Reproducibility and predictability of the Asian summer monsoon in the ECHAM4-GCM

The mean evolution of the Asian summer monsoon and its interannual variability have been studied using three simulations (from 1961 to 1994) with the ECHAM4 General Circulation Model (GCM). The results have been compared with observational data and with two reanalyses data sets: the ECMWF Reanalysis (ERA) and the NCEP-NCAR Reanalysis. The South Asian summer monsoon (SASM) has been studied in terms of mean precipitation and circulation patterns. The model is successful in simulating the mean circulation of the SASM, though precipitation is generally weaker than observed in India, but closer to the observed values over the Indian Ocean and the Philippines. The ECHAM4 model also shows a capability to capture the interannual variability of the monsoon as it is measured by two different indices, the EIMR (Extended Indian Monsoon Rainfall) index and the DMI (Dynamical Monsoon Index). An analysis of NINO3 SSTA anomalies and of the Asian summer monsoon indices showed that the model is able to capture rather well the interdecadal variation of the correlation between them. A large ensemble of 25 members, forced with interannually varying SST from 1979 to 1993, has been used to test the potential predictability of the Indian summer monsoon and the dependence of the skill on the ensemble size. Results indicate that a minimum ensemble size of 16 members is needed to capture the variability of the monsoon indices.     

Absorbing aerosols and pre-summer monsoon hydroclimate variability over the Indian subcontinent: The challenge in investigating links

The sub-monthly evolution of the interannual variations of absorbing aerosols and related hydrometeorology over South Asia in the pre-monsoon period is investigated from the analysis of pentad-resolution observational datasets.It is shown that pre-monsoon (late April–early May) variations are characterized by increased aerosols, reduced cloudiness and precipitation, and increased downward shortwave radiation. Lead-lag regressions indicate the significant influence of synoptic scale advection (and related vertical motion) in simultaneously shaping the aerosol distribution and associated significant hydroclimate (precipitation, cloudiness, surface shortwave radiation, and 2-m air temperature) over the Indo-Gangetic Plain.The above findings can be interpreted as a manifestation of the aerosol “semi-direct” effect if one is not mindful of the prevailing circulation anomalies and their concurrent impact on aerosol and hydroclimate. The complex interplay among aerosols, dynamics and precipitation also shows the challenge of extracting the aerosol impact from an observational analysis. Finally, the analysis points to the pitfalls of a columnar, circulation-blind framework in investigating aerosol–monsoon interactions, a concern of relevance in analyses of the impact of long-term aerosol trends, as well.     

Simulation of the anthropogenic aerosols over South Asia and their effects on Indian summer monsoon

A regional climate model coupled with a chemistry-aerosol model is employed to simulate the anthropogenic aerosols including sulfate, black carbon and organic carbon and their direct effect on climate over South Asia. The model is driven by the NCAR/NCEP re-analysis data. Multi-year simulations with half, normal and double emission fluxes are conducted. Results show that the model performs well in reproducing present climate over the region. Simulations of the aerosol optical depth and surface concentration of aerosols are also reasonable although to a less extent. The negative radiative forcing is found at the top of atmosphere and largely depended on emission concentration. Surface air temperature decreases by 0.1?C0.5°C both in pre-monsoon and monsoon seasons. The range and intensity of cooling areas enlarge while aerosol emission increases. Changes in precipitation are between ?25 and 25%. Different diversifications of rainfall are showed with three emission scenarios. The changes of precipitation are consistent with varieties of monsoon onset dates in pre-monsoon season. In the regions of increasing precipitation, monsoon onset is advanced and vice versa. In northeast India and Myanmar, aerosols lead the India summer monsoon onset advancing 1?C2 pentads, and delaying by 1?C2 pentads in central and southeast India. These changes are mainly caused by the anomaly of local Hadley circulations and enhancive precipitation. Tibetan Plateau played a crucial role in the circulation changes.     

Asian monsoon simulations by Community Climate Models CAM4 and CCSM4

This study examines the ability of Community Atmosphere Model (CAM) and Community Climate System Model (CCSM) to simulate the Asian summer monsoon, focusing particularly on inter-model comparison and the role of air–sea interaction. Two different versions of CAM, namely CAM4 and CAM5, are used for uncoupled simulations whereas coupled simulations are performed with CCSM4 model. Ensemble uncoupled simulations are performed for a 30 year time period whereas the coupled model is integrated for 100 years. Emphasis is placed on the simulation of monsoon precipitation by analyzing the interannual variability of the atmosphere-only simulations and sea surface temperature bias in the coupled simulation. It is found that both CAM4 and CAM5 adequately simulated monsoon precipitation, and considerably reduced systematic errors that occurred in predecessors of CAM4, although both tend to overestimate monsoon precipitation when compared with observations. The onset and cessation of the precipitation annual cycle, along with the mean climatology, are reasonably well captured in their simulations. In terms of monsoon interannual variability and its teleconnection with SST over the Pacific and Indian Ocean, both CAM4 and CAM5 showed modest skill. CAM5, with revised model physics, has significantly improved the simulation of the monsoon mean climatology and showed better skill than CAM4. Using idealized experiments with CAM5, it is seen that the adoption of new boundary layer schemes in CAM5 contributes the most to reduce the monsoon overestimation bias in its simulation. In the CCSM4 coupled simulations, several aspects of the monsoon simulation are improved by the inclusion of air–sea interaction, including the cross-variability of simulated precipitation and SST. A significant improvement is seen in the spatial distribution of monsoon mean climatology where a too-heavy monsoon precipitation, which occurred in CAM4, is rectified. A detailed investigation of this significant precipitation reduction showed that the large systematic cold SST errors in the Northern Indian Ocean reduces monsoon precipitation and delays onset by weakening local evaporation. Sensitivity experiments with CAM4 further confirmed these results by simulating a weak monsoon in the presence of cold biases in the Northern Indian Ocean. It is found that although the air–sea coupling rectifies the major weaknesses of the monsoon simulation, the SST bias in coupled simulations induces significant differences in monsoon precipitation. The overall simulation characteristics demonstrate that although the new model versions CAM4, CAM5 and CCSM4, are significantly improved, they still have major weaknesses in simulating Asian monsoon precipitation.     

The sensitivity of the Indian summer monsoon to a global warming of 2��C with respect to pre-industrial times

In this study the potential future changes in different aspects of the Indian summer monsoon associated with a global warming of 2°C with respect to pre-industrial times are assessed, focussing on the role of the different mechanisms leading to these changes. In addition, these changes as well as the underlying mechanisms are compared to the corresponding changes associated with a markedly stronger global warming exceeding 4.5°C, associated with the widely used SRES A1B scenario. The study is based on two sets of four ensemble simulations with the ECHAM5/MPI-OM coupled climate model, each starting from different initial conditions. In one set of simulations (2020?C2200), greenhouse gas concentrations and sulphate aerosol load have been prescribed in such a way that the simulated global warming dioes not exceed 2°C with respect to pre-industrial times. In the other set of simulations (1860?C2200), greenhouse gas concentrations and sulphate aerosol load have been prescribed according to observations until 2000 and according to the SRES A1B scenario after 2000. The study reveals marked changes in the Indian summer monsoon associated with a global warming of 2°C with respect to pre-industrial conditions, namely an intensification of the summer monsoon precipitation despite a weakening of the large-scale monsoon circulation. The increase in the monsoon rainfall is related to a variety of different mechanisms, with the intensification of the atmospheric moisture transport into the Indian region as the most important one. The weakening of the large-scale monsoon circulation is mainly caused by changes in the Walker circulation with large-scale divergence (convergence) in the lower (uppper) troposphere over the Indian Ocean in response to enhanced convective activity over the Indian Ocean and the central and eastern Pacific and reduced convective activity over the western tropical Pacific. These changes in the Walker circulation induce westerly (easterly) wind anomalies at lower (upper) level in the Indian region. The comparison with the changes in the Indian summer monsoon associated with a global warming of 4.5°C reveals that both the intensification of the monsoon precipitation and the weakening of the large-scale monsoon circulation (particularly in the lower troposphere) are relatively strong (with respect to the magnitude of the projected global warming by the end of the twentieth century for the two scenarios) in the scenario with a global warming of 2°C. The relatively strong intensification of the monsoon rainfall is related to rather strong increases in evaporation over the Arabian Sea and the Bay of Bengal, while a rather weak amplification of the meridional temperature gradient between the Indian Ocean and the land areas to the north contributes to the relatively strong reduction of the large-scale monsoon flow.     

Simulation of Indian summer monsoon: experiments with SSTs

Summary Hindcasts for the Indian summer monsoons (ISMs) of 2002 and 2003 have been produced from an ensemble of numerical simulations performed with a global model by changing SST. Two sets of ensemble simulations have been produced without vegetation: (i) by prescribing the weekly observed SST from ECMWF (European Centre for Medium Range Weather Forecasting) analyses, and (ii) by adding weekly SST anomalies (SSTA) of April to the climatological SST during the simulation period from May to August. For each ensemble, 10 simulations have been realized with different initial conditions that are prepared from ECMWF data with five each from April and May analyses of both the years. The predicted June–July monsoon rainfall over the Indian region shows good agreement with the GPCP (observed) pentad rainfall distribution when 5 member ensemble is taken from May initial conditions. The All-India June–July simulated rainfall time series matches favourably with the observed time series in both the years for the five member ensemble from May initial condition but drifts away from observation with April initial conditions. This underscores the role of initial conditions in the seasonal forecasting. But the model has failed to capture the strong intra-seasonal oscillation in July 2002. Heating over equatorial Indian Ocean for June 2002 in a particular experiment using 29th May 12 GMT as initial conditions shows some intra-seasonal oscillation in July 2002 rainfall, as in observation. Further evaluation of the seasonal simulations from this model is done by calculating the empirical orthogonal functions (EOFs) of the GPCP rainfall over India. The first four EOFs explain more than 80% of the total variance of the observed rainfall. The time series of expansion coefficients (principal components), obtained by projecting on the observed EOFs, provide a better framework for inter-comparing model simulations and their evaluation with observed data. The main finding of this study is that the All-India rainfall from various experiments with prescribed SST is better predicted on seasonal scale as compares to prescribed SST anomalies. This is indicative of a possible useful seasonal forecasts from a GCM at least for the case when monsoon is going to be good. The model responses do not differ much for 2002 and 2003 since the evolution of SST during these years was very similar, hence July rainfall seems to be largely modulated by the other feedbacks on the overall circulation.     

Prediction of summer monsoon rainfall over India using the NCEP climate forecast system

The performance of a dynamical seasonal forecast system is evaluated for the prediction of summer monsoon rainfall over the Indian region during June to September (JJAS). The evaluation is based on the National Centre for Environmental Prediction’s (NCEP) climate forecast system (CFS) initialized during March, April and May and integrated for a period of 9 months with a 15 ensemble members for 25 years period from 1981 to 2005. The CFS’s hindcast climatology during JJAS of March (lag-3), April (lag-2) and May (lag-1) initial conditions show mostly an identical pattern of rainfall similar to that of verification climatology with the rainfall maxima (one over the west-coast of India and the other over the head Bay of Bengal region) well simulated. The pattern correlation between verification and forecast climatology over the global tropics and Indian monsoon region (IMR) bounded by 50°E–110°E and 10°S–35°N shows significant correlation coefficient (CCs). The skill of simulation of broad scale monsoon circulation index (Webster and Yang; WY index) is quite good in the CFS with highly significant CC between the observed and predicted by the CFS from the March, April and May forecasts. High skill in forecasting El Nino event is also noted for the CFS March, April and May initial conditions, whereas, the skill of the simulation of Indian Ocean Dipole is poor and is basically due to the poor skill of prediction of sea surface temperature (SST) anomalies over the eastern equatorial Indian Ocean. Over the IMR the skill of monsoon rainfall forecast during JJAS as measured by the spatial Anomaly CC between forecast rainfall anomaly and the observed rainfall anomaly during 1991, 1994, 1997 and 1998 is high (almost of the order of 0.6), whereas, during the year 1982, 1984, 1985, 1987 and 1989 the ACC is only around 0.3. By using lower and upper tropospheric forecast winds during JJAS over the regions of significant CCs as predictors for the All India Summer Monsoon Rainfall (AISMR; only the land stations of India during JJAS), the predicted mean AISMR with March, April and May initial conditions is found to be well correlated with actual AISMR and is found to provide skillful prediction. Thus, the calibrated CFS forecast could be used as a better tool for the real time prediction of AISMR.     

Projected changes in South Asian summer monsoon by multi-model global warming experiments

South Asian summer monsoon (June through September) rainfall simulation and its potential future changes are evaluated in a multi-model ensemble of global coupled climate models outputs under World Climate Research Program Coupled Model Intercomparison Project (WCRP CMIP3) dataset. The response of South Asian summer monsoon to a transient increase in future anthropogenic radiative forcing is investigated for two time slices, middle (2031–2050) and end of the twenty-first century (2081–2100), in the non-mitigated Special Report on Emission Scenarios B1, A1B and A2 .There is large inter-model variability in the simulation of spatial characteristics of seasonal monsoon precipitation. Ten out of the 25 models are able to simulate space–time characteristics of the South Asian monsoon precipitation reasonably well. The response of these selected ten models has been examined for projected changes in seasonal monsoon rainfall. The multi-model ensemble of these ten models projects a significant increase in monsoon precipitation with global warming. The substantial increase in precipitation is observed over western equatorial Indian Ocean and southern parts of India. However, the monsoon circulation weakens significantly under all the three climate change experiments. Possible mechanisms for the projected increase in precipitation and for precipitation–wind paradox have been discussed. The surface temperature over Asian landmass increases in pre-monsoon months due to global warming and heat low over northwest India intensifies. The dipole snow configuration over Eurasian continent strengthens in warmer atmosphere, which is conducive for the enhancement in precipitation over Indian landmass. No notable changes have been projected in the El Niño–Monsoon relationship, which is useful for predicting interannual variations of the monsoon.     

中国区域气溶胶对东亚夏季风的可能影响（I）：硫酸盐气溶胶的影响

 利用NCAR的新一代GCM CAM3.0模式离线耦合一个气溶胶同化系统,模拟研究了中国区域硫酸盐气溶胶的直接气候效应对东亚夏季风及其降水的影响。结果显示：中国区域硫酸盐气溶胶引起全球平均的直接辐射强迫为-0.25 W/m²,中国内陆约25°N以北普遍降温,而海表温度升高。由此导致海陆温差缩小,东亚夏季风强度减弱,中国地区季风降水明显减少,而尤以积云降水减少起主要作用。硫酸盐气溶胶对中国地区的对流活动起抑制作用。     

On the Spatio-Temporal Variations of the Tropopause Height over India and Indian Summer Monsoon Activity

The spatio-temporal variation of the tropopause height (TH) over the Indian region (5°N-35°N, 70°E-95°E) has been studied using monthly mean TH data, for 22-year period, 1965 to 1986. The study revealed that the stations south of 20° showed maximum TH in April / May and minimum in September. This variation in TH has been attributed to the corresponding variation of average sea surface temperature (SST) over ± 20° latitudinal belt over Indian Ocean, Arabian Sea and Bay of Bengal. Further the stations north of 20°N showed maximum in June and minimum in October/ November. This maximum in TH has primarily been attributed to the increased insolation and convection. Furthermore it is noticed that the anomaly of TH moved northwards during the period April to July.The interannual variability of the Indian Summer Monsoon Activity (ISMA) has been studied in relation to all India mean TH (at 12 GMT) for six months April through September. The composites of mean TH for good and bad monsoon years showed that     

中国区域气溶胶对东亚夏季风的可能影响（I）：硫酸盐气溶胶的影响

利用NCAR的新一代GCM CAM3.0模式离线耦合一个气溶胶同化系统,模拟研究了中国区域硫酸盐气溶胶的直接气候效应对东亚夏季风及其降水的影响。结果显示：中国区域硫酸盐气溶胶引起全球平均的直接辐射强迫为-0.25 W/m²,中国内陆约25°N以北普遍降温,而海表温度升高。由此导致海陆温差缩小,东亚夏季风强度减弱,中国地区季风降水明显减少,而尤以积云降水减少起主要作用。硫酸盐气溶胶对中国地区的对流活动起抑制作用。     

Assessment of CORDEX-SA experiments in representing precipitation climatology of summer monsoon over India

The present work assesses the performance of 11 regional climate simulations in representing the precipitation patterns of summer monsoon over India for the period 1970–2005. These simulations have been carried out under Coordinated Regional Climate Downscaling Experiment–South Asia (CORDEX-SA) project. The regional climate models (RCMs) have been inter-compared as well as evaluated against the observation to identify the common weaknesses and differences between them. For this, a number of statistical analysis has been carried out to compare the model precipitation field with the corresponding observation. Model uncertainty has been also evaluated through bias studies and analysis of the spread in the ensemble mean (hereafter, ensemble). The models which perform better than the rest are identified and studied to look for any improvement in the ensemble performance. These better performing experiments (best RCM experiments) are further assessed over the monsoon core region (MCR) of India. This has been done to understand how well the models perform in a spatially homogeneous zone of precipitation which is considered to be a representative region of Indian summer monsoon characteristics. Finally, an additional analysis has been done to quantify the skill of models based on two different metrics—performance and convergence including a combination of the two. The experiment with regional model RegCM4 forced with the global model GFDL-ESM2M shows the highest combined mean skill in capturing the seasonal mean precipitation. In general, a significant dry bias is found over a larger part of India in all the experiments which seems most pronounced over the central Indian region. Ensemble on an average tends to outperform many of the individual experiments with bias of smaller magnitude and an improved spatial correlation compared with the observation. Experiments which perform better over India improve the results but only slightly in terms of agreement among experiments and bias.     

人为气溶胶对中国东部冬季风影响的模拟研究

采用美国国家大气研究中心(NCAR)的公共大气模式CAM5.1研究了人为气溶胶排放增加对中国东部冬季风的影响,同时通过对比中国东部地区不同人为气溶胶排放源的敏感性试验结果,探讨了人为硫酸盐、黑碳及总人为气溶胶(硫酸盐+黑碳)增加对东亚冬季风的影响。结果表明:冬季硫酸盐气溶胶排放增加的直接和第一间接效应减少了到达地表的短波辐射通量,引起了陆地地表和对流层低层降温,海平面气压升高,增加了海陆间气压梯度,使得东亚冬季风增强。其第二间接效应导致中国南部大尺度降水率减少;黑碳气溶胶排放增加导致到达地表的短波辐射通量减少和大气中短波辐射通量增加,其半直接效应部分抵消了直接效应,故地表温度变化微小且不显著。加热的对流层低层导致中国南部对流活动和对流降水率增加;总人为气溶胶排放增加导致的大气温度变化表现为弱的降温作用,引起中国北部对流和大尺度降水率减少,而南部对流降水率增加。总人为气溶胶和黑碳气溶胶排放增加是导致中国北(南)部的东亚冬季风增强(减弱)的重要因素。     

The role of land-surface processes in modulating the Indian monsoon annual cycle

The annual cycle of solar radiation, together with the resulting land–ocean differential heating, is traditionally considered the dominant forcing controlling the northward progression of the Indian monsoon. This study makes use of a state-of-the-art atmospheric general circulation model in a realistic configuration to conduct “perpetual” experiments aimed at providing new insights into the role of land–atmosphere processes in modulating the annual cycle of precipitation over India. The simulations are carried out at three important stages of the monsoon cycle: March, May, and July. Insolation and SSTs are held fixed at their respective monthly mean values, thus eliminating any external seasonal forcing. In the perpetual May experiment both precipitation and circulation are able to considerably evolve only by regional internal land–atmosphere processes and the mediation of soil hydrology. A large-scale equilibrium state is reached after approximately 270 days, closely resembling mid-summer climatological conditions. As a result, despite the absence of external forcing, intense and widespread rains over India are able to develop in the May-like state. The interaction between soil moisture and circulation, modulated by surface heating over the northwestern semi-arid areas, determines a slow northwestward migration of the monsoon, a crucial feature for the existence of desert regions to the west. This also implies that the land–atmosphere system in May is far from being in equilibrium with the external forcing. The inland migration of the precipitation front comprises a succession of large-scale 35–50 day coupled oscillations between soil moisture, precipitation, and circulation. The oscillatory regime is self-sustained and entirely due to the internal dynamics of the system. In contrast to the May case, minor changes in the land–atmosphere system are found when the model is initialized in March and, more surprisingly, in July, the latter case further emphasizing the role of northwestern surface heating.     

中国区域气溶胶对东亚夏季风的可能影响(Ⅱ)：黑碳气溶胶及其与硫酸盐气溶胶的综合影响

利用NCAR的新一代GCM CAM3.0模式耦合一个气溶胶同化系统,研究了中国区域黑碳气溶胶的直接气候效应。结果显示,中国区域黑碳气溶胶引起全球平均辐射强迫为0.13 W/m2,导致除了青藏高原和广西以外的中国大部分地区降温,其中东北、四川和内蒙古中北部降温最显著。由此造成海陆温差缩小,气压差降低,从而总体上使东亚夏季风减弱。但与硫酸盐气溶胶的影响相比,黑碳气溶胶使季风减弱的程度较小,长江中下游地区的降水有所增加。黑碳气溶胶加强了中国东南部地区的对流活动,这与硫酸盐气溶胶的作用相反。同时,探讨了中国区域硫酸盐和黑碳气溶胶的综合直接气候效应。结果表明,硫酸盐和黑碳气溶胶的综合作用与仅有硫酸盐气溶胶的情形十分相似,降水变化的区域也和硫酸盐的保持一致。     

中国区域气溶胶对东亚夏季风的可能影响(Ⅱ)：黑碳气溶胶及其与硫酸盐气溶胶的综合影响

 利用NCAR的新一代GCM CAM3.0模式耦合一个气溶胶同化系统,研究了中国区域黑碳气溶胶的直接气候效应。结果显示,中国区域黑碳气溶胶引起全球平均辐射强迫为0.13 W/m2,导致除了青藏高原和广西以外的中国大部分地区降温,其中东北、四川和内蒙古中北部降温最显著。由此造成海陆温差缩小,气压差降低,从而总体上使东亚夏季风减弱。但与硫酸盐气溶胶的影响相比,黑碳气溶胶使季风减弱的程度较小,长江中下游地区的降水有所增加。黑碳气溶胶加强了中国东南部地区的对流活动,这与硫酸盐气溶胶的作用相反。同时,探讨了中国区域硫酸盐和黑碳气溶胶的综合直接气候效应。结果表明,硫酸盐和黑碳气溶胶的综合作用与仅有硫酸盐气溶胶的情形十分相似,降水变化的区域也和硫酸盐的保持一致。     

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.     

Delay in the onset of South Asian summer monsoon induced by local black carbon in an AGCM

The influence of local black carbon (BC) on monthly mean precipitation and the associated circulation in South Asia is investigated. The results show that the amplified shortwave radiative heating rate by BC absorption enhances convective activity near the Himalayas and the southern Tibetan Plateau with increased rainfall anomalies in April and May, which is consistent with previous studies. However, the enhanced vertical motions and the precipitation ultimately lead towards cooling of the lower troposphere especially in May, which is extended till June. This favored negative rainfall responses in June, implying delay in the onset of South Asian summer monsoon rainfalls over Arabian Sea and western parts of the subcontinent. The negative precipitation response is further associated with the prevailing anomalous high pressure and the anticyclonic wind circulations induced by BC. Thus, we present here one different feature associated with BC-induced elevated heat pump-like circulations in South Asia.     

A coupled model study on the intensification of the Asian summer monsoon in IPCC SRES Scenarios

The Asian summer monsoon is an important part of the climate system. Investigating the response of the Asian summer monsoon to changing concentrations of greenhouse gases and aerosols will be meaningful to understand and predict climate variability and climate change not only in Asia but also globally. In order to diagnose the impacts of future anthropogenic emissions on monsoon climates, a coupled general circulation model of the atmosphere and the ocean has been used at the Max-Planck-Institute for Meteorology. In addition to carbon dioxide, the major well mixed greenhouse gases such as methane, nitrous oxide, several chlorofluorocarbons, and CFC substitute gases are prescribed as a function of time. The sulfur cycle is simulated interactively, and both the direct aerosol effect and the indirect cloud albedo effect are considered. Furthermore, changes in tropospheric ozone have been pre-calculated with a chemical transport model and prescribed as a function of time and space in the climate simulations. Concentrations of greenhouse gases and anthropogenic emissions of sulfur dioxide are prescribed according to observations （1860-1990） and projected into the future （1990-2100） according to the Scenarios A2 and B2 in Special Report on Emissions Scenarios （SRES, Nakcenovic et al., 2000） developed by the Intergovernmental Panel on Climate Change （IPCC）. It is found that the Indian summer monsoon is enhanced in the scenarios in terms of both mean precipitation and interannual variability. An increase in precipitation is simulated for northern China but a decrease for the southern part. Furthermore, the simulated future increase in monsoon variability seems to be linked to enhanced ENSO variability towards the end of the scenario integrations.     

Radiative effects of black carbon aerosols on Indian monsoon: a study using WRF-Chem model

The Weather Research and Forecasting model with Chemistry (WRF-Chem) is utilized to examine the radiative effects of black carbon (BC) aerosols on the Indian monsoon, for the year 2010. Five ensemble simulations with different initial conditions (1st to 5th December, 2009) were performed and simulation results between 1st January, 2010 to 31st December, 2010 were used for analysis. Most of the BC which stays near the surface during the pre-monsoon season gets transported to higher altitudes with the northward migration of the Inter Tropical Convergence Zone (ITCZ) during the monsoon season. In both the seasons, strong negative SW anomalies are present at the surface along with positive anomalies in the atmosphere, which results in the surface cooling and lower tropospheric heating, respectively. During the pre-monsoon season, lower troposphere heating causes increased convection and enhanced meridional wind circulation, bringing moist air from Indian Ocean and Bay of Bengal to the North-East India, leading to increased rainfall there. However, during the monsoon season, along with cooling over the land regions, a warming over the Bay of Bengal is simulated. This differential heating results in an increased westerly moisture flux anomaly over central India, leading to increased rainfall over northern parts of India but decreased rainfall over southern parts. Decreased rainfall over southern India is also substantiated by the presence of increased evaporation over Bay of Bengal and decrease over land regions.