The change in the East Asian summer monsoon simulated by the MIROC3.2 high-resolution coupled model under global warming scenarios

In order to investigate changes in the East Asian summer monsoon (EASM) under the global warming, the MIROC3.2 (hires) coupled general circulation model (CGCM) developed by the Center for Climate System Research is utilized. The outputs of MIROC3.2 (hires) model have been analyzed using two scenarios; the 20th Century Climate in Coupled Models (20C3M) scenario and the Special Reports for Emissions Scenarios A1B (SRES A1B). Eight Intergovernmental Panel on Climate Change (IPCC) models are also analyzed to compare model performances. It is shown that the simulation skill of MIROC3.2 (hires) for the EASM is relatively superior to these IPCC CGCMs. It has been found that the intensified rain band and the extended duration of the EASM are anticipated with MIROC3.2 (hires) under the global warming in well accordance with previous studies. Especially, the precipitation due to the cumulus convection is predicted to increase more significantly than the precipitation by the large-scale condensation. Due to the increased land-sea thermal contrast in summer under the global warming, water vapor fluxes in the lower troposphere are enhanced. Consequently, the convective instability may be strengthened and thus it leads to the increase of precipitation by cumulus convection. Moreover, the upper tropospheric circulations associated with the EU pattern would lead to the larger interannual variability of precipitation over the EASM region in the future warm climate. In addition, it is found that the relationship between the sea surface temperature over the tropical Pacific Ocean in the wintertime and the summer rainfall over the East Asia may be weakened, suggesting that the predictability of the EASM might become more difficult under the global warming.     

Time-dependent greenhouse warming computations with a coupled ocean-atmosphere model

Climate changes during the next 100 years caused by anthropogenic emissions of greenhouse gases have been simulated for the Intergovernmental Panel on Climate Change Scenarios A (business as usual) and D (accelerated policies) using a coupled ocean-atmosphere general circulation model. In the global average, the near-surface temperature rises by 2.6 K in Scenario A and by 0.6 K in Scenario D. The global patterns of climate change for both IPCC scenarios and for a third step-function 2 x CO₂ experiment were found to be very similar. The warming delay over the oceans is larger than found in simulations with atmospheric general circulation models coupled to mixed-layer models, leading to a more pronounced land-sea contrast and a weaker warming (and in some regions even an initial cooling) in the Southern Ocean. During the first forty years, the global warming and sea level rise due to the thermal expansion of the ocean are significantly slower than estimated previously from box-diffusion-upwelling models, but the major part of this delay can be attributed to the previous warming history prior to the start of present coupled ocean-atmosphere model integration (cold start).     

Use of variability modes to evaluate AR4 climate models over the Euro-Atlantic region

This paper analyzes the ability of the multi-model simulations from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) to simulate the main leading modes of variability over the Euro-Atlantic region in winter: the North-Atlantic Oscillation (NAO), the Scandinavian mode (SCAND), the East/Atlantic Oscillation (EA) and the East Atlantic/Western Russia mode (EA/WR). These modes of variability have been evaluated both spatially, by analyzing the intensity and location of their anomaly centres, as well as temporally, by focusing on the probability density functions and e-folding time scales. The choice of variability modes as a tool for climate model assessment can be justified by the fact that modes of variability determine local climatic conditions and their likely change may have important implications for future climate changes. It is found that all the models considered are able to simulate reasonably well these four variability modes, the SCAND being the mode which is best spatially simulated. From a temporal point of view the NAO and SCAND modes are the best simulated. UKMO-HadGEM1 and CGCM3.1(T63) are the models best at reproducing spatial characteristics, whereas CCSM3 and CGCM3.1(T63) are the best ones with regard to the temporal features. GISS-AOM is the model showing the worst performance, in terms of both spatial and temporal features. These results may bring new insight into the selection and use of specific models to simulate Euro-Atlantic climate, with some models being clearly more successful in simulating patterns of temporal and spatial variability than others.     

Multiyear hindcast simulations of summer monsoon over South Asia using a nested regional climate model��BCC_RegCM1.0

Multiyear (1983?C2006) hindcast simulation of summer monsoon over South Asia has been carried out using the regional climate model of the Beijing Climate Centre (BCC_RegCM1.0). The regional climate model (hereafter BCC RCM) is nested into the global climate model of the Beijing Climate Centre BCC_CGCM1.0 (here after CGCM). The regional climate model is initialized on 01 May and integrated up to the end of the September for 24?years. Compared to the driving CGCM the BCC RCM reproduces reasonably well the intensity and magnitude of the large-scale features associated with the South Asia summer monsoon such as the upper level anticyclone at 200?hPa, the mid-tropospheric warming over the Tibetan plateau, the surface heat low and the 850?hPa moisture transport from ocean to the land. Both models, i.e., BCC RCM and the driving CGCM overestimates (underestimates) the 850?hPa southwesterly flow over the northern (southern) Arabian Sea. Moreover, both models overestimate the seasonal mean precipitation over much of the South Asia region compared to the observations. However, the precipitation biases are significantly reduced in the BCC RCM simulations. Furthermore, both models simulate reasonably the interannual variability of the summer monsoon over India. The precipitation index simulated by BCC RCM shows significant correlation (0.62) with the observed one. The BCC RCM simulates reasonably well the spatial and temporal variation of the precipitation and surface air temperature compared to the driving CGCM. Further, the temperature biases are significantly reduced (1?C4°C) in the BCC RCM simulations. The simulated vertical structure of the atmosphere show biases above the four sub-regions, however, these biases are significantly reduced in the BCC RCM simulations compared to the driving CGCM. Compared to the driving CGCM, the evolution processes of the onset of summer monsoon, e.g., the meridional temperature gradient and the vertical wind shear are well simulated by the BCC RCM. The 24-year simulations also show that with a little exception the BCC RCM is capable to reproduce the monsoon active and break phases and the intraseasonal precipitation variation over the Indian subcontinent.     

EC-Earth V2.2: description and validation of a new seamless earth system prediction model

EC-Earth, a new Earth system model based on the operational seasonal forecast system of the European Centre for Medium-Range Weather Forecasts (ECMWF), is presented. The performance of version 2.2 (V2.2) of the model is compared to observations, reanalysis data and other coupled atmosphere–ocean-sea ice models. The large-scale physical characteristics of the atmosphere, ocean and sea ice are well simulated. When compared to other coupled models with similar complexity, the model performs well in simulating tropospheric fields and dynamic variables, and performs less in simulating surface temperature and fluxes. The surface temperatures are too cold, with the exception of the Southern Ocean region and parts of the Northern Hemisphere extratropics. The main patterns of interannual climate variability are well represented. Experiments with enhanced CO₂ concentrations show well-known responses of Arctic amplification, land-sea contrasts, tropospheric warming and stratospheric cooling. The global climate sensitivity of the current version of EC-Earth is slightly less than 1?K/(W?m^?2). An intensification of the hydrological cycle is found and strong regional changes in precipitation, affecting monsoon characteristics. The results show that a coupled model based on an operational seasonal prediction system can be used for climate studies, supporting emerging seamless prediction strategies.     

中国600个站气温和IPCC模式产品气温的比较

利用中国600个站1961～2000年的月平均气温和IPCC提供的7个全球海气耦合模式(CCC、CCSR、CSIRO、DKRZ、GFDL、HADL、NCAR)在同样时段只考虑03等温室气体的影响的气温产品，进行了对比分析。结果表明：(1)IPCC模式对中国区域气温有一定的模拟能力，考虑总体情况，HADL模式模拟效果最好，依次是模式CCSR、DKRZ、7个模式平均、模式CSIRO、NCAR、CCC、GFDL；(2)低纬度地区比中纬度地区和高纬度地区、中部地区比东部地区和西部地区的模式产品模拟效果更接近于中国600站的实况，另外中纬度地区模拟的效果好于高纬度地区，东部地区模拟效果好于西部地区；(3)中国600个站的气温与IPCC模式的气温产品在中国区域都有上升的趋势，与全球气候变暖的趋势相一致。     

气候变化的归因与预估模拟研究

本文总结了近五年来中国科学院大气物理研究所在气候变暖的归因模拟与预估研究上的主要进展。研究表明,利用海温、太阳辐射和温室气体等实际强迫因子驱动大气环流模式,能够较为合理地模拟全球平均地表气温在20世纪的演变,但是难以模拟出包括北大西洋涛动／北极涛动和南极涛动在内的高纬度环流的长期变化趋势。利用温室气体和硫酸盐气溶胶等“历史资料”驱动气候系统模式,能够较好地模拟出20世纪后期的全球增暖,但如果要再现20世纪前期(1940年代)的变暖,还需同时考虑太阳辐射等自然外强迫因子。20世纪中国气温演变的耦合模式模拟技巧,较之全球平均情况要低;中国气候在1920年代的变暖机理目前尚不清楚。对于近50年中国东部地区“南冷北暖”、“南涝北旱”的气候变化,基于大气环流模式特别是区域气候模式的数值试验表明,夏季硫酸盐气溶胶的负辐射效应超过了温室气体的增暖效应,从而对变冷产生贡献。但现有的数值模拟证据,不足以说明气溶胶增加对“南涝北旱”型降水异常有贡献。20世纪中期以来,青藏高原主体存在明显增温趋势,温室气体浓度的增加对这种增暖有显著贡献。多模式集合预估的未来气候变化表明,21世纪全球平均温度将继续增暖,增温幅度因不同排放情景而异;中国大陆年均表面气温的增暖与全球同步,但增幅在东北、西部和华中地区较大,冬季升温幅度高于夏季、日最低温度升幅要强于日最高温度;全球增暖有可能对我国中东部植被的地理分布产生影响。伴随温室气体增加所导致的夏季平均温度升高,极端温度事件增多;在更暖的气候背景下,中国大部分地区总降水将增多,极端降水强度加大且更频繁发生,极端降水占总降水的比例也将增大。全球增暖有可能令大洋热盐环流减弱,但是减弱的幅度因模式而异。全球增暖可能不是导致北太平洋副热带－热带经圈环流自20世纪70年代以来变弱的原因。文章同时指出了模式预估结果中存在的不确定性。     

Simulation of state-dependent high-frequency atmospheric variability associated with ENSO

High-frequency atmospheric variability depends on the phase of El Nino/Southern Oscillation (ENSO). Recently, there is increasing evidence that state-dependent high-frequency atmospheric variability significantly modulates ENSO characteristics. Hence, in this study, we examine the model simulations of high-frequency atmospheric variability and, further, its dependency on the El Nino phase, using atmospheric and coupled GCMs (AGCM and CGCM). We use two versions of physical packages here—with and without convective momentum transport (CMT)—in both models. We found that the CMT simulation gives rise to a large climatological zonal wind difference over the Pacific. Also, both the climate models show a significantly improved performance in simulating the state-dependent noise when the CMT parameterization is implemented. We demonstrate that the better simulation of the state-dependent noise results from a better representation of anomalous, as well as climatological, zonal wind. Our further comparisons between the simulations, demonstrates that low-frequency wind is a crucial factor in determining the state-dependency of high-frequency wind variability. Therefore, it is suggested that the so-called state-dependent noise is directly induced by the low-frequency wind anomaly, which is caused by SST associated with ENSO.     

Comparison of statistically downscaled precipitation in terms of future climate indices and daily variability for southern Ontario and Quebec,Canada

Given the coarse resolution of global climate models, downscaling techniques are often needed to generate finer scale projections of variables affected by local-scale processes such as precipitation. However, classical statistical downscaling experiments for future climate rely on the time-invariance assumption as one cannot know the true change in the variable of interest, nor validate the models with data not yet observed. Our experimental setup involves using the Canadian regional climate model (CRCM) outputs as pseudo-observations to estimate model performance in the context of future climate projections by replacing historical and future observations with model simulations from the CRCM, nested within the domain of the Canadian global climate model (CGCM). In particular, we evaluated statistically downscaled daily precipitation time series in terms of the Peirce skill score, mean absolute errors, and climate indices. Specifically, we used a variety of linear and nonlinear methods such as artificial neural networks (ANN), decision trees and ensembles, multiple linear regression, and k-nearest neighbors to generate present and future daily precipitation occurrences and amounts. We obtained the predictors from the CGCM 3.1 20C3M (1971–2000) and A2 (2041–2070) simulations, and precipitation outputs from the CRCM 4.2 (forced with the CGCM 3.1 boundary conditions) as predictands. Overall, ANN models and tree ensembles outscored the linear models and simple nonlinear models in terms of precipitation occurrences, without performance deteriorating in future climate. In contrast, for the precipitation amounts and related climate indices, the performance of downscaling models deteriorated in future climate.     

Tropical Pacific impacts of convective momentum transport in the SNU coupled GCM

Impacts of convective momentum transport (CMT) on tropical Pacific climate are examined, using an atmospheric (AGCM) and coupled GCM (CGCM) from Seoul National University. The CMT scheme affects the surface mainly via a convection-compensating atmospheric subsidence which conveys momentum downward through most of the troposphere. AGCM simulations—with SSTs prescribed from climatological and El Nino Southern Oscillation (ENSO) conditions—show substantial changes in circulation when CMT is added, such as an eastward shift of the climatological trade winds and west Pacific convection. The CMT also alters the ENSO wind anomalies by shifting them eastward and widening them meridionally, despite only subtle changes in the precipitation anomaly patterns. During ENSO, CMT affects the low-level winds mainly via the anomalous convection acting on the climatological westerly wind shear over the central Pacific—so that an eastward shift of convection transfers more westerly momentum toward the surface than would occur without CMT. By altering the low-level circulation, the CMT further alters the precipitation, which in turn feeds back on the CMT. In the CGCM, CMT affects the simulated climatology by shifting the mean convection and trade winds eastward and warming the equatorial SST; the ENSO period and amplitude also increase. In contrast to the AGCM simulations, CMT substantially alters the El Nino precipitation anomaly patterns in the CGCM. Also discussed are possible impacts of the CMT-induced changes in climatology on the simulated ENSO.     

ECHAM4/OPYC3海气耦合模式对东亚季风年循环及其未来变化的模拟

德国马普研究所海气耦合摸式ECHAM4/OPYC3对东亚地区2 m温度年循环的模拟尽管有一些偏差,但还是相当成功的.其模拟的东亚夏季风偏弱,而冬季风偏强,此偏差可能与2 m温度以及西太平洋副热带高压模拟偏差有关.该模式模拟的东亚季风区夏季降水量偏弱,这与上述夏季风环流的模拟结果是一致的.该模式较好地抓住了华北地区经向环流和降水量的年循环特征.利用最新的温室气体和SO2排放方案,即政府间气候变化委员会(IPCC)排放方案特别报告(SRES)的A2和B2方案,通过该模式111年的积分结果讨论了东亚季风气候在21世纪后30年中的变化,其主要结果为:全球变暖导致夏季海陆温差增大和冬季海陆温差减弱,进而使东亚季风环流在夏季加强,冬季减弱.长江流域和华北地区的夏季降水量显著增强,而后者的增强更为显著,使得东亚季风区的夏季多雨区向北延伸;东亚季风区9月份的降水量在两个方案中都显著增加,说明在全球变暖条件下东亚季风区的多雨季节将延迟一个月.     

Recent Rapid Regional Climate Warming on the Antarctic Peninsula

The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century.     

AMO’s structure and climate footprint in observations and IPCC AR5 climate simulations

This study aims to characterize the spatiotemporal features of the low frequency Atlantic Multidecadal Oscillation (AMO), its oceanic and atmospheric footprint and its associated hydroclimate impact. To accomplish this, we compare and evaluate the representation of AMO-related features both in observations and in historical simulations of the twentieth century climate from models participating in the IPCC’s CMIP5 project. Climate models from international leading research institutions are chosen: CCSM4, GFDL-CM3, UKMO-HadCM3 and ECHAM6/MPI-ESM-LR. Each model employed includes at least three and as many as nine ensemble members. Our analysis suggests that the four models underestimate the characteristic period of the AMO, as well as its temporal variability; this is associated with an underestimation/overestimation of spectral peaks in the 70–80 year/10–20 year range. The four models manifest the mid-latitude focus of the AMO-related SST anomalies, as well as certain features of its subsurface heat content signal. However, they are limited when it comes to simulating some of the key oceanic and atmospheric footprints of the phenomenon, such as its signature on subsurface salinity, oceanic heat content and geopotential height anomalies. Thus, it is not surprising that the models are unable to capture the majority of the associated hydroclimate impact on the neighboring continents, including underestimation of the surface warming that is linked to the positive phase of the AMO and is critical for the models to be trusted on projections of future climate and decadal predictions.     

Responses of East Asian summer monsoon to historical SST and atmospheric forcing during 1950–2000

The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR) community atmospheric model version 3 (CAM3) and the geophysical fluid dynamics laboratory (GFDL) atmospheric model version 2.1 (AM2.1) are analyzed. These simulations are forced separately with different historical forcing, namely tropical sea surface temperature (SSTs), global SSTs, greenhouse gases plus aerosols, and a combination of global SSTs and greenhouse gases plus aerosols. This study focuses on the relative roles of these individual forcings in causing the observed monsoon and rainfall changes over East Asia during 1950–2000. The simulations from both models show that the SST forcing, primarily from the Tropics, is able to induce most of the observed weakening of the EASM circulation, while the greenhouse gas plus (direct) aerosol forcing increases the land-sea thermal contrast and thus enhances the EASM circulation. The results suggest that the recent warming in the Tropics, especially the warming associated with the tropical interdecadal variability centered over the central and eastern Pacific, is a primary cause for the weakening of the EASM since the late 1970s. However, a realistic simulation of the relatively small-scale rainfall change pattern over East China remains a challenge for the global models.     

On the cold start problem in transient simulations with coupled atmosphere-ocean models

Finite computer resources force compromises in the design of transient numerical experiments with coupled atmosphere-ocean general circulation models which, in the case of global warming simulations, normally preclude a full integration from the undisturbed pre-industrial state. The start of the integration at a later time from a climate state which, in contrast to the true climate, is initially in equilibrium then induces a cold start error. Using linear response theory a general expression for the cold start error is derived. The theory is applied to the Hamburg CO₂ scenario simulations. An attempt to estimate the global-mean-temperature response function of the coupled model from the response of the model to a CO₂ doubling was unsuccessful because of the non-linearity of the system. However, an alternative derivation, based on the transient simulation itself, yielded a cold start error which explained the initial retardation of the Hamburg global warming curve relative to the IPCC results obtained with a simple box-diffusion-upwelling model. In the case of the sea level the behaviour of the model is apparently more linear. The cold start error estimations based on a CO₂ doubling experiment and on an experiment with gradually increasing CO₂ (scenario A) are very similar and explain about two thirds of the coupled model retardation relative to the IPCC results.     

Polar amplification: is atmospheric heat transport important?

Surface albedo feedback is widely believed to be the principle contributor to polar amplification. However, a number of studies have shown that coupled ocean-atmosphere models without ice albedo feedbacks still produce significant polar amplification in 2 × CO₂ runs due to atmospheric heat transports and their interaction with surface conditions. In this article, the relative importance of atmospheric heat transport and surface albedo is assessed using a conceptual 2-box energy balance model in a variety of different model climates. While both processes are shown to independently contribute to the polar amplified response of the model, formal feedback analysis indicates that a strong surface albedo response will tend to reduce the effect of atmospheric heat transport in the full model. We identify several scenarios near the present day climate in which, according to this formal feedback analysis, atmospheric heat transport plays no role in shaping the equilibrium warming response to uniform forcing. However, a closer analysis shows that even in these scenarios the presence of atmospheric heat transport feedback does play a significant role in shaping the trajectory by which the climate adjusts to its new equilibrium.     

Energy and water balance studies of a snow cover during snowmelt period at a high arctic site

Summary  The predicted global warming is supposed to have an enhanced effect on the arctic regions. How this will influence the water, carbon dioxide and methane balances in the European arctic tundra is the objective of the EU-funded project “Understanding Land Surface Physical Processes in the Arctic” (LAPP), to which where SINTEF is one of several contributors. The snow cover is one of the limiting factors for these exchange processes and knowledge of how it behaves and will behave under a different climate is important. Data collected for water and energy balance studies in an area close to Ny-?lesund at 79°N at Svalbard are the basis of this study. Measurements during the ablation periods since 1992 show an average air temperature for the periods of 2.1 °C, an average incoming shorwave radiation of 230 W/m² and an average measured runoff intensity of 14 mm/day with a maximum of 68 mm/day. Three models of different complexity are tested in order to simulate the water and energy balance of a snow cover on the arctic tundra. The three models are: a complex numerical model (CROCUS), a simple energy balance model and a temperature index model. The simulations were carried out for the melt periods in 1992 and 1996 as these two periods represent very different meteorological conditions. The results of these simulations exposed weaknesses in all the models. The energy balance model lacks calculation of cold content in the snowpack. This influences both the outgoing longwave radiation and the timing of the melt. Due to the effect of compensating errors in the simulations, CROCUS performed better than the simple energy balance model but also this model has problems with the simulation of outgoing longwave radiation. The temperature index model does not perform well for snowmelt studies in regions were radiation is the main driving energy source for the melt. Received September 28, 1999 Revised September 18, 2000     

CMIP5模式对中国东北气候模拟能力的评估

利用CN05观测资料和参与IPCC第五次评估报告的45个全球气候系统模式的模拟结果,分析了新一代全球气候模式对中国东北三省(1961~2005年)气温和降水的模拟能力。结果表明:1)绝大多数模式都能较好地模拟出研究区内显著增温的趋势,对气温的年际变化模拟能力则相对有限;2)所有模式均能很好地再现气温气候态的空间分布特征,且多模式集合模拟结果优于绝大多数单个模式,空间相关系数达到了0.96;3)对于降水的模拟结果,模式间差异较大,多模式集合能较好地再现其空间分布规律(空间相关系数为0.86),对降水年际变化及线性变化趋势的模拟能力则较差。总体来说,多模式集合对东北气候的时空变化特征具有一定的模拟能力,且对气温模拟效果优于降水,对空间分布的模拟能力优于时间变化。     

Simulation of the influence of solar radiation variations on the global climate with an ocean-atmosphere general circulation model

 Two simulations with a global coupled ocean-atmosphere circulation model have been carried out to study the potential impact of solar variability on climate. The Hoyt and Schatten estimate of solar variability from 1700 to 1992 has been used to force the model. Results indicate that the near-surface temperature simulated by the model is dominated by the long periodic solar fluctuations (Gleissberg cycle), with global mean temperatures varying by about 0.5 K. Further results indicate that solar variability and an increase in greenhouse gases both induce to a first approximation a comparable pattern of surface temperature change, i.e., an increase of the land-sea contrast. However, the solar-induced warming pattern in annual means and summer is more centered over the subtropics, compared to a more uniform warming associated with the increase in greenhouse gases. The observed temperature rise over the most recent 30 and 100 years is larger than the trend in the solar forcing simulation during the same period, indicating a strong likelihood that, if the model forcing and response is realistic, other factors have contributed to the observed warming. Since the pattern of the recent observed warming agrees better with the greenhouse warming pattern than with the solar variability response, it is likely that one of these factors is the increase of the atmospheric greenhouse gas concentration. Received: 14 October 1996 / Accepted: 9 May 1997     

Will the South Asian monsoon overturning circulation stabilize any further?

Understanding the response of the South Asian monsoon (SAM) system to global climate change is an interesting scientific problem that has enormous implications from the societal viewpoint. While the CMIP3 projections of future changes in monsoon precipitation used in the IPCC AR4 show major uncertainties, there is a growing recognition that the rapid increase of moisture in a warming climate can potentially enhance the stability of the large-scale tropical circulations. In this work, the authors have examined the stability of the SAM circulation based on diagnostic analysis of climate datasets over the past half century; and addressed the issue of likely future changes in the SAM in response to global warming using simulations from an ultra-high resolution (20 km) global climate model. Additional sensitivity experiments using a simplified atmospheric model have been presented to supplement the overall findings. The results here suggest that the intensity of the boreal summer monsoon overturning circulation and the associated southwesterly monsoon flow have significantly weakened during the past 50-years. The weakening trend of the monsoon circulation is further corroborated by a significant decrease in the frequency of moderate-to-heavy monsoon rainfall days and upward vertical velocities particularly over the narrow mountain ranges of the Western Ghats. Based on simulations from the 20-km ultra high-resolution model, it is argued that a stabilization (weakening) of the summer monsoon Hadley-type circulation in response to global warming can potentially lead to a weakened large-scale monsoon flow thereby resulting in weaker vertical velocities and reduced orographic precipitation over the narrow Western Ghat mountains by the end of the twenty-first century. Supplementary experiments using a simplified atmospheric model indicate a high sensitivity of the large-scale monsoon circulation to atmospheric stability in comparison with the effects of condensational heating.