A study on the distinct features of the Asian summer monsoon during the years of extreme monsoon activity over India

Summary In this paper, interseasonal characteristics of the Asian summer monsoon in the years of 1987 and 1988 are studied as 1987 is characterized by a large deficiency of monsoon rainfall (drought) and that of 1988 by a large excess monsoon rainfall (flood) over India. In order to compare the similarities and differences seen in the large scale dynamics and energetics of the Asian summer monsoon during the years of extreme monsoon activity, uninitialized analyses (12 Z) of the European Centre for Medium Range Weather Forecasts (ECMWF), U.K. are utilized in this study for the summer monsoon seasons of 1987 and 1988.It is found that the excess rainfall season (1988) is characterized by much stronger tropical easterly jet (TEJ) associated with the upper tropospheric easterlies and the East African low level jet (Somali Jet) associated with lower tropospheric westerlies. Such a feature mainly determines the strength of the reverse Hadley circulation which normally covers the South Asian continent during the northern summer. Further, the energetics of the TEJ show that the monsoon of 1988 has comparatively stronger zones of kinetic energy flux divergence (convergence) at its entrance (exit) regions. These zones of kinetic energy flux divergence are largely maintained by the adiabatic processes over the strong kinetic energy flux divergence zones over the Bay of Bengal and east central Arabian Sea as compared to that of 1987. Apart from this, both the zonal and meridional components of the ageostrophic flows are found to be stronger during 1988 monsoon season. Analysis of the vertically integrated thermodynamical features of the monsoon indicate that the monsoon of 1988 was characterized by an excess import of heat and moisture into the monsoon atmosphere as compared to that of 1987. Further, from the quantitative estimation of certain significant heat and moisture budget parameters during the contrasting monsoon seasons of 1987 and 1988, it becomes evident that considerable differences exist in the quantities of adiabatic production of heat energy, diabatic heating and the moisture source/sink.With 13 Figures     

A study on the variability of atmospheric and oceanic processes over the Arabian Sea during contrasting monsoons

Summary The atmospheric and oceanic conditions associated with the southwest monsoon during the contrasting monsoon years of 2002 and 2003 over the Arabian Sea have been analyzed in the present study. Early onset of southwesterlies and reduced net heat gain due to low solar radiation were responsible for low sea-surface temperatures (SSTs) over the Arabian Sea during 2002 pre-monsoon (particularly in May). Conversely, light winds and an increased net heat gain set up the pre-monsoon warming in 2003. The development and intensification of deep convection over a large area of the Arabian Sea prior to the onset of the monsoon was observed during 2003, but was absent in 2002. Weak cross equatorial flow and a weak low level jet over the Arabian Sea reduced moisture transport towards the Indian subcontinent in July 2002. This scenario helped to contribute to a prolonged break in monsoon conditions during July. However, no such break in conditions occurred during July 2003. In 2002, the summer monsoon cooling of the Arabian Sea occurred well before July, whereas in 2003 cooling occurred during July. Estimates of wind driven Ekman (horizontal) and vertical transports showed maximum values in the month of June (July) in 2002 (2003). These estimates clearly show the importance of horizontal and vertical advection in the summer cooling of the Arabian Sea. During the southwest monsoon period, the Arabian Sea was warmer in 2003 than in 2002. Late onset of the southwesterlies in June, late cooling of the Arabian Sea in July, and downwelling Rossby wave propagation were responsible for the warm SSTs in 2003. Weak wind stress curl in July dampened the westward propagating sea surface height anomaly signals (Rossby waves) before they reached the western Arabian Sea in 2002, whereas, in 2003 strong wind stress curl enhanced Rossby wave propagation. During the summer monsoon period, subsurface temperatures in the south central Arabian Sea were warmer in 2003 than in 2002, particularly in July and August. Strong Ekman convergence, solar penetration, and downwelling (downward velocities) are responsible for the enhanced subsurface warming in 2003.     

Atmospheric Moisture Distribution and Transport over the Tibetan Plateau and the Impacts of the South Asian Summer Monsoon

In this study, by using the ECMWF ERA-Interim reanalysis data from 1979 to 2010, the spatial distribution and transport of total atmospheric moisture over the Tibetan Plateau（TP） are analyzed, together with the associated impacts of the South Asian summer monsoon（SASM）. Acting as a moisture sink in summer, the TP has a net moisture flux of 2.59× 107kg s 1during 1979–2010, with moisture supplies mainly from the southern boundary along the latitude belts over the Bay of Bengal and the Arabian Sea. The total atmospheric moisture over the TP exhibits significant diferences in both spatial distribution and transport between the monsoon active and break periods and between strong and weak monsoon years. Large positive（negative） moisture anomalies occur over the southwest edge of the TP and the Arabian Sea, mainly due to transport of easterly（westerly） anomalies during the monsoon active（break） period. For the whole TP region, the total moisture supply is more strengthened than the climatological mean during the monsoon active period, which is mainly contributed by the transport of moisture from the south edge of the TP. During the monsoon break period, however, the total moisture supply to the TP is slightly weakened. In addition, the TP moisture sink is also strengthened（weakened） in the strong（weak） monsoon years, mainly attributed by the moisture transport in the west-east directions. Our results suggest that the SASM has exerted great impacts on the total atmospheric moisture and its transport over the TP through adjusting the moisture spatial distribution.     

南海夏季风期间水汽输送的气候特征

通过分析NCEP/NCAR 1973～1998年(共26年)4～8月的再分析比湿场和风场资料,研究了南海夏季风期间的水汽输送特征.夏季,东亚上空水汽水平输送特征在各月有很大差异,这是夏季风环流系统演变的结果.孟加拉湾南部地区是中国长江中下游和南海地区重要的水汽源地,来自上游孟加拉湾南部地区的水汽输送对南海季风的爆发具有重要意义.经向水汽输送主要有利于20～30°N之间华南地区的水汽辐合.从总的收支看,南海地区是一个水汽汇区.南海季风爆发早晚年的水汽输送通道存在明显差别.在爆发偏早年,从赤道印度洋到南海地区的输送通道建立早且维持时间长,4～5月南海易成为水汽辐合区;在偏晚年,南海地区水汽则是辐散的,不利于形成季风性降水.南海季风爆发早晚年与长江中下游旱涝年的水汽输送有一定联系.     

青藏高原汛期降水异常空间分布及水汽配置

青藏高原汛期(5—9月)降水具有南北反相的空间分布特征,利用青藏高原67个台站1967—2008年逐月降水资料,分别讨论了汛期各月降水的主要空间分布型以及初夏(5—6月)和盛夏(7—8月)对应的水汽配置和环流异常.结果表明:初夏高原降水以南北反相型(North-South Reverse Type,NSRT)为主,全区一致型(Whole Region Consistent Type,WRCT)次之;盛夏高原降水以WRCT为主.高原降水呈现NSRT分布时,初夏水汽由高原南部输向北部,而盛夏高原北部为水汽辐合区,南部为水汽辐散区.高原降水呈现WRCT分布时,初夏高原水汽主要来自西太平洋,盛夏水汽主要来自阿拉伯海向东转向的水汽输送,该水汽输送由高原西南地区进入高原.在500 hPa位势高度场上,初夏(盛夏)降水两种主要空间分布型的位势高度差异以经(纬)向差异为主,且影响高原降水异常分布的系统多为深厚系统.     

Moisture Transport in the Asian Summer Monsoon Region and Its Relationship with Summer Precipitation in China

The characteristics of moisture transport over the Asian summer monsoon region and its relationship with summer precipitation in China are examined by a variety of statistical methods using the NCEP/NC AR reanalysis data for 1948-2005.The results show that:1) The zonal-mean moisture transport in the Asian monsoon region is unique because of monsoon activities.The Asian summer monsoon region is a dominant moisture sink during summer.Both the Indian and East Asian monsoon areas have their convergence cente...     

Circumglobal wave train and the summer monsoon over northwestern India and Pakistan: the explicit role of the surface heat low

This study examines the influence of the mid-latitude circulation on the surface heat low (HL) and associated monsoon rainfall over northwestern India and Pakistan using the ERA40 data and high resolution (T106L31) climate model ECHAM5 simulation. Special emphasis is given to the surface HL which forms over Pakistan and adjoining areas of India, Iran and Afghanistan during the summer season. A heat low index (HLI) is defined to depict the surface HL. The HLI displays significant correlations with the upper level mid-latitude circulation over western central Asia and low level monsoon circulation over Arabian Sea and acts as a bridge connecting the mid-latitude wave train to the Indian summer monsoon. A time-lagged singular value decomposition analysis reveals that the eastward propagation of the mid-latitude circumglobal wave train (CGT) influences the surface pressure anomalies over the Indian domain. The largest low (negative) pressure anomalies over the western parts of the HL region (i.e., Iran and Afghanistan) occur in conjunction with the upper level anomalous high that develops over western-central Asia during the positive phase of the CGT. The composite analysis also reveals a significant increase in the low pressure anomalies over Iran and Afghanistan during the positive phase of CGT. The westward increasing low pressure anomalies with its north?Csouth orientation provokes enormous north?Csouth pressure gradient (lower pressure over land than over sea). This in turn enables the moist southerly flow from the Arabian Sea to penetrate farther northward over northwestern India and Pakistan. A monsoon trough like conditions develops over northwestern India and Pakistan where the moist southwesterly flow from the Arabian Sea and the Persian Gulf converge. The convergence in association with the orographic uplifting expedites convection and associated precipitation over northwestern India and Pakistan. The high resolution climate model ECHAM5 simulation also underlines the proposed findings and mechanism.     

AN EAST ASIAN SUBTROPICAL SUMMER MONSOON INDEX DEFINED BY MOISTURE TRANSPORT

Using daily NCEP/NCAR reanalysis dataset and observation rainfall data in China for the 1971- 2000 period, a subtropical summer monsoon index has been defined by meridional moisture transport of the total atmosphere column. Results show that the subtropical summer monsoon index defined by the difference of meridional moisture transport between South China and North China can be used to describe the intensity of the subtropical summer monsoon. High (low) index is corresponding to strong (weak) subtropical summer monsoon. And the new index is well related to the summer rainfall over the middle and lower reaches of Yangtze River. In addition, the convergence of moisture transport from the west Pacific via the South China Sea and that from the North China may be responsible for the anomalously excessive summer rainfall over the middle and lower reaches of Yangtze River.     

Dependence of Indian monsoon rainfall on moisture fluxes across the Arabian Sea and the impact of coupled model sea surface temperature biases

The Arabian Sea is an important moisture source for Indian monsoon rainfall. The skill of climate models in simulating the monsoon and its variability varies widely, while Arabian Sea cold sea surface temperature (SST) biases are common in coupled models and may therefore influence the monsoon and its sensitivity to climate change. We examine the relationship between monsoon rainfall, moisture fluxes and Arabian Sea SST in observations and climate model simulations. Observational analysis shows strong monsoons depend on moisture fluxes across the Arabian Sea, however detecting consistent signals with contemporaneous summer SST anomalies is complicated in the observed system by air/sea coupling and large-scale induced variability such as the El Ni?o-Southern Oscillation feeding back onto the monsoon through development of the Somali Jet. Comparison of HadGEM3 coupled and atmosphere-only configurations suggests coupled model cold SST biases significantly reduce monsoon rainfall. Idealised atmosphere-only experiments show that the weakened monsoon can be mainly attributed to systematic Arabian Sea cold SST biases during summer and their impact on the monsoon-moisture relationship. The impact of large cold SST biases on atmospheric moisture content over the Arabian Sea, and also the subsequent reduced latent heat release over India, dominates over any enhancement in the land-sea temperature gradient and results in changes to the mean state. We hypothesize that a cold base state will result in underestimation of the impact of larger projected Arabian Sea SST changes in future climate, suggesting that Arabian Sea biases should be a clear target for model development.     

2007年夏季淮河流域洪涝与亚洲地区大气低频振荡的关系

利用NCEP/NCAR再分析资料和我国地面观测站的逐日降水资料, 研究了2007年夏季淮河流域洪涝与亚洲地区大气低频振荡的联系, 通过分析研究表明: 2007年夏季淮河流域降水低频振荡的主要周期是30～70天, 该低频序列的方差大约占了总方差的47%; 在降水低频振荡的位相5～8 (1～4), 亚洲季风区从阿拉伯海北部经孟加拉湾到我国南海地区, 以及我国淮河流域经渤海到日本地区主要受低频热源 (热汇) 的控制; 并且在极大降水位相7, 我国东部地区 (10°N～45°N, 110°E～120°E), 从北到南, 〈Q1〉低频分量的分布呈负正相间的低频热汇、 热源、 热汇、 热源形式(位相3则呈相反的分布形式); 在位相5～8 (1～4), 亚洲季风区〈Q1〉低频分量的分布有利于 (不利于) 气流向淮河流域汇合并形成辐合上升; 受低频环流变化的影响, 在位相5～8, 大量的水汽被输送到淮河流域, 辐合上升形成降水; 相反, 在位相1～4, 来自西太平洋上的水汽在该地区辐散, 不利于降水的发生。     

A Study on Water Vapor Transport and Budget of Heavy Rain in Northeast China

The characteristics of moisture transport and budget of widespread heavy rain and local heavy rain events in Northeast China are studied using the NCEP--NCAR reanalysis 6-hourly and daily data and daily precipitation data of 200 stations in Northeast China from 1961--2005. The results demonstrate that during periods with widespread heavy rain in Northeast China, the Asian monsoon is very active and the monsoonal northward moisture transport is strengthened significantly. The widespread heavy rainfall obtains enhanced water vapor supply from large regions where the water vapor mainly originates from the Asian monsoon areas, which include the East Asian subtropical monsoon area, the South China Sea, and the southeast and southwest tropical monsoon regions. There are several branches of monsoonal moisture current converging on East China and its coastal areas, where they are strengthened and then continue northward into Northeast China. Thus, the enhanced northward monsoonal moisture transport is the key to the widespread heavy rain in Northeast China. In contrast, local heavy rainfall in Northeast China derives water vapor from limited areas, transported by the westerlies. Local evaporation also plays an important role in the water vapor supply and local recycling process of moisture. In short, the widespread heavy rains of Northeast China are mainly caused by water vapor advection brought by the Asian monsoon, whereas local heavy rainfall is mainly caused by the convergence of the westerly wind field.     

我国东、西部夏季水汽输送特征及其差异

本文利用ERA-40再分析每日资料分析了我国东部季风区与西北干旱—半干旱区夏季1971～2000年气候平均的水汽输送特征及其差异, 分析结果表明我国东部季风区与西北干旱—半干旱区夏季气候平均的水汽输送特征有明显的差异。由于亚洲夏季风从孟加拉湾、 南海和热带西太平洋输送大量水汽到我国东部季风区, 故在东部季风区夏季经向水汽输送通量比纬向水汽输送通量大。而西北干旱—半干旱区受中纬度西风带的影响, 夏季纬向水汽输送通量比经向水汽输送通量大, 且此区域夏季无论纬向或者经向水汽输送通量均比东部季风区的水汽输送通量小一量级。并且, 分析结果还表明: 我国东部季风区由于湿度大, 故夏季水汽输送通量的散度不仅依赖于湿度平流, 而且依赖于风场的辐合、 辐散, 而西北干旱—半干旱区夏季水汽输送通量的散度主要依赖于湿度平流。此外, 分析结果还表明了我国东部季风区的水分平衡与西北干旱—半干旱区的水分平衡也有明显的不同。     

Temperature variability over the Indian Ocean and its relationship with Indian summer monsoon rainfall

Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.     

年代际气候预测系统IAP DecPreS的海洋同化试验在西北太平洋的海温偏差及其对亚洲夏季风的影响

本文分析了中国科学院大气物理研究所年代际气候预测系统IAP DecPreS的海洋同化试验（简称EnOI-IAU试验）在西北太平洋地区的海表面温度（SST）年循环的模拟技巧,并通过对比IAP DecPreS系统自由耦合历史气候模拟试验结果,在包含海气耦合过程的框架下讨论了耦合模式中西北太平洋夏季SST模拟差异,及其对亚洲季风区夏季季风降水模拟的影响。结果表明,EnOI-IAU试验较好地模拟出了西北太平洋各个季节的SST空间分布,并显著减小了原存在于历史气候模拟试验中持续全年的SST冷偏差。混合层热收支诊断分析表明,包含同化过程在内的海洋过程的模拟差异对西北太平洋海温的模拟提升有重要贡献。夏季,EnOI-IAU试验模拟的印度季风伴随的低层西风较观测偏东、偏强,且高估了赤道西太平洋区域的降水量值、低估了印度洋区域的降水量值。水汽收支分析显示,气旋式环流异常造成的水汽辐合异常是造成亚洲季风区降水模拟差异的主要原因。研究表明,较之历史模拟试验,EnOI-IAU试验中夏季西北太平洋地区SST增暖造成局地对流增强,进而使得局地产生异常上升运动,水汽辐合增强,造成西北太平洋地区降水模拟偏多,激发出低层西风异常及赤道外气旋式环流异常。该低层西风异常导致了北印度洋地区低层辐散异常,减小了原存在于历史试验中印度洋地区的正降水偏差。西北太平洋气旋式环流异常一方面增强了印度夏季风伴随的低层西风,使得更多的水汽从阿拉伯海输送到西太平洋暖池区域,增强了该区域的降水量;另一方面,该气旋式环流异常减小了历史模拟试验中中国南部区域偏强的低层风速,进而提升了模式对东亚低层西南风的模拟能力。     

On the relationship between Indian Ocean Dipole events and the precipitation of Pakistan

This study investigated the relationship between the Indian Ocean Dipole (IOD) and the precipitation of Pakistan using data for the period of 1958–2010. The long-term evolution of the IOD index did not show interannual patterns similar to those of the annual precipitation of Pakistan. No linkage between the co-occurring trends of the IOD and the precipitation was traced during the period of investigation. The correlation between the IOD and the precipitation of Pakistan indicated a noteworthy impact over the monsoonal regions, especially the coastal area and the western region of Pakistan, which showed a significant positive correlation between the IOD index and annual and summer precipitation. A significant positive relationship was also revealed between the precipitation of the Balochistan Plateau and the IOD index for the summer monsoon season. No connection was observed between the IOD and the precipitation of the northern regions and the upper Indus Plain of Pakistan. Positive phases of the IOD have been noted to occur along with surplus precipitation during active monsoon conditions. The southeasterly wind moves from the Arabian Sea and transports additional moisture from the Arabian Sea to the coastal and southwestern parts of Pakistan during positive phases of the IOD.

     

Surface-level moisture transport over the Indian Ocean during southwest monsoon months using NOAA/HIRS data

Surface-level moisture transport over the Indian Ocean has been computed using NOAA/HIRS data for the years 1980, 1981 and 1984. The global relation between monthly mean surface-level humidity and precipitable water (Liu, 1986) has been applied for the computation of surface-level humidity using monthly mean satellite-derived water vapour. The monthly mean surface wind fields over the Indian ocean provided by Florida State University have been used for the surface-level moisture flux computations. Our analysis indicates net positive surface-level moisture flux divergence over the Arabian Sea and negative moisture flux divergence over the Bay of Bengal. It has also been found that evaporation over the Arabian Sea is a variable quantity and forms a significant part of the net moisture budget over the Arabian Sea. The relative contribution of cross-equatorial flux and evaporation from the Arabian Sea has been studied for all three years.     

The Impacts of Moisture Transport of East Asian Monsoon on Summer Precipitation in Northeast China

By using the ECMWF reanalysis daily data and daily precipitation data of 80 stations in Northeast China from 1961 to 2002, the impacts of moisture transport of East Asian summer monsoon on the summer precipitation anomaly in Northeast China, and the relationship between the variation of moisture budget and the establishment of East Asian summer monsoon in this region are studied. The results demonstrate that the moisture of summer precipitation in Northeast China mainly originates from subtropical, South China Sea, and South Asia monsoon areas. East China and its near coastal area are the convergent region of the monsoonal moisture currents and the transfer station for the currents continually moving northward. The monsoonal moisture transport, as an important link or bridge, connects the interaction between middle and low latitude systems. In summer half year, there is a moisture sink in Northeast China where the moisture influx is greater than outflux. The advance transport and accumulation of moisture are of special importance to pentad time scale summer precipitation. The onset, retreat, and intensity change of the monsoonal rainy season over Northeast China are mainly signified by the moisture input condition along the southern border of this area. The establishment of East Asian summer monsoon in this area ranges from about 10 July to 20 August and the onset in the west is earlier than that in the east. The latitude that the monsoon can reach is gradually northward from west to east, reaching 50°N within longitude 120°-135°E. In summer, the difference of air mass transport between summers with high and low rainfall mainly lies in whether more air masses originating from lower latitudes move northward through East China and its coastal areas, consequently transporting large amounts of hot and humid air into Northeast China.     

中国东部季风区夏季四类雨型的水汽输送特征及差异

利用1951~2015年NCEP/NCAR再分析逐日资料和中国160站月降水观测资料,及中国东部季风区夏季四类雨型（北方型、中间型、长江型和华南型）的划分结果,分析了东亚水汽输送与中国东部季风区夏季降水的关系,比较了四类雨型的水汽输送、收支特征及其差异,结果表明:（1）夏季影响中国东部季风区的水汽通道主要有以下6条:印度洋通道,表征印度季风区偏南的西风水汽输送;高原南侧通道,表征印度季风区偏北的西风水汽输送;太平洋通道,表征由西太平洋副热带高压（副高）带来的西太平洋的水汽;西风带通道,表征西风带的水汽输送;孟加拉湾通道,表征来自孟加拉湾向北的水汽输送;南海通道,表征来自印度洋和孟加拉湾在中南半岛转向及来自南海的水汽;与中国东部不同地区降水异常相联系的水汽通道存在明显的差异,且同一条水汽通道在夏季不同阶段与降水的关系也不尽相同。（2）四类雨型的水汽输送和收支特征有明显的差异,华北盛夏降水主要受亚洲季风水汽输送的影响,其次是西风带水汽输送,北方型年二者往往偏强,尤其是季风水汽输送增加一倍以上,贡献也明显增加,20世纪70年代中期之后,季风水汽输送显著减弱,西风带水汽输送的重要性相对增大;淮河流域夏季降水异常主要受太平洋通道水汽输送异常的主导,其次是高原南侧通道水汽输送,二者偏强并在淮河流域辐合时,淮河流域降水偏多形成中间型年;长江中下游地区夏季降水主要受太平洋通道水汽输送异常的主导,长江型年,副高西北侧的西南水汽输送异常加强,并与北方冷空气异常在长江中下游地区辐合,区域为正的水汽净收支;华南地区夏季降水则受印度洋通道、太平洋通道及南海通道的共同影响,当三条通道异常偏强,水汽与北方冷空气在华南地区辐合,形成华南型年。本研究所得结论加深了我们对四类雨型形成机理的认识,并为汛期主雨带的预测提供了参考。     

Simulation of the variability and extremes of daily rainfall during the Indian summer monsoon for present and future times in a global time-slice experiment

In this study the simulation of the variability and extremes of daily rainfall during the Indian summer monsoon for the present-day and the future climate is investigated. This is done on the basis of a global time-slice experiment (TSL) with the ECHAM4 atmospheric general circulation model (GCM) at a high horizontal resolution of T106. The first time-slice (period: 1970–1999) represents the present-day climate and the second (2060–2089) the future climate. Moreover, observational rainfall data from the Global Precipitation Climatology Project (GPCP, 1997–2002) and rainfall data from the ECMWF re-analysis (ERA, 1958–2001) are considered. ERA reveals serious deficiencies in its representation of the variability and extremes of daily rainfall during the Indian summer monsoon. These are mainly a severe overestimation of the frequency of wet days over the oceans and in the Himalayas, where also the rainfall intensity is overestimated. Further, ERA shows unrealistically heavy rainfall events over the tropical Indian Ocean. The ECHAM4 atmospheric GCM at a horizontal resolution of T106, on the other hand, simulates the variability and extremes of daily rainfall in good agreement with the observations. The only marked deficiencies are an underestimation of the rainfall intensity on the west coast of the Indian peninsula and in Bangladesh, an overestimation over the tropical Indian Ocean, due to an erroneous northwestward extension of the tropical convergence zone, and an overestimation of the frequency of wet days in Tibet. Further, heavy rainfall events are relatively strong in the centre of the Indian peninsula. For the future, TSL predicts large increases in the rainfall intensity over the tropical Indian Ocean as well as in northern Pakistan and northwest India, but decreases in southern Pakistan, in the centre of the Indian peninsula, and over the western part of the Bay of Bengal. The frequency of wet days is markedly increased over the tropical Indian Ocean and decreased over the northern part of the Arabian Sea and in Tibet. The intensity of heavy rainfall events is generally increased in the future, with large increases over the Arabian Sea and the tropical Indian Ocean, in northern Pakistan and northwest India as well as in northeast India, Bangladesh, and Myanmar.     

South Asian summer monsoon precipitation variability: Coupled climate model simulations and projections under IPCC AR4

Summary South Asian summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models assessed as part of the Intergovernmental Panel on Climate Change Fourth Assessment. Out of the 22 models examined, 19 are able to capture the maximum rainfall during the summer monsoon period (June through September) with varying amplitude. While two models are unable to reproduce the annual cycle well, one model is unable to simulate the summer monsoon season. The simulated inter-annual variability from the 19 models is examined with respect to the mean precipitation, coefficient of variation, long-term trends and the biennial tendency. The model simulated mean precipitation varies from 500 mm to 900 mm and coefficient of variation from 3 to 13%. While seven models exhibit long-term trends, eight are able to simulate the biennial nature of the monsoon rainfall. Six models, which generate the most realistic 20th century monsoon climate over south Asia, are selected to examine future projections under the doubling CO₂ scenario. Projections reveal a significant increase in mean monsoon precipitation of 8% and a possible extension of the monsoon period based on the multi-model ensemble technique. Extreme excess and deficient monsoons are projected to intensify. The projected increase in precipitation could be attributed to the projected intensification of the heat low over northwest India, the trough of low pressure over the Indo-Gangetic plains, and the land–ocean pressure gradient during the establishment phase of the monsoon. The intensification of these pressure systems could be attributed to the decline in winter/spring snowfall. Furthermore, a decrease of winter snowfall over western Eurasia is also projected along with an increase of winter snowfall over Siberia/eastern Eurasia. This projected dipole snow configuration during winter could imply changes in mid-latitude circulation conducive to subsequent summer monsoon precipitation activity. An increase in precipitable water of 12–16% is projected over major parts of India. A maximum increase of about 20–24% is found over the Arabian Peninsula, adjoining regions of Pakistan, northwest India and Nepal. Although the projected summer monsoon circulation appears to weaken, the projected anomalous flow over the Bay of Bengal (Arabian Sea) will support oceanic moisture convergence towards the southern parts of India and Sri Lanka (northwest India and adjoining regions). The ENSO-Monsoon relationship is also projected to weaken.