前冬印度洋海盆一致模对ENSO衰减期华南春季降水的影响

利用逐月台站观测降水、HadISST1.1海温和ERA5大气再分析资料,研究了前冬印度洋海盆一致模（Indian Ocean Basin,IOB）对华南春季降水（SCSR）与ENSO关系的影响,并分析了IOB通过调控ENSO环流异常进而影响SCSR的可能机制。结果表明:当前冬El Ni?o（La Ni?a）与IOB暖（冷）位相同时发生时,SCSR显著增多（减少）;而当El Ni?o或La Ni?a单独发生而IOB处于中性时,SCSR并无明显多寡倾向。其原因在于,当El Ni?o与IOB暖相位并存时,前冬热带印度洋和赤道中东太平洋均为正海温异常（Sea-Surface Temperature Anomaly,SSTA）,且印度洋SSTA强度可一直维持至春季。在对流层低层,春季赤道中东太平洋的正SSTA激发出异常西北太平洋反气旋（Western North Pacific Anticyclone,WNPAC）。而热带印度洋的正SSTA在副热带印度洋激发出赤道南北反对称环流,赤道以北的东风异常有利于异常WNPAC西伸;赤道以南的西风异常与来自赤道西太平洋的东风异常在东印度洋辐合上升,气流至西北太平洋下沉,形成经向垂直环流,有利于春季WNPAC维持。在对流层高层,印度洋的正SSTA在热带印度洋上空激发出位势高度正异常,随之形成的气压经向梯度加强了东亚高空副热带西风急流,进而在华南上空形成异常辐散环流。WNPAC的西伸和加强可为华南提供充足的水汽,同时高空辐散在华南引发水汽上升运动,共同导致SCSR正异常。而若El Ni?o发生时IOB处于中性状态,El Ni?o相关的SSTA衰减较快,春季WNPAC不显著,SCSR无明显多寡趋势。      

南半球环流异常与我国夏季旱涝分布关系及其影响机制

利用1951—2000年NCEP/NCAR风场和高度场再分析资料及全国160站降水量资料, 采用奇异值分解、相关和合成分析方法, 研究6—8月南半球500 hPa高度、高低层纬向风距平差异常 (Δu₈₅₀-Δu₂₀₀) 与我国夏季旱涝分布的关系及其影响机制。结果表明:当500 hPa澳大利亚高压脊偏强及西南太平洋热带地区高低层纬向风距平差为负值时, 来自南半球冷空气活动偏弱, 有利于西北太平洋副热带高压位置偏南, 热带季风偏弱, 我国夏季雨带偏南。反之, 当澳大利亚高压脊偏弱及西南太平洋热带地区高低层纬向风距平差为正值时, 我国北方降水偏多。同时, 定义了澳大利亚冬季风指数, 指出澳大利亚冬季风强年和弱年影响我国夏季旱涝分布异常的水汽输送型式不同。     

印度洋海温异常年际变率模态对中国东部地区夏季降水影响机制的数值试验

利用ECHAM5全球大气环流模式研究了印度洋海温异常年际变率模态从冬至夏的演变对我国东部地区夏季降水影响的机制。观测资料研究表明:对于正的印度洋海温异常年际变率模态,春、夏季热带印度洋和澳大利亚以西洋面(东极子)均为水汽的异常源区,向马达加斯加以东南洋面(西极子)及印度洋邻近大陆提供水汽。夏季,印度洋地区南极涛动、马斯克林高压加强;而印度季风低压和南亚高压均减弱,对应于印度夏季风减弱。夏季印度洋地区正压性的纬向风异常经向遥相关使热带印度洋地区出现西风异常,导致海洋性大陆地区对流活动减弱,而菲律宾海地区对流活动加强,进而导致西太平洋副热带高压偏弱、位置偏东北。对于负的印度洋海温异常年际变率模态,则反之。模式结果基本支持了已有的观测资料诊断结果。     

秋季南极涛动异常对冬季中国南方降水的影响

本文针对秋季南极涛动（AAO）和冬季中国南方降水的关系作了研究,发现两者之间存在显著的反向年际变化关系。AAO正（负）异常年,副热带西风急流显著增强（减弱）,欧洲西部槽、乌拉尔山高压脊和东亚沿岸大槽均偏强（偏弱）,阿留申低压、南支槽和西太平洋副高偏弱（偏强）,西南急流上的扰动不活跃（活跃）,我国大部分地区出现异常偏北风（偏南风）和OLR弱正距平,导致南方降水偏少（偏多）。两半球存在东亚—南半球高纬地区和纵贯太平洋南北的两个遥相关型,海洋性大陆对流活动可能是秋季AAO影响南方冬季降水的一个机制。因此,秋季南极涛动的异常很可能对预测我国南方冬季降水有显著指示意义。     

The Role of Land–sea Distribution and Orography in the Asian Monsoon. Part I： Land–sea Distribution

A number of AGCM simulations were performed by including various land--sea distributions (LSDs), such as meridional LSDs, zonal LSDs, tropical large-scale LSDs, and subcontinental-scale LSDs, to identify their effects on the Asian monsoon. In seven meridional LSD experiments with the continent/ocean located to the north/south of a certain latitude, the LSDs remain identical except the southern coastline is varied from 40^o to 4^oN in intervals of 5.6^o. In the experiments with the coastline located to the north of 21^oN, no monsoon can be found in the subtropical zone. In contrast, a summer monsoon is simulated when the continent extends to the south of 21^oN. Meanwhile, the earlier onset and stronger intensity of the tropical summer monsoon are simulated with the southward extension of the tropical continent. The effects of zonal LSDs were investigated by including the Pacific and Atlantic Ocean into the model based on the meridional LSD run with the coastline located at 21^oN. The results indicate that the presence of a mid-latitude zonal LSD induces a strong zonal pressure gradient between the continent and ocean, which in turn results in the formation of an East Asian subtropical monsoon. The comparison of simulations with and without the Indian Peninsula and Indo-China Peninsula reveals that the presence of two peninsulas remarkably strengthens the southwesterly winds over South Asia due to the tropical asymmetric heating between the tropical land and sea. The tropical zonal LSD plays a crucial role in the formation of cumulus convection.     

A comparative synoptic climatology of cool-season rainfall in major grain-growing regions of southern Australia

Two distinct synoptic weather systems, cut-off lows and fronts, deliver most of the cool-season rainfall to the cropping regions of southern Australia. A comparative synoptic climatology of daily rainfall events over approximately five decades reveals both spatial and temporal variations of the dominant synoptic types. The rainfall characteristics and associated large-scale drivers differ between the two synoptic types. Understanding regional rainfall depends on understanding these differences. Cut-off lows contribute one half of growing season rainfall in southeast Australia, while frontal systems associated with Southern Ocean depressions contribute about a third. The proportions are reversed in the Central Wheat Belt (CWB) of Western Australia where Southern Ocean fronts are the dominant source of growing season rainfall. In the southern island state of Tasmania, topography strongly influences the outcome with cut-off lows contributing about half the rainfall near the east coast and fronts dominating a short distance to the west. Cut-off lows generally contribute their highest proportion of rainfall in the austral autumn and spring while frontal rainfall is at its maximum in late winter. Cut-off low rainfall contributes more strongly in percentage terms to the recent decline in rainfall. The distribution of synoptic types is explained by the dominant long-wave structure in the winter half of the year. The major trough near Western Australia favours frontogenesis to the southwest of the CWB but fronts moving out of the region encounter a persistent meridional ridge in the Tasman Sea where there is a high frequency of blocking events.     

OLR与长江中游夏季降水的关联

用SVD方法分析了1、4、7月全球OLR与夏季(6—8月)中国华中区域降水场的关系,结果表明:若1月南非东部沿岸至西印度洋、北美北部OLR(Outgoing Longwave Radiation)偏低(偏高),或北非、美国西南沿岸及近海OLR偏高(偏低),则夏季长江中游降水将偏多(偏少)。若4月澳大利亚至东印度洋、日界线以东热带太平洋OLR偏低(偏高),或西北太平洋偏高(偏低),则夏季长江中游降水将偏多(偏少)。若7月东印度洋—澳大利亚大陆、东亚OLR偏低(偏高),则夏季华中区域长江及其以北降水将偏多(偏少),湖南和江西南部降水将偏少(偏多)。夏季长江中游旱、涝年前期OLR明显的区别在于热带太平洋:涝年1月东、西太平洋为明显负、正异常,4月这种异常进一步加剧;旱年1月正好相反,东、西太平洋为微弱的正、负异常,4月转为东、西太平洋为微弱的负、正异常。太平洋暖池OLR低值区(强对流区)4、7月持续偏南,是夏季长江中游降水偏多的另一重要信号。冬、春季OLR与夏季长江中游降水大尺度关联的可能机制为:若1月热带东、西太平洋OLR为明显负、正异常,4月这种异常进一步加剧,也即冬、春季热带太平洋Walker环流持续减弱,从而使夏季暖池对流活动减弱,热带辐合带偏南,Hadley环流偏弱,使夏季西太平洋副热带高压主体位置偏南,导致中国夏季主雨带不能北推至黄河流域,而长期滞留长江中下游,最后造成长江中游降水异常。     

Sub-seasonal interannual variability associated with the excess and deficit Indian winter monsoon over the Western Himalayas

During the winter season (Dec., Jan., and Feb.; DJF) the western Himalaya (WH) receives one-third of its annual precipitation due to Indian winter monsoon (IWM). The IWM is characterized by eastward-moving synoptic weather systems called western disturbances. Seasonal interannual precipitation variability is positively correlated with monthly interannual variabilities. However, it was found that the monthly interannual variabilities differ. The interannual variability for Jan. is negatively correlated with that for Dec. and Feb. Because the entire seasonal interannual variability is in phase with the El Niño Southern Oscillation, it is interesting to investigate such contrasting behavior. Composite analysis based on extreme wet and dry seasons indicates that Dec. and Feb. precipitation variabilities have a high positive (low negative) correlation with eastern (western) equatorial Pacific warming (cooling), whereas Jan. precipitation variability exhibits negligible correlations. Seasonal mid/upper tropospheric cooling over the Himalayas enhances anomalous cyclonic circulation, which along with suppressed convection over the western equatorial Pacific, shifts the 200-hPa subtropical westerly jet southward over the Himalayas. Due to the upper tropospheric anomalous cyclonic circulation, mass transfer favors anticyclone formation at the mid/lower troposphere, which is enhanced in Jan. due to a warmer mid troposphere and hence decreases precipitation compared with Dec. and Feb. Additionally, a weakening of meridional moisture flux transport from the equatorial Indian Ocean to WH is observed in Jan. Further analysis reveals that mid-tropospheric and surface temperatures over WH also play dominant roles, acting as local forcing where the preceding month’s surface temperature controls the succeeding month’s precipitation.     

我国南方地区初秋气温的年际变化特征及影响因子分析

利用1979–2021年NCEP2.5°×2.5°、MOHC1°×1°海洋等资料,通过经验正交函数（EOF）分解、合成分析和相关分析等方法,分析了我国南方地区初秋气温的年际变化特征及其相关的大气和海洋异常。结果表明：（1）我国南方地区初秋气温主要表现为一致变化型和经向偶极变化型两种模态。（2）一致变化空间型主要受到高纬度西伯利亚高压和东亚大槽以及中低纬度地区的副热带高压和近地面风的共同影响,而经向偶极变化型则主要受到我国东北地区与我国长江下游流域对流层位势高度反位相变化的影响。（3）一致变化空间型与前期冬季我国邻近海域以及赤道印度洋和东太平洋地区海温异常、鄂霍茨克海和拉布拉多海海冰异常有关,经向偶极变化型则与前期冬季赤道中东太平洋的海温异常、鄂霍茨克海和巴伦支海海冰异常有关。     

影响南海夏季风爆发年际变化的关键海区及机制初探

利用1958—2011年NCEP/ NCAR再分析资料和ERSST资料,采用Lanczos时间滤波器、相关分析、回归分析、合成分析和交叉检验等方法,研究了影响南海夏季风爆发年际变化的关键海区海温异常的来源与可能机制。结果表明,前冬(12—2月)热带西南印度洋和热带西北太平洋是影响南海夏季风爆发年际变化的关键海区。冬季热带西南印度洋(热带西北太平洋)的异常增暖是由前一年夏季El Ni?o早爆发(强印度季风异常驱动的行星尺度东-西向环流)触发、热带印度洋(西北太平洋)局地海气正反馈过程引起并维持到春季。冬季热带西北太平洋反气旋性环流(气旋性环流)及印度洋(热带西北太平洋)的暖海区局地海气相互作用使得印度洋(热带西北太平洋)海温异常维持到春末。春季,逐渐加强北移到10 °N附近的低层大气对北印度洋(热带西北太平洋)暖海温异常响应的东风急流(异常西风)及南海-热带西北太平洋维持的反气旋性环流(气旋性环流)异常,使得南海夏季风晚(早)爆发。      

亚洲和南半球大气冷(热)源对亚洲冬季风影响的数值试验

利用NCAR CAM3.1模式及NCEP/NCAR(version 1)再分析资料计算了几种现实大气热源分布情况,讨论了亚洲各地区和南半球上空冬季1月大气冷(热)源对东亚冬季风环流系统和印度冬季风环流系统形成的影响.结果表明:(1)冬季1月东亚地区和澳大利亚上空大气冷(热)源与东亚冬季风环流关系密切,南半球澳大利亚附近的非绝热加热可以激发出澳大利亚北部的热低压系统,东亚大陆东部的大气冷源可以使东亚大陆低空出现冷高压,基本上模拟出东亚季风系统冬季主要环流成员;(2)亚洲地区西部及其对应的南半球印度洋非绝热加热与印度冬季风环流关系密切,同样对东亚冬季风也有一定的影响,特别是亚洲大陆西部副热带地区的非绝热加热可以加强冬季南海的越赤道气流并能调整阿留申低压的位置.     

冬季和春季印度洋海温异常年际变率模态对中国东部夏季降水的可能影响过程

印度洋热力状况是影响全球气候变化和亚洲季风变异的一个重要的因素,但以往研究更多关注热带印度洋海温的变化,对南印度洋中高纬地区海温变化关注不够,由此限制了我们对印度洋的全面认识.本文研究了年际尺度上整个印度洋海温异常主导模态的特征及其对我国东部地区夏季降水的可能影响过程,以期望为气候变异研究及预测提供理论依据.研究结果表明:全印度洋海温异常年际变率的主导模态特征是在南印度洋副热带地区海温异常呈现西南—东北反向变化的偶极子模态,西极子位于马达加斯加以东南洋面,东极子位于澳大利亚以西洋面;同时,热带印度洋海温异常与东极子一致.当西极子为正的海温异常,东极子、热带印度洋为负异常时定义为正的印度洋海温异常年际变率模态;反之,则为负的印度洋海温异常年际变率模态.从冬至春,印度洋海温异常年际变率模态具有较好的季节持续性;与我国长江中游地区夏季降水显著负相关,而与我国华南地区夏季降水显著正相关.其可能的影响过程为:对于正的冬、春季印度洋海温异常年际变率模态事件,印度洋地区异常纬向风的经向大气遥相关使得热带印度洋盛行西风异常,导致春、夏季海洋性大陆对流减弱,使夏季西太平洋副热带高压强度偏弱、位置偏东偏北,造成华南地区夏季降水增多,长江中游地区降水减少;反之亦然.同时,印度洋海温异常年际变率模态可通过改变印度洋和孟加拉湾向长江中游地区的水汽输送而影响其夏季降水.     

海温异常对东亚夏季风强度先兆信号的影响

利用ERA-Interim再分析资料、NOAA海温资料、CMAP格点降水资料和中国气象站降水资料,通过合成、相关和回归分析等方法研究了1979—2012年东亚夏季风强度与其先兆信号的关系,并分析了热带海温异常的可能影响。研究表明:东亚夏季风先兆指数反映了2月200 hPa纬向风距平的主要模态特征 (EOF1),前冬热带中东太平洋海温偏低 (高),2月亚洲地区西风急流位置偏北 (偏南),东亚夏季风先兆指数偏强 (弱)。前期热带海温异常对东亚夏季风强度有明显影响,前冬热带中东太平洋海温偏低 (高) 有利于东亚夏季风偏强 (弱)。2月亚洲中纬度地区纬向风异常特征在春季不能持续,先兆信号与东亚夏季风强度的联系主要源自热带海洋。     

Interannual Variability of Autumn Precipitation over South China and its Relation to Atmospheric Circulation and SST Anomalies

The interannual variability of autumn precipitation over South China and its relationship with atmospheric circulation and SST anomalies are examined using the autumn precipitation data of 160 stations in China and the NCEP-NCAR reanalysis dataset from 1951 to 2004. Results indicate a strong interannual variability of autumn precipitation over South China and its positive correlation with the autumn western Pacific subtropical high （WPSH）. In the flood years, the WPSH ridge line lies over the south of South China and the strengthened ridge over North Asia triggers cold air to move southward. Furthermore, there exists a significantly anomalous updraft and cyclone with the northward stream strengthened at 850 hPa and a positive anomaly center of meridional moisture transport strengthening the northward warm and humid water transport over South China. These display the reverse feature in drought years. The autumn precipitation interannual variability over South China correlates positively with SST in the western Pacific and North Pacific, whereas a negative correlation occurs in the South Indian Ocean in July. The time of the strongest lag-correlation coefficients between SST and autumn precipitation over South China is about two months, implying that the SST of the three ocean areas in July might be one of the predictors for autumn precipitation interannual variability over South China. Discussion about the linkage among July SSTs in the western Pacific, the autumn WPSH and autumn precipitation over South China suggests that SST anomalies might contribute to autumn precipitation through its close relation to the autumn WPSH.     

Extreme monsoons over East Asia: Possible role of Indian Ocean Zonal Mode

Summary The influence of the Indian Ocean Zonal Mode on the extreme summer monsoon rainfall over East Asia (China, Korea, Japan) has been investigated applying simple statistical techniques of correlation and composite analysis. While the observed rainfall data are used as a measure of rainfall activity, the NCEP-NCAR Reanalysis data are used to examine the circulation features associated with the extreme monsoon phases and the dynamics of the zonal mode – monsoon variability connections. The data used covers the period 1960 to 2000.The equatorial Indian Ocean is dominated by westerly winds blowing towards Indonesia. However, during the positive phase of the zonal mode, an anomalous, intensified easterly flow prevails, consistent with the positive (negative) sea surface temperature anomalies over the western (southeastern) equatorial Indian Ocean. This positive phase of the zonal mode enhances summer monsoon activity over China, but suppresses the monsoon activity over the Korea-Japan sector, 3 to 4 seasons later. The relationship is more consistent and stronger over the Korea-Japan region than over China.The Indian Ocean influences the monsoon variability over East Asia via the northern hemisphere mid-latitudes or via the eastern Indian Ocean/west Pacific route. The monsoon-desert mechanism induces strong subsidence northwest of India due to the anomalous convection over the Indian Ocean region associated with the positive phase of the zonal mode. This induces a zonal wave pattern over the mid-latitudes of Asia propagating eastwards and displacing the north Pacific subtropical high over East Asia. The warming over the eastern Indian Ocean/west Pacific inhibits the westward extension of the north Pacific sub-tropical high. The location and shape of this high plays a dominant role in the monsoon variability over East Asia. The memory for delayed impact, three to four seasons later, could be carried by the surface boundary conditions of Eurasian snow cover via the northern channel or the equatorial SSTs near the Indonesian Through Flow via the southern channel.     

一个海洋环流模式模拟的北印度洋经向环流及其热输送

对比两个同化资料GODAS（Global Ocean Data Assimilation System）和SODA（Simple Ocean Data Assimilation）,考察中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室发展的气候系统海洋模式LICOM（LASG/IAP Climate system Ocean Model）模拟的北印度洋经向环流及热输送的气候态。LICOM能抓住北印度洋大尺度环流的季节变化特征,模拟的年平均越赤道热输送为-0.24 PW （1 PW=1015W）,较之以往的数值模式结果更接近观测和同化资料。与同化资料的差异主要体现在季节变化强度,北半球夏季在赤道以南偏弱0.5 PW,这与模式夏季的纬向风应力偏弱,热输送中的大项Ekman热输送模拟偏弱,从而模拟的经圈翻转环流较浅有关。     

The Indian Ocean subtropical dipole mode simulated in the CMIP3 models

Using observation data and outputs from the “twentieth-century climate in coupled models” (20c3m) control runs of coupled general circulation models submitted to the Coupled Model Intercomparison Project, phase 3 (CMIP3), the ability of CMIP3 models to simulate the Indian Ocean subtropical dipole (IOSD) and its influence on the rainfall anomaly over the southern African region is investigated. Many models simulate the IOSD, but the location and shape of the sea surface temperature anomaly vary among models. This model bias is closely linked to the bias in simulating the anomalous strengthening and southward shift of the subtropical high. Almost all models fail to simulate the rainfall anomaly associated with the IOSD owing to the inaccurate simulation of the location of sea surface temperature and sea level pressure anomalies.     

2014年夏季浙江低温多雨的大尺度环流特征及与海温异常关系

利用NECP/NCAR再分析资料、国家气候中心和NOAA相关资料,研究了与2014年浙江夏季低温多雨事件相关的大尺度环流特征和海温因子。结果表明:中纬度我国东部到日本南部气旋性环流异常的存在有利于浙江夏季出现低温多雨,异常偏强偏南的西太平洋副热带高压（简称副高）是8月罕见低温多雨的直接原因;东亚-太平洋型遥相关（EAP）和欧亚型遥相关（EU）是影响浙江夏季低温阴雨的主要遥相关型,当EAP负位相和EU正位相时,冷空气容易堆积和南下,与暖湿气流交汇,有利于降水降温,8月罕见低温阴雨是EAP负位相和EU正位相协同作用的结果。进一步的分析表明ENSO暖位相激发了西太平洋上空强烈的异常下沉气流和反气旋,使得副高偏南偏强,东亚地区呈EAP波列型响应;热带印度洋海温全区一致模态（IOBW）暖位相的维持进一步减弱了8月海洋性大陆地区的对流活动;北大西洋中部海温季内的变化或许与EU位相的转变有联系。     

Moisture transport between the South Atlantic Ocean and southern Africa: relationships with summer rainfall and associated dynamics

Moisture exchange between the South Atlantic and southern Africa is examined in this study through zonal moisture transport. Along the west coast of southern Africa, a multivariate analysis of the zonal flow of moisture computed from NCEP-DOE AMIP II Re-analyses reveals a primary mode of variability typical of variations in intensity and of the latitudinal migration of the circulation associated with the midlatitude westerlies and the South Atlantic anticyclone. In austral summer (January–February), this mode, referred to as the South Atlantic midlatitude mode, is found to be well correlated with rainfall over southern Africa (i.e. to the south of the upper lands surrounding the Congo basin). Its positive/negative phases are found to correspond with surface pressures changes over the South Atlantic region in austral summer when the South Atlantic anticyclone is shifted northward/southward respectively. Such changes are accompanied by dipole-like SST anomalies in the midlatitude South Atlantic Ocean, while simultaneous SST anomalies with a similar structure are also found over South Indian Ocean regions. In January–February, positive/negative events linked to the South Atlantic midlatitude mode are marked by meridional shifts (northward/southward) and weakening/strengthening of the ITCZ over the southern tropics, together with modulations in intensity (weakened/sustained) of the Angola low, which could act as a tropical source of moisture for Tropical Temperate Troughs (TTTs). In association with a strengthened/weakened zonal component of the southern extension of the African Easterly Jet (AEJ), this could modulate the meridional transfer of moisture south of 15°S to the advantage/detriment of Angolan coastal regions, where above/below rainfall are expected. Variations in the latitudinal position (northward/southward) of the South Atlantic anticyclone, and thus of the midlatitude westerlies, are also found to reduce/favour moisture advection towards southern Africa subtropics allowing the southern Indian trades to penetrate less/more over the subcontinent south of 25°S. This would create a situation where convection processes are inhibited/supported within the SICZ/TTTs region resulting in drier/wetter conditions locally for positive/negative events respectively.     

Sea surface temperature associations with the late Indian summer monsoon

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.