features over the affected monsoon regions. Dry conditions are typically coincident with low-level anticyclonic anomalies, and vice versa. These circulation centers are situated to the northwest and southwest of a prominent precipitation anomaly situated near 120°150°E at the equator, which corresponds to the western half of a dipolar heating pattern resulting from eastwest displacements of the ascending branch of the Walker circulation during ENSO. The large-scale anomalous circulation over the monsoon regions is similar to that of a Rossby wave pattern associated with a condensational heat source or sink in the western equatorial Pacific.
Diagnosis of the output from the TOGA-ML experiment reveals that variations in the circulation and cloud cover accompanying ENSO-induced monsoon anomalies could modulate the latent heat and shortwave radiative fluxes at the airsea interface in the Indian Ocean, thereby changing the SST conditions in that basin. These simulated SST anomalies compare well with observational results. The local atmospheric response to these SST anomalies opposes the remote response of the south Asian monsoon flow to SST anomalies in the tropical Pacific, thus leading to a negative feedback loop in the airsea coupled system.
Klein, Stephen A., Brian J Soden, and Ngar-Cheung Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. Journal of Climate, 12(4), 917-932. Abstract PDF
In an El Niño event, positive SST anomalies usually appear in remote ocean basins such as the South China Sea, the Indian Ocean, and the tropical North Atlantic approximately 3 to 6 months after SST anomalies peak in the tropical Pacific. Ship data from 1952 to 1992 and satellite data from the 1980s both demonstrate that changes in atmospheric circulation accompanying El Niño induce changes in cloud cover and evaporation which, in turn, increase the net heat flux entering these remote oceans. It is postulated that this increased heat flux is responsible for the surface warming of these oceans. Specifically, over the eastern Indian Ocean and South China Sea, enhanced subsidence during El Niño reduces cloud cover and increases the solar radiation absorbed by the ocean, thereby leading to enhanced SSTs. In the tropical North Atlantic, a weakening of the trade winds during El Niño reduces surface evaporation and increases SSTs. These relationships fit the concept of an "atmospheric bridge" that connects SST anomalies in the central equatorial Pacific to those in remote tropical oceans.
Lau, Ngar-Cheung, and Mary Jo Nath, 1999: Observed and GCM-simulated westward-propagating, planetary-scale fluctuations with approximately three-week periods. Monthly Weather Review, 127(10), 2324-2345. Abstract PDF
The structural characteristics and vorticity dynamics of westward-traveling patterns (WTP) in the troposphere are examined using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalyses based on observations for the 1973-95 period, as well as the output from a 100-yr integration of a general circulation model (GCM) with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels. An identical set of diagnostic tools, including progressive/retrogressive variance analysis, cross-spectra, and complex empirical orthogonal functions (EOFs), are applied to the reanalysis and GCM datasets for 300-mb height. These diagnoses all indicate that the WTP are most prominent during the cold season in the high-latitude zone extending westward from northwestern Canada to northeastern Siberia, with a typical period of ~22 days. Outstanding episodes are identified on the basis of the temporal coefficients of the leading complex EOF. Composite charts of the anomalous 300-mb height, sea level pressure, and 850-mb temperature fields at various phases of these events are constructed. The typical circulation changes accompanyi ng the passage of the WTP are similar to those associated with well-known regional weather phenomena such as amplified pressure ridges over Alaska, cold air outbreaks over western North America and east Asia, and heavy snowfall over the Pacific Northwest. The occurrence of the WTP over the North Pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity.
The contributions of relative vorticity advection, planetary vorticity advection (the "ß effect"), and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300, 500, and 850 mb are investigated. In the mid- and upper troposphere, the vorticity dynamics of the WTP is similar to that of free external Rossby waves, with the ß effect (which leads to westward propagation) being the dominant term, whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the Alaska-Siberia sector.
Most of the essential characteristics of the observed WTP deduced from the NCEP-NCAR reanalyses are well reproduced by the GCM. The realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the GCM as a tool for studying the impact of WTP on intraseasonal atmospheric variability in extended model experiments, and for assessing the dependence of the locality and activity level of the WTP on various states of the ambient circulation.
Surface air temperatures from a 1000-yr integration of a coupled atmosphere-ocean model with constant forcing are analyzed by using a method that decomposes temperature variations into a component associated with a characteristic spatial structure and a residual. The structure function obtained from the coupled model output is almost identical to the so-called cold ocean-warm land (COWL) pattern based on observations, in which above-average spatial mean temperature is associated with anomalously cold oceans and anomalously warm land. This pattern features maxima over the high-latitude interiors of Eurasia and North America. The temperature fluctuations at the two continental centers exhibit almost no temporal correlation with each other. The temperature variations at the individual centers are related to telecommunication patterns in sea level pressure and 500-mb height that are similar to those identified in previous observational and modeling studies. As in observations, variations in the polarity and amplitude of this structure function are an important source of spatially averaged surface air temperature variability.
Results from parallel integrations of models with more simplified treatments of the ocean confirm that the contrast in thermal inertia between land and ocean is the primary factor for the existence of the COWL pattern, whereas dynamical air-sea interactions do not play a significant role. The internally generated variability in structure function amplitude in the coupled model integration is used to assess the importance of the upward trend in the amplitude of the observed structure function over the last 25 years. This trend, which has contributed to the accelerated warming of Northern Hemisphere temperature over recent decades, is unusually large compared with the trends generated internally by the coupled model. If the coupled model adequately estimates the internal variability of the real climate system, this would imply that the recent upturn in the observed structure function may not be purely a manifestation of unforced variability. A similar monotonic trend occurs when the same methodology is applied to a model integration with time-varying radiative forcing based on past and future CO2 and sulfate aerosol increases. This finding illustrates that this decomposition methodology yields ambiguous results when two distinct spatial patterns, the "natural" COWL pattern (i.e., that associated with internally generated variability) and the anthropogenic fingerprint, are present in the simulated climate record.
Trenberth, K E., G Brantstator, D J Karoly, Arun Kumar, Ngar-Cheung Lau, and C F Ropelewski, 1998: Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures. Journal of Geophysical Research, 103(C7), 14,291-14,324. Abstract PDF
The primary focus of this review is tropical-extratropical interactions and especially the issues involved in determining the response of the extratropical atmosphere to tropical forcing associated with sea surface temperature (SST) anomalies. The review encompasses observations, empirical studies, theory and modeling of the extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding. In the tropical atmosphere, anomalous SSTs force anomalies in convection and large-scale overturning with subsidence in the descending branch of the local Hadley circulation. The resulting strong upper tropospheric divergence in the subtropics act as a Rossby wave source. The climatological stationary planetary waves and associated jet streams, especially in the northern hemisphere, can make the total Rossby wave sources somewhat insensitive to the position of the tropical heating that induces them and thus can create preferred teleconnection response patterns, such as the Pacific-North American (PNA) pattern. However, a number of factors influence the dispersion and propagation of Rossby waves through the atmosphere, including zonal asymmetries in the climatological state, transients, and baroclinic and nonlinear effects. Internal midlatitude sources can amplify perturbations. Observations, modeling, and theory have clearly shown how storm tracks change in response to changes in quasi-stationary waves and how these changes generally feedback to maintain or strengthen the dominant perturbations through vorticity and momentum transports. The response of the extratropical atmosphere naturally induces changes in the underlying surface, so that there are changes in extratropical SSTs and changes in land surface hydrology and moisture availability that can feedback and influence the total response. Land surface processes are believed to be especially important in spring and summer. Anomalous SSTs and tropical forcing have tended to be strongest in the northern winter, and teleconnections in the southern hemisphere are weaker and more variable and thus more inclined to be masked by natural variability. Occasional strong forcing in seasons other than winter can produce strong and identifiable signals in the northern hemisphere and, because the noise of natural variability is less, the signal-to-noise ratio can be large. The relative importance of tropical versus extratropical SST forcings has been established through numerical experiments with atmospheric general circulation models (AGCMs). Predictability of anomalous circulation and associated surface temperature and precipitation in the extratropics is somewhat limited by the difficulty of finding a modest signal embedded in the high level of noise from natural variability in the extratropics, and the complexity and variety of the possible feedbacks. Accordingly, ensembles of AGCM runs and time averaging are needed to identify signals and make predictions. Strong anomalous tropical forcing provides opportunities for skillful forecasts, and the accuracy and usefulness of forecasts is expected to improve as the ability to forecast the anomalous SSTs improves, as models improve, and as the information available from the mean and the spread of ensemble forecasts is better utilized.
Lau, Ngar-Cheung, 1997: Interactions between global SST anomalies and the midlatitude atmospheric circulation. Bulletin of the American Meteorological Society, 78(1), 21-33. Abstract PDF
A review is given of the processes contributing to variability of the atmosphere-ocean system on interannual timescales. Particular emphasis is placed on the relationships between midlatitude atmospheric fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Various hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946-88 in different parts of the world's oceans. It is demonstrated that tropical Pacific SST fluctuations associated with El Niño-Southern Oscillation (ENSO) episodes produce a strong extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is less robust. Analysis of a 100-yr control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial fraction of the simulated atmospheric variability may be attributed to internal dynamical processes alone.
The observed coexistence of tropical ENSO events with SST anomalies in the extratropical North Pacific is successfully reproduced by forcing the model atmosphere with tropical Pacific SST variations and allowing the atmospheric perturbations thus generated to drive a simple ocean mixed layer model inserted at ocean grid points outside the tropical Pacific. This simulation affirms the role of the atmospheric circulation as a "bridge" linking SST changes in different parts of the world's oceans. The midlatitude model responses in the presence of local air-sea interactions are noticeably stronger than the corresponding responses without such interactions. This finding is indicative of the positive feedback processes inherent in extratropical air-sea coupling.
Lau, Ngar-Cheung, and M W Crane, 1997: Comparing satellite and surface observations of cloud patterns in synoptic-scale circulation systems. Monthly Weather Review, 125(12), 3172-3189. Abstract PDF
The propagation characteristics and spatial distributions of cloud cover associated with synoptic-scale circulation systems are studied using both satellite data processed by the International Satellite Cloud Climatology Project (ISCCP) and surface observations taken at weather stations or ships of opportunity. These two independent datasets produce comparable climatological patterns of the preferred direction and speed of the cloud movements. Such "propagation vectors" based on the ISCCP products depict cloud motions at a higher altitude than that of the corresponding movements deduced from surface synoptic reports.
By application of a similar composite procedure to the ISCCP and surface datasets, a detailed comparison is made between the satellite and surface observations of the organization of various cloud types in midlatitude cyclones. The relationships of these cloud patterns with the concurrent atmospheric circulation are illustrated by superposing the composites of wind and geopotential height on the cloud data. Both the satellite and surface observations yield well-defined cloud patterns organized about frontal zones. These cloud patterns are consistent with the composite distributions of weather types as coded in the surface reports. Combination of findings from the surface reports (which provide information on the altitude of the cloud base) and those from the satellite observations (which offer estimates of cloud-top heights) is found to be helpful for protraying the vertical distribution of cloud cover in various sectors of the extratropical cyclones. The ISCCP dataset underestimates the amount of low stratiform clouds lying under the high, optically thick cloud shield in the warm sector. Comparison between the composite results for cyclones occurring over the North Atlantic and those over the North American continent indicates the occurrence of larger anomalous cloud amounts in the continental systems. The more widespread cover by nimbostratus cloud in the northwestern sector of the surface low pressure center over land is particularly noteworthy. The distinction between the cloud organization in maritime and continental cyclones is seen to be related to the stronger, frictionally induced cross-isobaric flow over land surfaces.
Analogous composite charts are constructed for the convective disturbances occurring over the tropical western Pacific by using both satellite and surface cloud observations. Some degree of consistency is again discernible among the cloud patterns based on the two datasets. The ISCCP data tend to underestimate the amount of cumulus clouds situated below the anvils at the trailing end of these convective systems.
Peng, P, and Ngar-Cheung Lau, 1997: The impact of the monthly-varying global SST anomalies on the Northern Hemisphere circulation as seen in an ensemble of NCEP-GCM simulations In Proceedings of the Twenty-First Annual Climate Diagnostics and Prediction Workshop, Springfield, VA, NTIS, 102-105.
Lau, Ngar-Cheung, 1996: Variability of the midlatitude atmospheric circulation in relation to tropical and extratropical sea surface temperature anomalies 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, 549-572. Abstract
A review is given of the processes contributing to atmospheric and oceanic variability on inter-annual time scales. Particular emphasis is placed on the observations and theories of the origin of midlatitude atmospheric fluctuations, as well as relationships between such fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Several hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946 - 1988 in different maritime regions. It is demonstrated that tropical Pacific SST fluctuations associated with El Niño-Southern Oscillation (ENSO) episodes produce the strongest extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is considerably weaker. Analysis of an 100-year control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial portion of the simulated atmospheric variability may be attributed to internal dynamical processes alone.
The observed coexistence of tropical ENSO events with SST anomalies in the extratropical North Pacific is successfully reproduced by forcing the model atmosphere with tropical Pacific SST variations, and allowing the atmospheric perturbations thus generated to drive a simple ocean mixed layer model inserted at ocean grid points outside the tropical Pacific. This simulation affirms the role of the atmospheric circulation as a 'bridge' linking SST changes in different parts of the world's oceans. The midlatitude model responses in the presence of local air-sea interactions are noticeably stronger than the corresponding responses without such interactions. This finding implies that positive feedback processes are inherent in extratropical air-sea coupling.
Lau, Ngar-Cheung, and M W Crane, 1996: A satellite view of the synoptic-scale organization of cloud properties in midlatitude and tropical circulation systems In International Workshop on Research Uses of ISCCP Datasets, World Climate Research Programme, WCRP-97, WMO/TD No. 790, World Meteorological Organization, 11.20.
Lau, Ngar-Cheung, and Mary Jo Nath, 1996: The Role of the "Atmospheric Bridge" in Linking Tropical Pacific ENSO Events to Extratropical SST Anomalies. Journal of Climate, 9(9), 2036-2057. Abstract PDF
The role of the atmospheric circulation as a "bridge" between sea surface temperature (SST) anomalies in the tropical Pacific and those in the midlatitude northern oceans is assessed. The key processes associated with this atmospheric bridge are described using output from four independent simulations with a general circulation model subjected to month to month SST variations observed in the tropical Pacific during the 1946-1988 period and to climatological SST conditions elsewhere (the "TOGA" runs). In episodes with prominent SST anomalies in the tropical Pacific, extratropical perturbations in the simulated atmospheric temperature, humidity, and wind fields induce changes in the latent and sensible heat fluxes across the air-sea interface of the midlatitude oceans. These anomalous fluxes in turn lead to extratropical SST changes.
The relevance of the atmospheric bridge mechanism is evaluated by driving a motionless, 50-m deep oceanic mixed layer model at individual grid points with the local surface fluxes generated in the TOGA runs. The negative feedback of the mixed layer temperature anomalies on the imposed flux forcing is taken into account by introducing a linear damping term with a 5-month dissipative time scale. This simple system reproduces the basic spatial and temporal characteristics of the observed SST variability in the North Pacific and western North Atlantic.
The two-way air-sea feedbacks associated with the atmospheric bridge are investigated by performing four additional 43-year runs of a modified version of the TOGA Experiment. These new "TOGA-ML" runs predict the ocean temperature outside the tropical Pacific by allowing the atmosphere to interact fully with the same mixed layer model mentioned above. The results support the notion that midlatitude ocean-atmosphere interaction can be modeled as a first-order Markov process, in which the red-noise response of mixed layer temperature is driven by white-noise atmospheric forcing in the presence of linear damping.
The amplitude of near-surface atmospheric anomalies appearing in the TOGA-ML runs is higher than that in the TOGA runs. This finding implies that, in the TOGA-ML scenario, the midlatitude oceanic responses to atmospheric driving could exert positive feedbacks on the atmosphere, thereby reinforcing the air-sea coupling. The enhanced atmosphere-ocean interactions operating in TOGA-ML prolong the duration of persistent meteorological episodes in that experiment. A comprehensive survey is conducted of the persistence characteristics simulated in TOGA, TOGA-ML, and several other experiments subjected to prescribed SST forcing at various sites. Model scenarios in which observed tropical Pacific SST anomalies act in conjunction with SST perturbations in midlatitudes (either prescribed or predicted) are seen to produce the highest frequency of persistent events.
Peng, P, and Ngar-Cheung Lau, 1996: Zonal mean flow/eddy relationship in a perpetual January GCM experiment In Proceedings of the 20th Annual Climate Diagnostics Workshop, U.S. Dept. of Commerce/NOAA/NWS, 216-219.
Although the distribution of sunshine is symmetrical about the equator, the earth's climate is not. Climatic asymmetries are prominent in the eastern tropical Pacific and Atlantic Oceans where the regions of maximum sea surface temperature, convective cloud cover, and rainfall are north of the equator. This is the result of two sets of factors: interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located. The ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to shoal in the eastern equatorial Pacific and Atlantic, but not in the eastern Indian Ocean, because easterly trade winds prevail over the tropical Atlantic and Pacific whereas monsoons, with a far larger meridional component, are dominant over the Indian Ocean. That is how the global distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands of longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the slope of the western coast of the Americas relative to meridians. Low-level stratus clouds over cold waters are crucial to the maintenance of the asymmetries.
Lau, Ngar-Cheung, and M W Crane, 1995: A satellite view of the synoptic-scale organization of cloud properties in midlatitude and tropical circulation systems. Monthly Weather Review, 123(7), 1984-2006. Abstract PDF
The spatial and temporal variability of various cloud types and cloud optical thickness are investigated using daily global analyses produced by the International Satellite Cloud Climatology Project (ISCCP) for the 1983-90 period. The climatological patterns of the relative abundance of individual cloud types are closely related to prevalent circulation regimes. Composite charts at various time lags relative to selected cloudy episodes are used to describe the representative shape and propagation of the local cloud patterns. These satellite-based findings are consistent with published results obtained from analyses of geopotential height and wind data.
The midlatitude baroclinic cyclones along the wintertime storm tracks over the North and South Atlantic, and the summertime synoptic-scale disturbances occurring over the tropical western Pacific, are selected for more in-depth investigation. The variations of different cloud types in the selected sites are examined in conjunction with the concurrent three-dimensional atmospheric structure and dynamical processes, as deduced from the daily operational analyses generated at the European Centre for Medium-Range Weather Forecasts (ECMWF). Results for the extratropical storm track regions are in agreement with traditional conceptual frameworks for the organization of cloud properties near warm and cold frontal zones. These midlatitude cloud patterns are linked to the vertical circulation induced by the advection of temperature and vorticity in developing baroclinic waves. Results for the tropical western Pacific reveal cloud structures similar to those occurring in squall lines, with low-top clouds in advance of the approaching deep convection zone, and high-top, optically thinner cloud elements in the rear anvil region.
The spatial correspondence between composite features obtained from the independent ISCCP and ECMWF analyses lends credence to the reliability of both datasets, especially in regions with sparse in situ observations. The consistency of the ISCCP cloud patterns with the ambient atmospheric structure and dynamics demonstrates the applicability of satellite data products in advancing our understanding of different types of circulation systems.
Lau, Ngar-Cheung, and Mary Jo Nath, 1995: A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system In Proceedings of the International Scientific Conference on Tropical Ocean Global Atmosphere [TOGA] Programme, WCRP-91, WMO/TD No. 717, Geneva, Switzerland, World Meteorological Organization, 578-582.
Lau, Ngar-Cheung, and Mary Jo Nath, 1994: A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system. Journal of Climate, 7(8), 1184-1207. Abstract PDF
In three parallel experiments, an atmospheric general circulation model has been subjected to observed, monthly varying sea surface temperature (SST) conditions in each of the following domains: near-global ocean (GOGA run), tropical Pacific (TOGA run), and midlatitude North Pacific (MOGA run). Four independent realizations were obtained for the model response to the sequence of SST anomalies during the 1946-88 period in each of the above regions.
The principal modes of coupling between the imposed SST forcing and the simulated Northern Hemisphere wintertime 515-mb height field in various experiments have been identified using a singular value decomposition (SVD) procedure. The leading SVD mode for the GOGA experiment is qualitatively similar to that based on observational data, although the amplitudes of the simulated height anomalies are notably lower than the observed values. The SST pattern of this mode resembles that associated with El Niño events. The accompanying 515-mb height anomaly is dominated by a wavelike pattern in the North Pacific/North American sector. TOGA experiment reproduces many of the atmosphere-ocean relationships discerned from the GOGA output. Conversely, the MOGA run yields a much weaker and less reproducible response. The contrast between the TOGA and MOGA runs is indicative of the primacy of tropical Pacific SST anomalies in forcing the midlatitude atmospheric circulation.
In the TOGA experiment, the remote atmospheric response to tropical Pacific SST anomalies influence the energy exchange across the local air-sea interface, and could thereby perturb the SST field outside of the tropical Pacific. Through this "atmospheric bridge", the tropical Pacific could set the pace for variability of the global ocean. Analysis of the TOGA output indicates that, over the North Pacific, changes in the surface energy fluxes are mainly determined by the surface wind speed and by the strength of temperature and moisture advection. Over the Indian Ocean, variations in the incident solar radiation due to changes in cloud cover also affect the surface fluxes. The worldwide SST tendencies inferred from the variations in surface fluxes simulated in the TOGA experiment are in good agreement with the local observed SST anomalies.
Ever since its completely unexpected discovery in 1985, the spectacular loss of atmospheric ozone near the South Pole has become an urgent issue confronting atmospheric scientists, policy makers, and the world community at large. This phenomenon offers a striking illustration of the fragility of our environment, and raises serious concerns about the potential impact of human activities on the atmosphere. The objective of this article is to present in simple terms an account of our current scientific understanding of ozone depletion, and the international actions being taken to tackle this problem.
Ting, Mingfang, and Ngar-Cheung Lau, 1993: A diagnostic and modeling study of the monthly mean wintertime anomalies appearing in a 100-year GCM experiment. Journal of the Atmospheric Sciences, 50(17), 2845-2867. Abstract PDF
The nature of simulated atmospheric variability on monthly time scales has been investigated by analyzing the output from a 100-year integration of a spectral GCM with rhomboidal wavenumber 15 truncation. In this experiment, the seasonally varying, climatological sea surface temperature was prescribed throughout the world oceans. The principal modes of variability in the model experiment were identified by applying a rotated empirical orthogonal function (EOF) analysis to the Northern Hemisphere monthly averaged 515-mb geopotential height for the winter season (November through March). The individual leading spatial modes are similar to the observed north-south dipoles over the North Atlantic and North Pacific, as well as wavelike patterns in the Pacific/North American and Northern Asian sectors.
Quasigeostrophic geopotential tendencies forced by synoptic-scale (2.5-6 day) eddy vorticity and heat fluxes were computed for those months when the individual EOF modes are particularly active. The composite patterns of the eddy-induced tendencies were compared with the corresponding monthly mean anomaly patterns. It is seen that the forcing due to eddy vorticity transports exhibits a distinctive barotropic character, and reinforces the monthly averaged geopotential height anomalies throughout the tropospheric column. On the other hand, the eddy heat fluxes lead to dissipation of the monthly mean height anomalies in the upper troposphere, and enhancement of the height anomalies in the lower troposphere. Hence, the eddy heat fluxes exert a strong impact on the baroclinic component of the circulation by destroying the concurrent local monthly mean temperature and geopotential thickness anomalies. The above relationships based on model data are in agreement with the corresponding observational results.
A linear stationary wave model was then used to mimic the individual EOF modes appearing in the GCM experiment, and to diagnose the relative importance of different types of forcing in the generation of such modes. As suggested by the tendency calculations, the transient eddy forcing due to heat fluxes was parameterized as a thermal diffusion mechanism in the stationary wave model. When the model was linearized about the climatological zonally averaged basic state, it failed to reproduce the EOF patterns appearing in the GCM experiment. However, when the same model was linearized about the zonally varying GCM climatology, the response to the total forcing (which includes vorticity fluxes by eddies on submonthly time scales, diabatic heating, and nonlinearity in those months when the individual EOF modes are active) bears a considerable resemblance to the corresponding anomaly patterns in the GCM. By evaluating the individual contributions of each of the three forcing mechanisms to the total linear model solution, it is concluded that the transient eddy vorticity fluxes exert the strongest influences. The response to nonlinear effects is negligible, while the forcing due to diabatic heating is weak and acts in opposition to the anomaly patterns in the upper troposphere. The forcing associated with vorticity fluxes by synoptic-scale transient eddies accounts for approximately half of the total vorticity forcing due to all submonthly fluctuations.
Both the tendency calculations and the stationary wave model results indicate the crucial role of vorticity transports by transient eddies. The linear model solutions also illustrate the importance of incorporating the climatological stationary waves in the basic state. These findings hence suggest that the monthly mean anomalies in this GCM experiment are intimately linked to barotropic interactions between transient fluctuations of different time scales, and between the monthly mean anomalies and the climatological stationary waves.
Wu, Guoxiong, and Ngar-Cheung Lau, 1993: Reply. Monthly Weather Review, 121(7), 2144-2146. PDF
Kushnir, Y, and Ngar-Cheung Lau, 1992: The general circulation model response to a North Pacific SST anomaly: Dependence on time scale and pattern polarity. Journal of Climate, 5(4), 271-283. Abstract PDF
A general circulation model was integrated with perpetual January conditions and prescribed sea surface temperature (SST) anomalies in the North Pacific. A characteristic pattern with a warm region centered northeast of Hawaii and a cold region along the western seaboard of North America was alternately added to and subtracted from the climatological SST field. Long 1350-day runs, as well as short 180-day runs, each starting from different initial conditions, were performed. The results were compared to a control integration with climatological SSTs.
The model's quasi-stationary response does not exhibit a simple linear relationship with the polarity of the prescribed SST anomaly. In the short runs with a negative SST anomaly over the central ocean, a large negative height anomaly, with an equivalent barotropic vertical structure, occurs over the Gulf of Alaska. For the same SST forcing, the long run yields a different response pattern in which an anomalous high prevails over northern Canada and the Alaskan Peninsula. A significant reduction in the northward heat flux associated with baroclinic eddies and a concomitant reduction in convective heating occur along the model's Pacific storm track. In the runs with a positive SST anomaly over the central ocean, the average height response during the first 90-day period of the short runs is too weak to be significant. In the subsequent 90-day period and in the long run an equivalent barotropic low occurs downstream from the warm SST anomaly. All positive anomaly runs exhibit little change in baroclinic eddy activity or in the patterns of latent heat release. Horizontal momentum transports by baroclinic eddies appear to help sustain the quasi-stationary repsonse in the height field regardless of the polarity of the SST anomaly. These results emphasize the important role played by baroclinic eddies in determining the quasi-stationary response to midlatitude SST anomalies. Differences between the response patterns of the short and long integrations may be relevant to future experimental design for studying air-sea interactions in the extratropics.
Lau, A K., and Ngar-Cheung Lau, 1992: The energetics and propagation dynamics of tropical summertime synoptic-scale disturbances. Monthly Weather Review, 120(11), 2523-2539. Abstract PDF
Periods of enhanced synoptic activity in the tropical western Pacific, Bay of Bengal-northeastern India, and African-Atlantic regions are identified by extended empirical orthogonal function analysis. Composite meteorological fields for such active periods at various sites are constructed using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses for the northern summers of 1980-1987. These composite data form the basis for evaluating the contributions of different dynamical processes to local balances of heat, moisture, vorticity, enstrophy, and energy, so that the propagation dynamics and principal energy sources of the tropical disturbances may be studied in detail.
In all three tropical regions considered here, the westward propagation of the synoptic-scale disturbances is attributed mostly to vorticity advection by both the time-mean flow and the transient fluctuations.
In the western Pacific and Indian sectors, condensation heating associated with cumulus convection is seen to be the most significant energy source for the tropical disturbances. The stretching effect associated with large-scale convective activity is the most important mechanism for the generation of eddy enstrophy in these maritime disturbances. There is substantial barotropic conversion of enstrophy and kinetic energy from the time-mean flow to the transient fluctuations.
In the African-Atlantic sector, the disturbances along the more prominent northern track at approximately 20°N are accompanied by dry desert-type convection. Vortex stretching associated with the dry convection is still the most important process for the generation of eddy enstrophy in these disturbances. However, the main source of available potential energy for these North African disturbances is the baroclinic conversion from the time-mean flow to the transient fluctuations along the zone of strong temperature gradients south of the Sahara. The dynamics and energetics of the weaker southern disturbances along 10°N in the African-Atlantic sector are similar to the moist disturbances found in the western Pacific and Indian sectors.
Lau, Ngar-Cheung, 1992: Climate variability simulated in GCMs In Climate System Modeling, Cambridge, UK, Cambridge University Press, 617-642.
Lau, Ngar-Cheung, 1992: Review of book, "Teleconnections linking worldwide climate anomalies. Scientific basis and societal impact". Science, 256(5057), 682.
Lau, Ngar-Cheung, and K M Lau, 1992: Simulation of the Asian summer monsoon in a 40-year experiment with a general circulation model In Proceedings of the 2nd International Conference on East Asia and Western Pacific Meteorology and Climate, W. J. Kyle, and C. P. Chang, eds., World Scientific Publishing Co, 49-56.
A 140-year simulation of the ocean-atmosphere climate system has been performed by the GFDL Climate Dynamics Project using a low-resolution coupled general circulation model (GCM). The model was subjected to annually averaged insolation throughout the integration. This coupled system exhibits well-defined fluctuations in the tropical Pacific, with a preferred time scale of 3-4 years. The characteristics of these recurrent anomalies were examined by applying an extended empirical orthogonal function (EEOF) analysis to selected model variables. These results indicate that the simulated oscillations are accompanied by coherent changes in the atmospheric and oceanic circulation.
The spatial patterns associated with the leading EEOF mode indicate that SST anomalies make their first appearance off the Peru-Ecuador coast and then migrate steadily westward, with an average transit time of 12-15 months. The arrival and eventual decay of SST fluctuations in the western Pacific is typically followed by the initiation of anomalies of the opposite polarity along the American coasts. The space-time evolution of various meteorological and oceanographic signals exhibits well-defined phase relationships with the SST perturbations. Some aspects of the model behavior during these warm and cold episodes are reminiscent of observed phenomena associated with the El Niño-Southern Oscillation (ENSO).
Analysis of the climatological heat budget for the top ocean layer indicates a near balance between horizontal and vertical temperature advection by the time-mean flow, vertical diffusion, and heat input from the overlying atmosphere. Contributions of transient effects to this balance are negligible. The principal mechanisms associated with the simulated ENSO-like cycles were then studied by examining the local heat budget for the SST perturbations. It is shown that the relative importance of various linear advective processes in the heat budget exhibits a notable dependence on geographical location and on the specific phase of the ENSO-like cycle.
Neelin, J D., Ngar-Cheung Lau, and S G H Philander, et al., 1992: Tropical air-sea interaction in general circulation models. Climate Dynamics, 7, 73-104. Abstract
An intercomparison is undertaken of the tropical behavior of 17 coupled ocean-atmosphere models in which at least one component may be termed a general circulation model (GCM). The aim is to provide a taxonomy - a description and rough classification - of behavior across the ensemble of models, focusing on interannual variability. The temporal behavior of the sea surface temperature (SST) field along the equator is presented for each model, SST being chosen as the primary variable for intercomparison due to its crucial role in mediating the coupling and because it is a sensitive indicator of climate drift. A wide variety of possible types of behavior are noted among the models. Models with substantial interannual tropical variability may be roughly classified into cases with propagating SST anomalies and cases in which the SST anomalies develop in place. A number of the models also exhibit significant drift with respect to SST climatology. However, there is not a clear relationship between climate drift and the presence or absence of interannual oscillations. In several cases, the mode of climate drift within the tropical Pacific appears to involve coupled feedback mechanisms similar to those responsible for El Niño variability. Implications for coupled-model development and for climate prediction on seasonal to interannual time scales are discussed. Overall, the results indicate considerable sensitivity of the tropical coupled ocean-atmosphere system and suggest that the simulation of the warm-pool/cold-tongue configuration in the equatorial Pacific represents a challenging test for climate model parameterizations.
A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10°N to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west have a very different shape-an approximate Gaussian shape centered on the equator.
The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s-1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This "delayed oscillator" mechanism that sustains interannual oscillations in the model differs in its details from that previously discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.
Wu, Guoxiong, and Ngar-Cheung Lau, 1992: A GCM simulation of the relationship between tropical-storm formation and ENSO. Monthly Weather Review, 120(6), 958-977. Abstract PDF
A low-resolution Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model has been integrated for 15 years. In the course of this experiment, the observed month-to-month sea surface temperature (SST) variations in the tropical Pacific Ocean were incorporated in the lower boundary condition. The output from this model run was used to investigate the influence of El Niño-Southern Oscillation (ENSO) events on the variability of tropical-storm formation.
Criteria for detecting tropical cyclogenesis and tropical-storm formation were developed for the model. Tropical storms appearing in the model atmosphere exhibit many typhoonlike characteristics: strong cyclonic vorticity and convergence in the lower troposphere, strong anticyclonic vorticity and divergence near the tropopause, and intense precipitation. It is demonstrated that, despite its coarse resolution, the model is capable of reproducing the observed geographical distribution and seasonal variation of tropical-storm formation.
The relationship between simulated tropical-storm formation and ENSO was explored using correlation statistics, composite fields for the warm and cold phases of ENSO, and individual case studies. Significant correlations were found between eastern equatorial Pacific SST anomalies and tropical-storm formation over the western North Pacific, western South Pacific, and western North Atlantic. In these areas, below-normal frequency of tropical-storm formation was simulated in warm El Niño years, whereas more tropical storms occurred in La Niña years. The correlation between tropical-storm formation and equatorial SST changes is particularly high for fluctuations on time scales of less than 3-4 years. During the boreal summer months (June-October), there exists a seesaw in the frequency of tropical-storm formation between western and central North Pacific: while more tropical storms were generated over western North Pacific during La Niña years, less tropical storms were detected over central North Pacific. The reverse situation prevails in El Niño years. Over the Indian Ocean, the relationship between storm formation and ENSO exhibits a seasonal dependence.
Kushnir, Y, and Ngar-Cheung Lau, 1991: The response of a GCM atmosphere to extratropical SST anomalies In Proceedings of the Fourteenth Annual Climate Diagnostics Workshop, Springfield, VA, NTIS, 289-294.
Lau, Ngar-Cheung, 1991: Variability of the baroclinic and barotropic transient eddy forcing associated with monthly changes in the midlatitude storm tracks In Fifth Conference on Climate Variations, Boston, MA, American Meteorological Society, 59-62.
Lau, Ngar-Cheung, and Mary Jo Nath, 1991: Variability of the baroclinic and barotropic transient eddy forcing associated with monthly changes in the midlatitude storm tracks. Journal of the Atmospheric Sciences, 48(24), 2589-2613. Abstract PDF
The heat and vorticity transports by synoptic-scale eddies at various levels between 1000 and 100 mb have been compiled for each winter month of the 1966-84 period using time-filtered daily analyses produced by the U.S. National Meteorological Center. These circulation statistics were used to compute the three-dimensional distributions of the quasigeostrophic geopotential tendency and vertical motion induced by baroclinic and barotropic eddy processes in individual months. The latter fields serve as the basis for describing the synoptic-scale eddy forcing associated with the leading modes of month-to-month variability of the storm tracks over the North Pacific and North Atlantic. These modes are associated with the meridional displacements of the storm-track axes from their climatological positions.
The placement of a storm track at a certain latitude phi in a certain month is accompanied by enhanced convergence of eddy heat fluxes poleward of phi. In the tropospheric column poleward of the storm track, this baroclinic eddy forcing leads to positive geopotential tendency near the tropopause and negative geopotential tendency near sea level, as well as strong positive temperature tendency and rising motion. In the same month, the convergence of eddy vorticity transport is also enhanced poleward of phi. This barotropic forcing results in negative geopotential tendency throughout the troposphere, as well as rising motion and weak negative temperature tendency poleward of phi. All of these features appear with reversed polarity in latitudes equatorward of phi.
In the upper troposphere, the geopotential tendency induced by vorticity fluxes is stronger than the opposing effects due to heat fluxes, so that the net eddy forcing retains most of the characteristics of the forcing associated with barotropic processes alone, but with considerably reduced amplitudes. Near sea level, the geopotential tendencies induced by heat and vorticity fluxes reinforce each other and are comparable in amplitude. Throughout the troposphere, the patterns of net geopotential tendency exhibit a positive spatial correlation with those of the concurrent monthly averaged height anomaly. The characteristic time scale associated with this constructive eddy forcing in the storm-track region ranges from several days at 1000 mb, to 1-2 months near the tropopause. On the other hand, the eddy-induced temperature tendency is negatively correlated with the local monthly mean temperature anomaly. The dissipative time scale for this thermal forcing in the storm-track region is ~10 days at 850 mb.
The barotropic geopotential tendency and the baroclinic tendency are essentially determined by the convergences of vorticity and heat fluxes, respectively. The eddy-induced secondary circulation plays a minor role in these tendencies.
Kushnir, Y, and Ngar-Cheung Lau, 1990: The response of a GCM atmosphere to extratropical SST anomalies In Proceedings of the Fourteenth Annual Climate Diagnostics Workshop, U.S. Dept. of Commerce/NOAA/NWS, 289-294.
Lau, Ngar-Cheung, 1990: GCM simulations of the influences of tropical and extratropical SST changes on the atmospheric circulation In Bureau of Meteorology Research Centre (BMRC) Research Report No. 21 - The Role of Sea Surface Temperatures in Numerical Modelling of the Atmosphere, Papers Presented At the First BMRC Modelling Workshop, J. D. Jasper, ed., Melbourne, Victoria; Austr, BMRC, 1-8.
Lau, A K., and Ngar-Cheung Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances In Internationational Conference on East Asia and Western Pacific Meteorology and Climate, Teaneck, NJ, World Scientific Publishing Co, 48-57.
Lau, A K., and Ngar-Cheung Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances. Monthly Weather Review, 118(9), 1888-1913. Abstract PDF
The three-dimensional structre and propagation characteristics of tropical synoptic scale transients during the northern summer are studied with twice daily ECMWF global gridded analyses for the 1980-1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations.
The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over several days. The disturbances in all active regions tend to travel west/northwestward. The eastern portion of each active site is characterized by rapid growth of the disturbances, whereas decay typically occurs in the western portion.
The transient behavior throughout the tropics is also investigated using Extended Empirical Orthogonal Function (EEOF) techniques. The sites of activity thus identified coincide with the location inferred from the lag-correlation analyses. Using time series of the EEOF coefficients as a reference, the temporal evolution as well as the horizontal and vertical structure of the disturbances occurring in each active region are delineated by composites of selected meteorological variables. Well-defined changes in vorticity, vertical velocity, temperature and humidity at various tropospheric levels, as well as convective activity (deduced from the outgoing longwave radiation field), are discernible in the disturbances at various sites. Phase relationships among different variables are interpreted in terms of dynamical and physical processes operating within disturbances. The horizontal phase tilt of the fluctuations and their positions relative to the ambient mean circulation suggest a tendency for kinetic energy transfer from the quasi-stationary flow to the transient eddies. Most of the findings reported here are in accord with previous investigations based on different analysis tools and more limited datasets.
Whereas considerable similarities are noted among disturbances occurring over various active maritime sites, the perturbations over central and western Africa exhibit structural characteristics that are unique to that region. Specifically, two propagation tracks are identified in the African sector. The northern track along southern Sahara consists mostly of eddies commonly found over arid zones, with ascent of warm and dry air over surface troughs. The southern track is collocated with the climatological rainfall maximum at about 10 degrees N, and is associated with moist convective systems.
Lau, Ngar-Cheung, and Mary Jo Nath, 1990: A general circulation model study of the atmospheric response to extratropical SST anomalies observed in 1950-79. Journal of Climate, 3(9), 965-989. Abstract PDF
The principal modes of variability of the seasonally averaged 515 mb height and SST fields have been identified using rotated principal component (RPC) analysis. The extrema of the first atmospheric mode reside over the North Atlantic and Eurasia, whereas the second mode is associated with height anomalies in the North Pacific/North American sector. Cross-correlation analysis reveals that these two atmospheric modes are linked to leading patterns of the SST field in the North Atlantic and North Pacific, respectively. It is also demonstrated that the extrema in leading RPC patterns of the SST field in the northern oceans are almost coincident with the sites of maximal covariability between the SST and 515 mb height fields.
Regression charts of selected model parameters versus the SST variations off the Newfoundland coast and northwest of Hawaii have been constructed. These two reference maritime sites have been identified by the RPC and cross-correlation analyses as being correlated with the strongest atmospheric signals. The model fields examined in this manner include the geopotential height at various pressure levels, precipitation, heat flux across the air-sea interface, as well as temporal variance and covariance statistics. These regression maps indicate that the atmospheric response to midlatitude SST anomalies has an equivalent barotropic structure. The presence of SST perturbations in the extratropics are associated with displacements of the storm track axes, and with relocations of the midlatitude rainbelts and preferred sites of heat transfer from the underlying ocean. The changes in the locality of synoptic scale eddy activity are accompanied by alterations in the transient eddy forcing of the quasi-stationary flow. The geopotential height tendencies associated with these anomalous eddy effects exhibit a positive spatial correlation with the seasonally averaged, downstream atmospheric response. The time scale for the eddy induced tendencies to produce such seasonal height anomalies is on the order of several days. These findings suggest that the transient disturbances act as an essential intermediary between the extratropical SST forcing and the time-mean atmospheric response.
The principal atmospheric anomaly pattern in the North Atlantic/Eurasian sector exhibits substantial correlations with SST fluctuations in the tropical South Atlantic; whereas oceanic anomalies in the equatorial Pacific are only weakly associated with atmospheric circulation changes in the North Pacific/North American region.
The temporal lead/lag relationships between the simulated atmospheric anomalies and the prescribed SST changes have been explored.
Two different coupled ocean-atmosphere models simulate irregular interannual fluctuations that in many respects resemble El Niño Southern Oscillation phenomena. For example, the spatial structure of various fields at the peaks of the warm El Niño and cold La Niña phases of the oscillation are realistic. This success indicates that the models capture certain aspects of the interactions between the ocean and atmosphere that cause the Southern Oscillation. The principal difference between the models, namely the prominence of oceanic Kelvin waves in one but not the other, causes the two models to differ significantly in the way El Niño episodes evolve, and in the mechanisms that cause a turnabout from El Niño to La Niña and vice versa. It is possible that the different processes that determine the properties of the simulated oscillations all play a role in reality, at different times and in different regions. Each of the models captures some aspects of what is possible. However, reality is far more complex than any model developed thus far and additional processes not yet included are also likely to have a significant influence on the observed Southern Oscillation.
Lau, Ngar-Cheung, 1988: Modeling of ENSO phenomena at GFDL In Meteorological Research Report, Japan-U.S. Workshop on the El Niño-Southern Oscillation Phenomenon, Japan: Div. of Meteorology, Geop, Univ.of Tokyo, 160-168.
Lau, Ngar-Cheung, 1988: Variability of the observed midlatitude storm tracks in relation to low-frequency changes in the circulation pattern. Journal of the Atmospheric Sciences, 45(19), 2718-2743. Abstract PDF
The principal modes of month-to-month variability of the wintertime storm tracks over the North Pacific and North Atlantic are identified by empirical orthogonal function analysis of the root-mean-square statistics of bandpass (2.5-6 day) filtered geopotential height data for 19 yrs. One of the two leading modes depicts fluctuations in the level of synoptic-scale activity without any noticeable spatial displacement of the storm track axes, whereas the other mode is associated with meridional shifts of the storm tracks from their time-averaged positions. Higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions.
It is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns. The dipolar western Pacific and western Atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans, whereas the more wave-like Pacific/North American and eastern Atlantic teleconnection patterns are coincident with north-south displacements of the storm track axes over the eastern oceans. The representative synoptic scenarios for various storm track modes are portrayed using composite charts. These patterns illustrate the strong modulation of the trajectory of weather systems by the intensity and steering action of the monthly averaged flow field, so that the storm tracks are preferentially located at and slightly downstream of the quasi-stationary troughs.
The shape and propagation of the synoptic scale eddies along the changing storm tracks, as well as the barotropic interactions between these disturbances and the monthly mean flow, are diagnosed using composite patterns of extended Eliassen-Palm vectors and eddy-induced geopotential tendencies at 300 mb. It is seen that the synoptic-scale fluctuations are typically crescent-shaped, and sometimes undergo noticeable deformation when they encounter quasi-stationary ridges. In the upper troposphere, enhanced eddy activity is accompanied locally by eastward acceleration, as well as by positive geopotential tendency immediately to the south, and negative geopotential tendency to the north, and vice versa. The distributions of eddy-induced geopotential tendency for individual storm track modes indicate a near inphase relationship between the synoptic scale barotropic forcing and the quasi-stationary flow pattern at 300 mb. The characteristic time scale for this forcing is approximately 7-10 days.
The characteristic circulations at sea level associated with various storm track modes are examined using composite charts of the sea level pressure field. Some of these composites resemble the patterns associated with the North Pacific and North Atlantic Oscillations.
Lau, Ngar-Cheung, Isaac M Held, and J David Neelin, 1988: The Madden-Julian Oscillation in an idealized general circulation model. Journal of the Atmospheric Sciences, 45(24), 3810-3832. Abstract PDF
The structure of the intraseasonal oscillations in the tropics of an idealized general circulation model with a zonally symmetric climate is described. Space-time spectra show a peak in zonal winds and velocity potential at the equator in zonal wavenumbers 1 and 2, corresponding to eastward-propagating power at phase speeds of ~ 18 m s-1. This speed is significantly greater than that of the observed oscillation but comparable to that obtained in similar models by Hayashi and Sumi and Swinbank et al. The corresponding eastward-propagating power in the precipitation spectrum is concentrated in wavenumbers 2-5. A composite procedure is used to describe the three-dimensional structure of the model's oscillation. The oscillation is characterized by circulation cells oriented along the equatorial zonal plane, with enhanced precipitation in the region of rising motion. Zonal wind changes tend to be positively correlated with geopotential height changes at the same level. Positive perturbations in the water vapor mixing ratio, evaporation, and lower tropospheric horizontal moisture convergence all exhibit distinct eastward displacements from the center of convection.
Two different linear models are used to interpret the GCM results. The response to the GCM's composited diabatic heating field is first computed using a linear primitive equation model on the sphere. This linear model requires strong damping above the heated region, as well as near the surface, to produce a pattern in rough agreement with the GCM. A simple Kelvin wave-CISK model, in which the vertical structure of the heating is taken from the composite, is then shown to be capable of reproducing the phase speed simulated in the GCM.
Philander, S G., and Ngar-Cheung Lau, 1988: Predictability of El Niño In Physically-based Modelling and Simulation of Climate and Climatic Change, Part II, Dordrecht, The Netherlands, Kluwer Academic Publishers, 967-982.
Lau, Ngar-Cheung, and Mary Jo Nath, 1987: Frequency dependence of the structure and temporal development of wintertime tropospheric fluctuations- Comparison of a GCM simulation with observations. Monthly Weather Review, 115(1), 251-271. Abstract PDF
The three-dimensional structure and temporal evolution of tropospheric fluctuations appearing on various time scales in observed and model-simulated atmospheres are investigated using cross-spectral analyses. The datasets examined include NMC analyses of the 500 mb height and sea level pressure fields for 18 winters, as well as a 12-winter simulation of the same fields by a 15-wavenumber general circulation model at GFDL. Statistically significant phase differences between 500 mb height fluctuations at selected centers of action and the corresponding fluctuations at all other grid points are displayed for various frequency bands using a vectorial format. Similar plots are constructed to elucidate the vertical phase structure in the middle and lower troposphere at individual grid points, as well as the propagation characteristics of the sea level pressure field in the vicinity of sloping terrain. It is demonstrated that these phase/coherence diagrams offer a useful alternative for quantifying the lead/lag relationships between different anomaly centers associated with some of the well-known teleconnection patterns.
The spectral results presented here indicate that the spatial and temporal behavior of the Pacific/North American, Atlantic and Northern Asian Patterns, as documented in various recent studies, exhibit a notable frequency dependence in both real and model atmospheres. For periods of 27-80 days, the atmospheric vairability over the Pacific and Atlantic Basins is organized in north-south oriented dipoles, with an almost 180 degree out-of-phase relationship between oscillations at the oppposite poles. As attention is shifted to the 20- and 10-day time scales, the north-south seesaw pattern gradually weakens, and all three teleconnection patterns mentioned above are characterized by successive downstream development from west to east of alternating troughs and ridges. Fluctuations with periods longer than 20 days acquire an equivalent barotropic structure over much of the northern oceans, with in-phase variations at 500 mb and at sea level.
The eddy behavior undergoes still further changes as one considers the 4-day period band. The high frequency disturbances tend to be elongated in the meridional direction. The corresponding horizontal phase variations are indicative of continuous eastward propagation across the midlatitude oceans and northern Siberia. The vertical phase variations suggest a systematic transition from a distinctly baroclinic structure at the starting points of such cyclone tracks, to a more barotropic structure in regions farther east.
The perturbations near the eastern and northern peripheries of the Tibetan Plateau are noted for their weak coherence in the vertical direction. Horizontal phase diagrams based on sea level pressure data reveal that the path of near-surface fluctuations tends to be aligned parallel to the local topographic contours in this region.
Kang, I-S, and Ngar-Cheung Lau, 1986: Principal modes of atmospheric variability in model atmospheres with and without anomalous sea surface temperature forcing in the tropical Pacific. Journal of the Atmospheric Sciences, 43(22), 2719-2735. Abstract PDF
Principal modes of low-frequency atmospheric variability and the influence of sea surface temperature anomalies on such modes are investigated by examining the output from two general circulation model experiments. In the first experiment (the "control run"), all boundary forcings were constrained to evolve through 15 identical annual cycles. In the second experiment (the "SST runs"), the sea surface temperature conditions in the tropical Pacific Basin were prescribed to follow the actual month-to-month changes observed during the period 1962-76, which encompasses several El Niño events. The analysis tools employed here include empirical orthogonal functions, teleconnections and composite charts.
Two prominent modes of variability have been identified in the wintertime Northern Hemisphere eddy streamfunction of the SST experiment. The first mode bears a strong spatial resemblance to the observed characteristic circulation pattern over the North Pacific-North American sector. It is demonstrated that this mode is highly correlated with the changing SST forcing imposed over the tropical Pacific, and with various meteorological phenomena accompanying El Niño-Southern Oscillation (ENSO). The second mode exhibits no significant correlation with the SST forcing, but is linked instead to a characteristic structure of the zonally averaged zonal wind. The circulation features related to this mode are reminiscent of observed anomalies accompanying the North Atlantic Oscillation, as well as variations of the "zonal index." The fluctuations associated with the first and second modes in the SST experiment are of comparable amplitudes. It is further demonstrated that the essential characteristics of the first and second modes are highly reproducible in another 15-year simulation initiated from a completely independent set of atmospheric conditions.
A parallel diagnosis of the model behavior in the control run reveals no signal of the first mode (i.e., that related to ENSO) as previously described. However, circulation features accompanying the second mode are still discernible in the control experiment. It is noted that the predominant anomalous phenomena in the control run are manifestations of the zonal/eddy relationship associated with the second mode. The mode appearing in both the control and SST runs is likely related to internal dynamical processes in the model atmosphere subjected to a fixed boundary forcing.
Lau, Ngar-Cheung, 1986: The influences of orography on large-scale atmospheric flow simulated by a general circulation model In Proceedings of the International Symposium on the Qinghai-Xizang Plateau and Mountain Meteorology, Beijing; Science Press, Boston, American Meteorological Society, 241-269.
Lau, Ngar-Cheung, and K M Lau, 1986: The structure and propagation of intraseasonal oscillations appearing in a GFDL general circulation model. Journal of the Atmospheric Sciences, 43(19), 2023-2047. Abstract PDF
The three-dimensional structure and temporal evolution of quasi-periodic, planetary-scale tropospheric oscillations simulated by a 15-wavenumber GCM are investigated by applying cross-spectral, eigenvector, composite and temporal correlation techniques to 12 years of model output. Evident from this diagnostic study is the presence in the model tropics of well-defined eastward traveling features with spatial scales of zonal wavenumbers 1 and 2, and with temporal scales of 25-40 and 15-20 days, respectively. The flow pattern associated with the oscillations of both spatial scales is characterized by circulation cells oriented along the equatorial zonal plane, with the zonal wind and geopotential height fluctuations near the sea level being negatively correlated with the corresponding fluctuations in the upper troposphere. The movement of these zonal circulation cells along the equatorial belt is accompanied by systematic migration of the global-scale horizontal divergence field, and by dipole-like precipitation structures within the Indonesian/Pacific sector. The preferred sites for such oscillatory behavior exhibit a notable seasonal dependence, with the most active zonal circulation cells being located in the summer hemisphere.
During the northern summer, the 25-40 day oscillations coincide with the occurrence of northward moving, zonally elongated rainbands over the monsoon region of South Asia. During the northern winter, the 25-40 day phenomena in the tropics are linked to well-organized extratropical wave trains spanning the Eurasian and Pacific/North American sectors.
The principal characteristics of the model-generated phenomena analyzed in this study are compared with corresponding results reported in the observational literature. Although the period of the simulated wavenumber-1 phenomena is somewhat shorter than the corresponding observed values, it is demonstrated that the spatial structure, propagation characteristics and seasonal dependence of the model features are consistent with observations. The model findings are also interpreted in terms of current theoretical understanding of tropical and extratropical motions.
Lau, Ngar-Cheung, 1985: Modeling the seasonal dependence of the atmospheric response to observed El Niños in 1962-76. Monthly Weather Review, 113(11), 1970-1996. Abstract PDF
Two 15-year atmospheric GCM integrations are conducted with the lower boundary over the tropical Pacific being forced by observed month-to-month sea surface temperature (SST) changes during the period 1962-76. A descriptive account is given on selected aspects of the 30-year model climatology, as well as the anomalous model behavior through the life cycles of El Niño-Southern Oscillation (ENSO) episodes centered in the years 1965, 1969 and 1972. These model results are compared with available observations reported in the published literature. Particular attention is devoted to the timing of various simulated meteorological phenomena with respect to the spatially and temporally evolving SST forcing, and to the climatological seasonal cycle.
An assessment is made of the capability of the model to simulate the seasonal dependence of various climatological features relevant to ENSO. The phenomena examined include the flow field and rainfall in different monsoon regions, the planetary scale waves in the extratropics, and the low-level convergence zones in the tropical Pacific Basin.
The evolutionary response of the model atmosphere in a typical ENSO event is examined using time series of selected circulation indices, composite charts and Hovmoller diagrams. As the warm SST anomaly appears in the eastern equatorial Pacific during the boreal spring and subsequently spreads across the ocean basin, a well-defined sequence of meteorological events is evident in the model atmosphere. The most notable atmospheric response over the tropical Pacific Basin includes weakening of the east-west surface pressure gradient and easterly trades, eastward displacement of the South Pacific Convergence Zone, southward displacement of the Intertropical Convergence Zone, above normal precipitation at and east of the date line, and below normal precipitation over the Indonesian Archipelago. The strongest anomalies are simulated in the northern winter following a warming off the Peruvian coast. The model response in this mature stage is characterized by tropospheric warming throughout the entire tropical zone, and by the appearance in the tropical upper troposphere of a pair of Pacific anticyclones straddling the equator. These anticyclonic centers appear as the starting points of well-organized wave trains spanning the midlatitude zones of both hemispheres. The Northern Hemisphere wave pattern in the Pacific-North American sector bears a strong resemblance to that reported in recent observational studies.
The warm Pacific SST anomaly tends to be replaced a year later by a cold anomaly. The polarities of meteorological anomalies simulated during the cold phase of the ENSO cycle are mostly opposite to those occurring during the warm phase.
Time series analysis of different circulation indices, as well as comparison between simulated amplitudes of atmospheric variability in this experiment and in a "control" experiment without any prescription of interannual SST variations, indicate that the impact of equatorial Pacific SST anomalies on the tropical circulation is much greater than that on the flow patterns in middle latitudes. In particular, the temporal variance of 200 mb height in this perturbed SST experiment is larger than the corresponding quantity in the control experiment by a factor of 2-6 over the tropics; whereas the same SST fluctuations are much less effective in enhancing the variability in middle and higher latitudes. Moreover, perturbations in the equatorial Pacific SST are more strongly correlated with circulation changes in the tropical atmosphere than with changes in the extratropics.
Lau, Ngar-Cheung, 1985: Publication of circulation statistics based on FGGE level III-b analyses produced by GFDL and ECMWF. Bulletin of the American Meteorological Society, 66(10), 1293-1301.
Lau, Ngar-Cheung, and Abraham H Oort, 1985: Response of a GFDL general circulation model to SST fluctuations observed in the tropical Pacific Ocean during the period 1962-1976 In Coupled Ocean-Atmosphere Models, Amsterdam; The Netherlands, Elsevier Science Publishers, 289-302. Abstract
The results of a special 15-year integration of an atmospheric general circulation model are compared with observations. In the 30 degrees S - 30 degrees N strip over the Pacific, the lower boundary of the model is forced by sea-surface temperatures which vary continuously according to actual observations during the period January 1962 - December, 1976. Everywhere else, the sea-surface temperatures follow a normal annual cycle without year-to-year variations. Using global teleconnection maps and time series of certain tropical circulation indices, such as the low-level and upper-level zonal winds, the surface pressure and the 200-mb height, it is shown that the dominant spatial modes of atmospheric variability in the tropics are very well simulated. Some of these modes are conspicuously absent from an earlier integration in which the surface temperatures everywhere were prescribed to follow the normal seasonal cycle.
Rosen, R D., D A Salstein, J P Peixoto, Abraham H Oort, and Ngar-Cheung Lau, 1985: Circulation statistics derived from level III-b and station-based analyses during FGGE. Monthly Weather Review, 113(1), 65-88. Abstract PDF
A number of Northern Hemisphere circulation fields and statistics are derived for the months of January and June 1979 from level III-b analyses produced by GFDL using a 4-dimensional data assimilation scheme which incorporates measurements from a wide variety of sources. In particular, hemispheric maps and zonal cross sections of the wind, specific humidity, and the eddy fluxes of momentum, heat and moisture are examined. Certain quantities related to the atmosphere's energy cycle are also considered. These fields and statistics are compared with those derived from analyses that rely solely on the conventional rawinsonde station data taken during the same months. In the case of the monthly mean zonal and meridional winds, we also present results based on the level III-b analyses of the ECMWF. The station-based analyses yield zonal mean statistics and hemispheric integrals that are generally comparable to those from the level III-b analyses. For example, the intensity of the Northern Hemisphere Hadley cell in January produced by the station analyses lies between those of the III-b analyses, which differ by as much as 35%. On regional scales, however, there are some large differences in the circulation fields between the station-based and level III-b analyses over areas of sparse station coverage. For example, the station-based analysis of the 200 mb field of transient eddy momentum flux in January does not include a significant region of northward flux over the northeast Pacific that is contained in the GFDL analysis. It is not yet clear, though, to what extent model biases may be affecting the GFDL analysis in this or in other station-sparse areas. In the case of the subtropical Pacific jet in January, the station-based analysis appears to underestimate its extent, but there are also considerable differences between the two level III-b analyses in this region. In addition, the GFDL analyses often appear to be noisy. Improvements in the level III-b analyses need to be made before full confidence can be placed in results based on modern data assimilation techniques.
Wallace, J M., and Ngar-Cheung Lau, 1985: On the role of barotropic energy conversions in the general circulation. Advances in Geophysics, 28A, 33-74.
Lau, Ngar-Cheung, 1984: Circulation Statistics Based on FGGE Level III-B Analysis Produced by GFDL, NOAA Data Report ERL GFDL-5: U. S. Dept. of Commerce / NOAA, 427 pp.
Lau, Ngar-Cheung, 1984: A Comparison of Circulation Statistics Based on FGGE Level III-B Analyses Produced by GFDL and ECMWF for the Special Observing Periods, NOAA Data Report ERL GFDL-6: U. S. Dept. of Commerce / NOAA, 237 pp.
Lau, Ngar-Cheung, and E O Holopainen, 1984: Transient eddy forcing of the time-mean flow as identified by geopotential tendencies. Journal of the Atmospheric Sciences, 41(3), 313-328. Abstract PDF
The forcing of the time-mean flow by transient eddies is examined within the framework of a quasi-geostrophic equation relating the geopotential tendency to the convergence of transient eddy transports of heat and vorticity. The forcing functions of this equation are computed using observed circulation statistics for the wintertime Northern Hemisphere, and solutions are sought for the three-dimensional structure of geopotential and temperature tendencies associated with eddies of different time scales.
In general, the geopotential tendencies associated with vorticity fluxes are of the same sign within a given atmospheric column; whereas the polarity of the geopotential tendencies associated with heat fluxes in the lower troposphere is opposite to that in the upper troposphere. The geostrophic wind tendencies associated with synoptic-scale eddies with periods between 2.5 and 6 days are strongest in the vicinity of the oceanic storm tracks. The enhanced poleward heat transports by active disturbances in these regions lead to eastward accelerations of the geostrophic flow in the lower troposphere, westward accelerations in the upper troposphere, and hence a reduction in the vertical shear of the eastward flow along the storm tracks. The vorticity transports by eddies with synoptic time scales are associated with eastward accelerations throughout the troposphere over the storm tracks. The geostrophic wind tendencies associated with the vorticity fluxes tend to dominate in the upper troposphere, so that the combined effect of the eddy transports of heat and vorticity by synoptic-scale eddies is to accelerate the eastward current at all vertical levels in middle latitudes. The geopotential tendencies associated with eddies with periods between 10 days and a season are generally stronger than those associated with synoptic-scale disturbances. In the upper troposphere, the transports of both heat and vorticity by the low-frequency eddies are accompanied by tendencies which act to destroy the departure from zonal symmetry of the time-averaged geopotential height field. The forcing of the geopotential height field due to vorticity transports by low-frequency eddies is stronger than the corresponding forcing due to heat transports.
The temperature tendencies associated with eddy heat transports are much stronger than those associated with eddy vorticity transports. The thermal forcing due to synoptic-scale disturbances is characterized by dipole-like structures over the western oceans, with positive temperature tendencies (warming) north of the cyclonic tracks and negative tendencies (cooling) further south. In the lower troposphere, the tendencies associated with low-frequency eddies act to destroy the zonally asymmetric component of the stationary temperature field. The typical magnitude of temperature tendencies as computed using the present method, which implicitly takes into account the combined effects of eddy flux convergences and the associated secondary circulations, is about 60-70% of the corresponding values obtained by considering the convergence of eddy heat fluxes alone.
The effects of transient disturbances as depicted by tendencies associated with eddy fluxes are contrasted with earlier results based on eddy transports of quasi-geostrophic potential vorticity. The distinction between these two approaches is discussed.
Lau, Ngar-Cheung, and K M Lau, 1984: The structure and energetics f midlatitude disturbances accompanying cold-air outbreaks over East Asia. Monthly Weather Review, 112(7), 1309-1327. Abstract PDF
The onset dates for 11 individual cold-air outbreaks over the East Asian seaboard during the Winter Monsoon Experiment (1 December 1978-28 February 1979) are used for constructing composite synoptic charts. The three-dimensional structure and energetics of disturbances with time scales shorter than ~ 5 days are distinguished from the corresponding properties of more slowly varying fluctuations by using time-filtering techniques.
It is seen that the high-frequency disturbances accompanying the cold surges experience systematic structural changes as they migrate along a well-defined storm track from East Asia to the Gulf of Alaska. The typical life cycle of such extratropical storms is characterized by a baroclinic growth phase coinciding with the polar outbreaks, and a decay phase in which barotropic processes play an active role. The propagation of low-frequency fluctuations is oriented toward lower latitudes, with new vorticity centers developing downstream and equatorward of the primary disturbances associated with the outbreaks. The shapes of the disturbances appearing in the composite charts indicate that a strong degree of anisotropy exists in both the high-frequency and low-frequency disturbances. The fluctuations with short time scales are elongated in the meridional direction, whereas those with long time scales are elongated in the zonal direction.
The findings of this composite study are seen to be consistent with circulation statistics derived from continuous climatological records. The behavior of the fluctuations with short and long time scales is also reminiscent of the characteristics of baroclinically unstable waves and Rossby-wave trains, respectively, appearing in model experiments.
Lau, Ngar-Cheung, 1983: Mid-latitude wintertime circulation anomalies appearing in a 15-year GCM experiment In Large-scale Dynamical Processes in the Atmosphere, New York, NY, Academic Press, 111-125.
Lau, Ngar-Cheung, 1983: The structure of stationary and quasi-stationary flows in general circulation models and their seasonal dependence In IAMAP-WMO Symposium on Maintenance of the Quasi-Stationary Components of the Flow in the Atmosphere and in Atmospheric Models, Geneva, Switzerland, World Meteorological Organization, 109.
Holopainen, E O., L Rontu, and Ngar-Cheung Lau, 1982: The effect of large-scale transient eddies on the time-mean flow in the atmosphere. Journal of the Atmospheric Sciences, 39(9), 1972-1984. Abstract PDF
The effect of horizontal transports of momentum and heat by transient eddies (TE) on the time-mean flow is studied by examining the relevant terms in a local budget of quasi-geostrophic potential vorticity. Two long-term observational data sets are used, and results for the Northern Hemisphere winter are presented.
The results indicate that eddy heat fluxes in the free atmosphere exert a dissipative influence on both the zonally averaged flow and the stationary waves. On the other hand, eddy momentum transports tend to force cyclonic circulations over the semi-permanent Icelandic and Aleutian surface lows, and anticyclonic circulations over the oceanic high pressure cells in the subtropics. The forcing of the time-mean flow arising from horizontal TE heat transports is generally stronger than the forcing associated with eddy momentum transports. The net effect of eddy transports of heat and momentum is to dissipate the potential enstrophy of the stationary waves. The characteristic time scale associated with this dissipative effect is of the order of 4-5 days.
The relative contribution to the eddy forcing by low-frequency fluctuations (with periods between 10 days and a season) and by synoptic-scale fluctuations (with periods between 2.5 and 6 days) are examined. The forcing associated with low-frequency eddies generally dominates. The forcing associated with synoptic-scale eddies is concentrated in the cyclone tracks near the east coasts of Asia and North America, where a certain degree of counterbalancing between the heat flux forcing and the momentum flux forcing takes place.
Lau, Ngar-Cheung, and Abraham H Oort, 1982: A comparative study of observed northern hemisphere circulation statistics based on GFDL and NMC analyses. Part II: Eddy statistics and the energy cycle. Monthly Weather Review, 110(8), 889-906. Abstract PDF
The comparison between two sets of observed circulation statistics undertaken by Lau and Oort (1981) is continued in this study by examining the temporal variance and covariance statistics in these sets. The first (GFDL) set is compiled by interpolating monthly averaged station statistics. The second set is based on twice-daily operational NMC analyses. The statistics for six winter and six summer seasons within the 1966-73 period are compared. The hemispheric fields examined include transient eddy kinetic energy at 300 mb, root-mean-squares of geopotential height and temperature at 300 and 850 mb, respectively, the horizontal transport by transient eddies of westerly momentum and geopotential height at 300 mb, and of heat at 850 mb. The patterns of horizontal eddy transports are presented in a vectorial format to delineate local relationships with the time-mean flow and the centers of eddy activity. Latitude-height distributions for zonally-averaged patterns of the above statistics are also presented.
The transient eddy statistics in the two sets are in good agreement over regions with adequate data coverage. The NMC set generally gives relatively higher eddy amplitudes and stronger eddy transports over the data-sparse oceans. The maximum deviations between the two sets in these regions are about 20-30%.
The two sets of analyses are further used to calculate the spatial integrals for the energy reservoirs and various energy conversion rates in the atmosphere. The transient and stationary eddies are treated separately in the formulation of the energy cycle. The largest differences are found in the transfer rate of kinetic energy from the stationary waves to the transient disturbances, and for the terms associated with the conversion of available potential energy into kinetic energy. The GFDL and NMC estimates of the other components of the energy cycle do not differ from each other by > 20%. The results from both sets of analyses imply that the transient eddies are very efficient in depleting the available potential energy of the stationary waves through their ability to transport heat down the local temperature gradient. The dissipative time scale associated with this mechanism is several days.
Lau, Ngar-Cheung, and Abraham H Oort, 1981: A comparative study of observed northern hemisphere circulation statistics based on GFDL and NMC analyses. Part I: The time-mean fields. Monthly Weather Review, 109(7), 1380-1403. Abstract PDF
Two sets of monthly mean analyses based essentially on the same observational data, but employing different analysis procedures, are compared. The first set was compiled at the Geophysical Fluid Dynamics Laboratory and consists of horizontal interpolations of monthly averaged circulation statistics accumulated at individual rawinsonde stations. The second set was derived from twice-daily gridded analyses produced by the National Meteorological Center on an operational basis. The data used cover nine winters and nine summers within the 1963-73 period. The spatial domain extends in latitude from 20 degrees N to 90 degrees N, and in the vertical from 850 to 100 mb. The circulation statistics examined include 1) hemispheric distributions of 9-year averages as well as month-to-month standard deviations for the horizonal wind components and geopotential heights at 850, 500 and 200 mb, and the temperature at 850 mb; and 2) latitude-height sections for the zonally averaged wind and temperature, the standing eddy variances of zonal and meridional wind components, geopotential height and temperature, and the meridional transports of westerly momentum, geopotential energy and heat by standing waves.
Over certain data-sparse regions, the two analyses are further compared with actual values reported in Monthly Climatic Data for the World by the few rawinsonde stations located in those regions.
The time-mean fields in the two data sets are found to be generally in excellent agreement over the North American and Eurasian continents, where a dense observing network exists. The deviations between the data sets are large over the oceans and northern Africa, where the GFDL analyses give relatively weaker zonal wind speeds in the jet exit regions, stronger ageostrophic motions in the meridional direction, lower temperatures in the subtropical lower troposphere, and higher temperatures above the subtropical tropopause. The maximum local deviations are on the order of 10-15 m s-1 for zonal wind, 6-8 m s-1 for meridional wind, 50-70 m for geopotential height, and 2-4 degrees C for temperature. These discrepancies are associated with much weaker standing eddy kinetic energy, and much stronger equatorward transports of geopotential energy by the stationary waves in the GFDL analyses. The inter-monthly variability of the monthly mean fields in the GFDL set is generally weaker over the oceans.
The spatial correlation coefficients for the monthly mean fields in the two data sets do not exhibit any discernible trends during the 9-year period. This suggest that the procedural changes in the NMC analysis system during this period did not result in serious inhomogeneities in the time series of the NMC fields.
Lau, Ngar-Cheung, 1981: A diagnostic study of recurrent meteorological anomalies appearing in a 15-year simulation with a GFDL general circulation model. Monthly Weather Review, 109(11), 2287-2311. Abstract PDF
The spatial structure and temporal characteristics of prominent anomalies occurring in a 15-year simulation with a GFDL spectral general circulation model are examined using empirical orthogonal functions, teleconnection patterns, composite charts, lagged correlation functions and frequency spectra.
Despite the absence of any nonseasonal perturbation in the prescribed forcing such as sea surface temperature, insolation and cloud cover, the simulated circulation exhibits an appreciable degree of temporal variability on monthly time scales. The standing oscillation in the Northern Hemisphere winter which accounts for the largest fraction of this variance has a coherent three-dimensional structure. In the middle and upper troposphere, this preferred mode of oscillation is characterized by a wavelike pattern with multiple centers of action. The corresponding anomaly pattern at the sea level is dominated by north-south pressure seesaws over the North Atlantic and North Pacific. The flow patterns associated with these pressure anomalies are consistent with the principal temperature anomaly pattern in the lower troposphere. The large-scale features of the above anomaly patterns are similar to those associated with the most prevalent standing oscillation observed in the atmosphere. The synoptic behavior and hydrological processes in the model atmosphere during the outstanding anomalous episodes are internally consistent.
The spatial structure of the principal mode in the simulation is rather insensitive to the averaging period of the model data. The auto correlation function and frequency spectrum of the first principal component, as determined from daily data, are characteristic of persistent phenomena with no preferred periodicity. The autocorrelation time scale associated with this anomaly pattern is estimated to be ~15 days.
The principal anomaly pattern in the Northern Hemisphere summer is relatively less organized, while those for the Southern Hemisphere and the tropics are noted for their zonal symmetry. The east-west sea level pressure seesaw associated with the observed Southern Oscillation over the Pacific is not simulated by the model, thus suggesting the potential role of nonseasonal forcing mechanisms (such as sea surface temperature anomalies) in that phenomenon.
Blackmon, M L., and Ngar-Cheung Lau, 1980: Regional characteristics of the Northern Hemisphere wintertime circulation: A comparison of the simulation of a GFDL general circulation model with observations. Journal of the Atmospheric Sciences, 37(3), 497-514. Abstract PDF
The hemispheric distributions of a selected set of temporal mean, variance and covariance statistics produced by a general circulation model developed at the Geophysical Fluid Dynamics Laboratory are compared with observations. The fields presented include 1) the seasonally averaged 300 mb geopotential height and zonal wind speed, sea level pressure and 500 mb vertical velocity; 2) the root-mean-squares of 500 and 1000 mb heights, and of 850 mb temperature; 3) the correlation coefficient between the 1000 and 500 mb heights, and 4) the horizontal and vertical transports of heat by transient eddies in the lower troposphere, and the horizontal eddy transports of momentum and potential vorticity near the tropopause. The partitioning of the variance of 500 mb height according to fluctuations of different temporal and spatial scales is examined. By making use of time filters which retain fluctuations with periods between 2.5 and 6 days, the characteristics of synoptic-scale disturbances appearing in the model are studied.
The regional contrasts of the observed wintertime circulation are simulated by the model. In particular, the transport properties of transient disturbances over the oceanic storm tracks and the locations of these centers of activity relative to the stationary flow field are reproduced. The agreement between model and observations substantiates some of our interpretations of the observed circulation presented in earlier works.
The model simulation differs from the observed atmosphere in the following aspects: 1) the amplitude of the simulated transient fluctuations in the upper troposphere is too weak; 2) the geographical distribution of the variance associated with low-frequency planetary-scale disturbances in the model bears little resemblance to the observed pattern; 3) the observed longitudinal variations of eddy activity in the middle and high latitudes are less evident in the model results; and 4) the simulated surface lows over Iceland and the Aleutians are too deep, so that the midlatitude westerlies over most of the Western Hemisphere are too strong, and the surface circulation over the North American continent is not realistic.
Holopainen, E O., Ngar-Cheung Lau, and Abraham H Oort, 1980: A diagnostic study of the time-averaged budget of atmospheric zonal momentum over North America. Journal of the Atmospheric Sciences, 37(10), 2234-2242. Abstract PDF
The terms in the time-mean zone momentum equation which depend only on the large-scale motions are evaluated for the North American continent during the winter season. The computations are based on two different sets of upper air atmospheric circulation statistics between 850 and 200 mb. The first (GFDL) data set consists of global objective horizontal analyses of monthly circulation statistics evaluated at individual stations; the second (NMC) data set was compiled by processing twice-daily synoptic analyses on a hemispheric grid. The mean residual forces needed for balance are discussed assuming that they represent the effects of horizontal and vertical subgrid-scale processes.
The results derived from the two largely independent data sets are similar. Zonal momentum is produced in the free atmosphere over North America by a local, thermally direct meridional circulation, with mean poleward ageostrophic flow above 700 mb. This production is partially counterbalanced by a net export of zonal momentum from the region. The flux divergence associated with large-scale vertical transports appears to be insignificant.
A residual force of considerable magnitude is needed for balance. In the lower troposphere, this force has an accelerating effect on the zonal flow almost everywhere over the eastern portion of the North American continent, and a decelerating effect over the western mountainous region where, however, the results are less certain. In the upper troposphere, the residual force over the entire continent has a net decelerating effect, but significant geographical differences appear to occur.
A tentative interpretation of the results for the residual force is offered.
Lau, Ngar-Cheung, 1980: Diagnostic study of the local sources and sinks of momentum, kinetic energy, vorticity, and heat in the observed Northern Hemisphere winter-time circulation In Workshop on Diagnostics of Diabatic Processes, Reading, England, European Centre for Medium Range Weather Forecasts, 33-60.
Lau, Ngar-Cheung, 1980: The three-dimensional structure of monthly anomalies appearing in a 15-year simulation of a GFDL general circulation model In Proceedings of the Fourth Annual Climate Diagnostics Workshop, Rockville, MD, NOAA, 403-412.
Lau, Ngar-Cheung, 1979: The observed structure of tropospheric stationary waves and the local balances of vorticity and heat. Journal of the Atmospheric Sciences, 36, 996-1016. Abstract PDF
The physical structure and the associated transport properties of stationary waves in the troposphere are described using circulation statistics compiled from twice-daily hemispheric analyses covering 11 winters. The distributions of standing eddy meridional transports in middle latitudes are characterized by momentum flux convergence and equatorward geopotential energy transports in the upper troposphere, and by poleward heat fluxes at the lower levels.
The contributions of steady and transient motions to the local, time-averaged budget of vorticity at 300 mb are evaluated. The dominant terms in the time-averaged vorticity equation are the local advection of relative vorticity by the stationary flow and the divergence term. The advection of planetary vorticity by the mean flow (the B-effect) and the convergence of vorticity fluxes by transient eddies appear to be of secondary importance. The hemispheric distributions of the principal terms in the vorticity balance are closely related to topographical features at the lower boundary.
The hemispheric field of stationary flow divergence at various levels is determined as a residual in the time-averaged vorticity balance. This diagnosed divergence field is used to 1) demonstrate the feasibility of retrieving essential stationary flow features in the upper troposphere through solution of the linearized vorticity equation with a prescribed divergence forcing; 2) deduce the velocity potential field; and 3) compute the mean vertical motion field through vertical integration of the continuity equation.
Standing eddy statistics involving vertical motions are described. Mean vertical motions in middle latitudes are found to be positively correlated with mean meridional motions and with mean temperature. The distributions of meridional and vertical transports of geopotential energy and westerly momentum in the meridional plane are presented in a vectorial format. The pattern depicting geopotential energy fluxes suggests that the enhanced standing wave kinetic energy over the subtropics is maintained by geopotential energy transports which originate from higher latitudes.
A diagnosis of the local, time-averaged balance of heat at 1000 and 700 mb is performed. The heat transports by the transient eddies in the lower troposphere exhibit a strong tendency to destroy the zonally asymmetric component of the stationary temperature field. This dissipative mechanism acts on a time scale of several days. The hemispheric distributions of the diabatic heating deduced from the heat budget are indicative of the central role of geogaphically fixed influences such as ocean currents and sea-land contrast.
Lau, Ngar-Cheung, 1979: On the distribution of horizontal transports by transient eddies in the northern hemisphere wintertime circulation In Proceedings of the (Thirteenth) Stanstead Seminar held at Bishop's University, Lennoxville, Québec, Canada, Publication in Meteorology No. 123, McGill University, Montréal, Canada, 58-62.
Lau, Ngar-Cheung, 1979: The structure and energetics of transient disturbances in the northern hemisphere wintertime circulation. Journal of the Atmospheric Sciences, 36, 982-995. Abstract PDF
The hemispheric distributions of wintertime circulation statistics derived from the forecast fields of vertical motion are presented. The dominant features in the pattern for time-mean vertical velocity are consistent with the existence of thermally direct meridional circulations over the entrance regions of the principal jet streams, and thermally indirect circulations over the jet exit regions. Rising motions in the transient disturbances are seen to display positive temporal correlations with temperature and geopotential height over the major oceanic storm track regions. On the other hand, the western portion of the continents and the adjacent oceanic areas are characterized by downward eddy transports of geopotential energy at 850 and 500 mb, as well as much reduced temporal correlations between the vertical motion and temperature fields.
The vertical phase structure of the transient disturbances at various geographical locations is studied by performing a cross-spectral analysis of the time series of geopotential height fields at 850, 500 and 250 mb. The local geopotential height fluctuations at different pressure levels are strongly coherent. Over the sites characterized by enhanced development of transient waves, geopotential height perturbations of synoptic temporal scales are seen to lag by about 60 degrees (1/6 cycle) between the tropopause and 850 mb levels. The corresponding phase lag is reduced to about 25 degrees over the western portion of the continents, and the disturbances acquire a barotropic character at these longitudes.
The results of a detailed diagnosis of the local, time-averaged budgets of time-mean and transient eddy kinetic energy at 300 mb are discussed. The kinetic energy of the intensified time-mean flow at the jet stream cores is primarily maintained by the local, time-averaged ageostrophic circulations, which dominate over the effects due to eddy-mean flow interactions. The energy generated in these source regions is transported by the time-mean flow to the jet exit regions, where the thermally indirect circulations function as local sinks of mean kinetic energy. Analogously, eddy kinetic energy at the jet stream level is generated by ageostrophic motions in the transient disturbances over the western oceans, it is then advected to the western portion of the continents by the time-averaged flow, and is eventually dissipated by the supergeostrophic flow in the eddies at those longitudes.
The regional character of the transient eddy statistics presented in this and earlier papers is interpreted in the light of the results from a recent modeling study by Simmons and Hoskins (1978) on the life cycle of nonlinear baroclinic waves.
Lau, Ngar-Cheung, and J M Wallace, 1979: On the distribution of horizontal transports by transient eddies in the northern hemisphere wintertime circulation. Journal of the Atmospheric Sciences, 36(10), 1844-1861. Abstract PDF
Horizontal fluxes in geopotential, heat, zonal momentum, relative vorticity and potential vorticity by the transient eddies on selected pressure surfaces are computed on the basis of data from twice daily synoptic charts for the Northern Hemisphere, objectively analyzed at the National Meteorological Center. The distribution of fluxes is resolved into nondivergent and irrotational parts and displayed in a vectorial format.
The nondivergent flux of geopotential closely parallels contours of constant temporal variance of geopotential. Nondivergent transient eddy fluxes of heat, relative vorticity and potential vorticity, all evaluated in the vicinity of the tropopause level, bear a similar but less exact relation to the distribution of the temporal variance of geopotential. These relationships are shown to exist because 1) the wind field responsible for the fluxes is quasi-geostrophic and 2) the instantaneous distributions of temperature, relative vorticity and potential vorticity tend to be rather similar to that of the geopotential field in the vicinity of the tropopause level. For these three parameters and for geopotential, the nondivergent fluxes at the tropopause level tend to be considerably larger than the corresponding irrotational fluxes.
The distribution of transient eddy heat flux in the lower troposphere is primarily irrotational and directed down the local horizontal gradient of the time-averaged temperature field. The magnitudes of these fluxes are comparable to those of corresponding fluxes associated with horizontal temperature advection by the time-averaged flow. There does not appear to be any simple functional relationship between the scalar magnitudes of the fluxes and local mean temperature gradients. The irrotational transient eddy heat fluxes at 300 mb resemble the distribution of total transient eddy heat flux at 850 mb. At the 200 mb level these fluxes are primarily countergradient in middle latitudes.
The irrotational transient eddy flux of zonal momentum at the jet stream level is much smaller than the corresponding flux associated with momentum advection by the time-averaged flow and it is directed into regions of low zonal wind speed. The irrotational flux of geopotential is directed out of regions of decaying transient disturbances and into regions of cyclogenesis.
The irrotational fluxes of vorticity and potential vorticity near the jet stream level are very similar. Their distribution appears to be strongly related to the time-averaged sea level pressure field, with fluxes out of regions of high sea level pressure and into regions of low pressure. These transports appear to be in the proper sense to fulfill the balance requirements for vorticity and potential vorticity. The divergence of the transient eddy flux of potential vorticity is weaker than the horizontal advection of potential vorticity by the time-averaged flow. It is suggested that the observed distribution of potential vorticity flux is imposed on the transient eddies by the distribution of sources and sinks of potential vorticity at the earth's surface which are closely related to the sea level pressure distribution.
