Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Nino Events

The inverse relationship between the warm phase of the El Nino Southern Oscillation(ENSO)and the Indian Summer Monsoon Rainfall(ISMR)is well established.Yet,some El Nino events that occur in the early months of the year(boreal spring)transform into a neutral phase before the start of summer,whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season.This study investigates the distinct influences of an exhausted spring El Nino(springtime)and emerging summer El Nino(summertime)on the regional variability of ISMR.The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Nino-3.4 region in the Pacific.The ISMR’s dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations,vertically integrated moisture flux convergence(VIMFC),wind shear,and upper atmospheric circulation.The monsoon circulation features are remarkably different in response to the exhausted spring El Nino and emerging summer El Nino phases,which distinctly dictate regional rainfall variability.The dynamic and thermodynamic responses reveal that exhausted spring El Nino events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India.In contrast,emerging summer El Nino events negatively impact the seasonal rainfall over the country,except for a few regions along the west coast and northeast India.

Interannual to Interdecadal Variation of East Asian Summer Monsoon and its Association with the Global Atmospheric Circulation and Sea Surface Temperature

The East Asian summer monsoon (EASM) underwent an interdecadal variation with interannual variations during the period from 1958 to 1997, its index tended to decline from a higher stage in the mid-1960,s until it reached a lower stage after 1980/s. Correlation analysis reveals that EASM is closely related with the global atmospheric circulation and sea surface temperature (SST). The differences between the weak and strong stage of EASM shows that, the summer monsoon circulation over East Asia and North Africa is sharply weakened, in the meantime, the westerlies in high latitudes and the trade-wind over the tropical ocean are also changed significantly. Over the most regions south of the northern subtropics, both air temperature in the lower troposphere and SST tended to rise compared with the strong stage of EASM. It is also revealed that the ocean-atmosphere interaction over the western Pacific and Indian Ocean plays a key role in interannual to interdecadal variation of EASM, most probably, the subtropical indian Ocean is more important. On the other hand, the ENSO event is less related to EASM at least during the concerned period.

Monsoon Break over the South China Sea during Summer: Statistical Features and Associated Atmospheric Anomalies

This study identifies break events of the South China Sea(SCS)summer monsoon(SCSSM)based on 42 years of data from 1979 to 2020,and investigates their statistical characteristics and associated atmospheric anomalies.A total of 214 break events are identified by examining the convection evolution during each monsoon season.It is found that most events occur between June and September and show a roughly even distribution.Short-lived events(3–7 days)are more frequent,accounting for about two thirds of total events,with the residual one third for long-lived events(8–24 days).The SCSSM break is featured by drastic variations in various atmospheric variables.Particularly,the convection and precipitation change from anomalous enhancement in adjoining periods to a substantial suppression during the break,with the differences being more than 60 W m−2 for outgoing longwave radiation(OLR)and 10 mm d−1 for precipitation.This convection/precipitation suppression is accompanied by an anomalous anticyclone in the lower troposphere,corresponding to a remarkable westward retreat of the monsoon trough from the Philippine Sea to the Indochina Peninsula,which reduces the transportation of water vapor into the SCS.Besides,the pseudo-equivalent potential temperature()declines sharply,mainly attributable to the local specific humidity reduction caused by downward dry advection.Furthermore,it is found that the suppressed convection and anomalous anticyclone responsible for the monsoon break form near the equatorial western Pacific and then propagate northwestward to the SCS.

Differences in Atmospheric Heat Source between the Tibetan Plateau–South Asia Region and the Southern Indian Ocean and Their Impacts on the Indian Summer Monsoon Outbreak

In this paper, the NCEP-NCAR daily reanalysis data are used to investigate the characteristics of the atmospheric heat source/sink （AHSS） over South Asia （SA） and southern Indian Ocean （SIO）. The thermal differences between these two regions and their influence on the outbreak of the Indian summer monsoon （ISM） are explored. Composite analysis and correlation analysis are applied. The results indicate that the intraseasonal variability of AHSS is signi- ficant in SA but insignificant in the SIO. Large inland areas in the Northern Hemisphere still behave as a heat sink in March, similar to the situation in winter. Significant differences are found in the distribution of AHSS between the ocean and land, with distinct land-ocean thermal contrast in April, and the pattern presents in the transitional period right before the ISM onset. In May, strong heat centers appear over the areas from the Indochina Peninsula to the Bay of Bengal and south of the Tibetan Plateau （TP）, which is a typical pattern of AHSS distribution during the monsoon season. The timing of SA-SIO thermal difference turning positive is about 15 pentads in advance of the onset of the ISM. Then, after the thermal differences have turned positive, a pre-monsoon meridional circulation cell develops due to the near-surface heat center and the negative thermal contrast center, after which the meridional circulation of the ISM gradually establishes. In years of early （late） conversion of the SASIO thermal difference turning from neg- ative to positive, the AHSS at all levels over the TP and SIO converts later （earlier） than normal and the establish- ment of the ascending and descending branches of the ISM＇s meridional circulation is later （earlier） too. Meanwhile, the establishment of the South Asian high over the TP is later （earlier） than normal and the conversion of the Mas- carene high from winter to summer mode occurs anomalously late （early）. As a result, the onset of the ISM is later （earlier） than normal. However, the difference in vorticity between early and late conversion only shows in the changes of strong vorticity centers＇ location in the upper and lower troposphere.

Strong/Weak Summer Monsoon Activity over the South China Sea and Atmospheric Intraseasonal Oscillation

The circulation pattern corresponding to the strong / weak summer monsoon in the South China Sea (SCS) region and the associated characteristics of the abnormal rainfall in Eastern China have been studied by using the NECP reanalysis data and precipitation data in China. The results show that the climate variations in China caused by the strong / weak summer monsoon are completely different (even in opposite phase). The analyses of atmospheric intraseasonal oscillation (ISO) activity showed that the atmospheric ISO at 850 hPa near the SCS region is strong (weak) corresponding to the strong (weak) SCS summer monsoon. And the analyses of the circulation pattern of the atmospheric ISO showed that the strong / weak SCS summer monsoon circulation (200 hPa and 850 hPa) result mainly from abnormal atmospheric ISO. This study also reveals that the atmospheric ISO variability in the South China Sea region is usually at opposite phase with one in the Jiang-huai River basin. For example, strong (weak) atmospheric ISO in the SCS region corresponds to the weak (strong) atmospheric ISO in the Jiang-huai River basin. As to the intensity of atmospheric ISO, it is generally exhibits the local exciting characteristics, the longitudinal propagation is weak. Key words The SCS summer monsoon - Atmospheric intraseasonal oscillation - Circulation pattern This was supported by National Key Basic Science Program in China (G1998040903) and State Key Project-SCSMEX.

CHARACTERISTICS OF ATMOSPHERIC HEAT SOURCE ASSOCIATED WITH THE SUMMER MONSOON ONSET OVER THE SOUTH CHINA SEA AND THE POSSIBLE MECHANISM RESPONSIBLE FOR ITS LATE OR EARLY ONSET

The characteristics of atmospheric heat source associated with the summer monsoon onset in the South China Sea (SCS) are studied using ECMWF reanalysis data from 1979 to 1993. A criterion of the SCS summer monsoon onset is defined by the atmospheric heat source. Applying this criterion to the 15-year (1979 – 1993) mean field, the onset of the SCS summer monsoon is found to occur in the fourth pentad of May. And this criterion can also give reasonable results for the onset time of the SCS summer monsoon on a year-to-year basis. In addition, pretty high correlation has been found between the onset time of the SCS summer monsoon and the zonal mean vertically integrated heat source <Q1> at 40°S in April. The causes for the late or early onset of the SCS summer monsoon and the close relationship between the onset time and the zonal mean vertically integrated heat source <Q1> at 40 °S in April might be explained by the variations in intensity of the Hadley circulation.

Relationship between Indian and East Asian Summer Rainfall Variations

The Indian and East Asian summer monsoons are two components of the whole Asian summer monsoon system. Previous studies have indicated in-phase and out-of-phase variations between Indian and East Asian summer rainfall. The present study reviews the current understanding of the connection between Indian and East Asian summer rainfall. The review covers the relationship of northern China, southern Japan, and South Korean summer rainfall with Indian summer rainfall; the atmospheric circulation anomalies connecting Indian and East Asian summer rainfall variations; the long-term change in the connection between Indian and northern China rainfall and the plausible reasons for the change; and the influence of ENSO on the relationship between Indian and East Asian summer rainfall and its change. While much progress has been made about the relationship between Indian and East Asian summer rainfall variations, there are several remaining issues that need investigation. These include the processes involved in the connection between Indian and East Asian summer rainfall, the non-stationarity of the connection and the plausible reasons, the influences of ENSO on the relationship, the performance of climate models in simulating the relationship between Indian and East Asian summer rainfall, and the relationship between Indian and East Asian rainfall intraseasonal fluctuations.

Definition of the South China Sea summer monsoon onset

We analyze statistically different definitions of the South China Sea summer monsoon(SCSSM) onset are to establish a SCSSM onset time series that is more recognizable by a majority of indicators.With the acknowledged index,we determine a key area(105°E-112.5°E,7.5°N-12.5°N) and define the zonal wind component in this key area as a new SCSSM onset index,using daily mean reanalysis data of the National Center for Environmental Prediction/National Center for Atmospheric Research.The atmospheric circulations before and after the onset of the SCSSM determined using the index defined in this paper are preliminarily studied.Results show that the Somalia cross-equatorial flow is enhanced,the strongest westerly wind in the tropical Indian Ocean shifts northward,the cyclone couple in the Bay of Bengal and the Southern Hemisphere weaken and move eastward,convection over the South China Sea increases,and the subtropical high retreats from the South China Sea after the outbreak of the SCSSM.By analyzing the atmospheric circulation,it is found that in 1984 and 1999,the SCSSM broke out in pentads 29 and 23,respectively,which is consistent with the onset times determined using our index.

2021年5月江西降水异常偏多特征及成因分析

2021年5月江西省降水异常偏多,多项降水指数突破历史极值,异常的降水在累计降水量、降水强度、日降水量等方面表现出极端性。利用1961—2021年江西省83站逐日降水资料和NCEP/NCAR再分析资料,分析了2021年5月江西降水异常偏多的主要原因。结果表明:5月大气环流异常是造成江西降水偏多的直接原因,欧亚中高纬500 hPa层环流呈“两脊一槽”的分布特征,受北大西洋三极子正位相影响,中国东北及周围地区位势高度为负距平,东北冷涡异常活跃,中高纬环流经向度增加,冷空气不断南下为降水提供了环流条件。西太平洋副热带高压月内加强西伸,为降水提供充足的水汽条件。此外,南海夏季风爆发偏晚和5月在印度洋活跃的热带季节内振荡对5月江西降水偏多有一定的指示意义。

Interdecadal Variation of the Atmospheric Heat Source over the Tibetan Plateau and Surrounding Asian Monsoon Region: Impact on the Northern Hemisphere Summer Circulation

We use 71-yr(1948–2018) reanalysis data to investigate the interdecadal variation in the atmospheric heat source(Q1) over the Tibetan Plateau and surrounding Asian monsoon region(AMTP) and its effect on the Northern Hemisphere summer circulation. The large-scale circulation driven by Q1 over the AMTP is characterized by a center of convergent(divergent) or low(high) potential wind function in the lower(upper) troposphere. Q1 over the AMTP shows a clear interdecadal variation(with positive–negative–positive phases) and these three phases correspond to the time periods 1948–1972, 1973–2005, and 2006–2018, respectively. The thermal circulation has a corresponding interdecadal variation as a response to the interdecadal variation in Q1. An enhanced Q1 leads to an increase in the conversion of the total potential energy to non-divergent wind kinetic energy via the divergent wind velocity. The maximum conversion occurs in the tropopause. The primary thermal forcing for Q1 is produced by the intense, large volume precipitation of the summer monsoon. This induces a response in the large-scale circulation, leading to largescale divergence patterns. The synergistic effects of Pacific Decadal Oscillation(PDO) and North Atlantic Multidecadal Oscillation(AMO) influence Q1 over the AMTP, which is ultimately responsible for the modulation of variations in the global divergent circulation. The global divergent circulation in summer is therefore essentially a direct thermodynamic circulation driven by the strong Q1 over the AMTP.

Recognition of two dominant modes of EASM and its thermal driving factors based on 25 monsoon indexes

Based on three reanalysis datasets—ERA-Interim,NCAR–NCEP and JRA-55—the classification of25 commonly used indexes of the East Asian summer monsoon(EASM)was investigated.The physical nature of two categories of monsoon index,together with their circulation pattern,climate anomalies,and driving factors,were investigated.Results suggest that the selected 25 monsoon indexes can be classified into two typical categories(CategoryⅠandⅡ),which are dominated by interannual and decadal variabilities of the EASM,respectively.The anomalous circulation patterns and summer rainfall patterns related to the two categories of index also exhibit evident differences.CategoryⅠis closely linked to the low-latitude circulation system and the anomalous circulation pattern is a typical East Asia–Pacific teleconnection pattern.The summer rainfall anomaly exhibits a typical tripole pattern.However,CategoryⅡmainly reflects the impacts of the middle–high latitude circulation system on the summer monsoon and is closely linked to a typical Eurasian teleconnection pattern,which corresponds to a dipole of summer rainfall anomalies.Further analysis suggests that the underlying thermal driving factors of the two categories of monsoon are distinct.The main driving factors of CategoryⅠare the tropical sea surface temperature anomalies(SSTAs),especially ENSO-related SSTAs in the preceding winter and summer SSTAs in the tropical Indian Ocean.The winter signal of Category II summer monsoon anomalous activity mainly originates from the polar region and the middle and high latitudes of the Eurasian continent.CategoryⅡmonsoon activity is also associated with summer SSTAs in the equatorial central Pacific.

青藏高原三江源和河湾区夏季降水变化特征及对高原夏季风的响应

本文利用1981~2020年观测数据和ERA5再分析资料,将青藏高原腹地三江源和东南重要水汽通道河湾区作为典型研究区域,分析了降水不同时间尺度变化特征及其典型强弱年对高原季风环流系统的响应,结果表明:(1)三江源和河湾区降水的季节变化均呈双峰型分布,峰值出现在7月初和8月下旬。夏季降水在21世纪初发生年代际转折,尤其是三江源降水量在近20年增加明显。两个高原季风指数DPMI(Dynamic Plateau Monsoon Index)和ZPMI(Zhou Plateau Monsoon Index)的夏季风爆发时间均超前于河湾区和三江源降水的明显增加期。三江源夏季降水年际变化与两个高原夏季风指数有较好的相关性。三江源与河湾区虽然相邻很近,但三江源夏季降水受高原季风影响程度远大于河湾区。当高原夏季风增强(减弱)时,三江源降水量偏多(少)。(2)三江源降水偏多年,南亚高压偏东偏强,低层高原主体低压异常,有利于西南风和东南风在三江源区域交汇,南方暖湿空气能够深入高原腹地导致水汽辐合偏强。河湾区降水偏多年,河湾区及整个高原主体附近高度场并没有明显异常,河湾区的水汽输送主要有两条路径,一条来自孟加拉湾沿高原南坡的西南路径,另一条来自中亚地区穿过高原上空的西北路径,两条路径在高原东侧汇合继续向东输送。

Relationship between the Tibetan Plateau-tropical Indian Ocean thermal contrast and the South Asian summer monsoon

The impact of land-sea thermal contrast on the South Asian summer monsoon(SASM)was investigated by calculating the atmospheric heat sources(AHS)and baroclinic component with ERA5 data for the period 1979-2019.Using diagnostic and statistical methods,it was found that the thermal contrast between the Tibetan Plateau(TP)and the tropical Indian Ocean(TIO)affects the South Asian monsoon circulation through the meridional temperature gradient in the upper troposphere.The seasonal changes of the AHS of the TP and TIO are reversed.In summer,the TP is the strongest at the same latitude whereas the TIO is the weakest,and the thermal contrast is the most obvious.The heat sources of the TP and TIO are located on the north and south side of the strong baroclinic area of the SASM region,respectively,and both of which are dominated by deep convective heating in the upper troposphere.The TP-TIO regional meridional thermal contrast index(QI)based on the AHS,and the SASM index(MI)based on baroclinicity were found to be strongly positively correlated.In years of abnormally high QI,the thermal contrast between the TP and TIO is strong in summer,which warms the upper troposphere over Eurasia and cools it over the TIO.The stronger temperature gradient enhances the baroclinicity in the troposphere,which results in a strengthening of the low-level westerly airflow and the upper-level easterly airflow.The anomalous winds strengthen the South Asian high(SAH),with the warmer center in the upper troposphere,and the enhanced Walker circulation over the equatorial Indian Ocean.Finally,the anomalous circulation leads to much more precipitation over the SASM region.The influence of abnormally low QI is almost the opposite.

亚洲夏季风对偶极子型人为气溶胶排放变化的响应特征与机理

利用第六次国际耦合模式比较计划(Coupled Model Intercomparison Project Phase 6,CMIP6)中的两组子试验,结合线性斜压模式模拟的结果,研究了近年来亚洲内部出现的东亚减少、南亚增加的偶极子型人为气溶胶排放变化调控亚洲夏季风响应的特征及物理机制。对东亚夏季风而言,在考虑海洋-大气耦合作用的气候系统总响应中,东亚夏季风环流和降水显著地加强;在不考虑海洋调控作用的大气直接响应过程中,东亚人为气溶胶排放减少导致的陆地升温使得海陆温差增大,进而通过引起东亚陆地上的气旋式环流异常加强东亚夏季风环流和降水。对南亚夏季风而言,其在偶极子型人为气溶胶强迫下呈现出更为复杂的变化特征。在大气直接响应过程中,人为气溶胶强迫引起的海陆热力差异变化导致南亚夏季风环流减弱、降水减少。在考虑海洋-大气耦合过程的总响应中,南亚夏季风环流表现出微弱增强,同时印度次大陆的南亚夏季风降水也出现增多的异常变化。这表明,局地和海洋-大气动力耦合过程在区域气候对人为气溶胶强迫的响应中扮演着非常重要的角色。此外,通过线性斜压模式发现,东亚和南亚局地的人为气溶胶强迫导致的大气加热场异常不仅能影响局地的夏季风环流,还可以通过引起大范围的表面气压异常进而调控整个亚洲夏季风环流的变化。

2012年海洋和大气环流异常及其对中国气候的影响

文章主要对2011/2012年冬季至2012年秋季的海洋和大气环流异常进行分析,并讨论这些异常特征对中国气温和降水的主要影响。分析表明:2012年3月拉尼娜事件结束,赤道中东太平洋在7—8月出现明显暖水波动,之后进入正常状态。暖水波动使9—10月西太副高偏强偏西控制长江以南大部,造成该地温高雨少:8—9月,热带印度洋呈显著的偶极子正位相模态,在热带东太平洋激发出异常反气旋,其西北侧西南气流有利于暖湿气流影响中国华西南部出现明显秋雨。2012年南海夏季风爆发偏早1候,结束偏晚2候,强度偏弱;东亚夏季风为1951年以来第四强,使得东亚夏季风雨带位置偏北,中国北方大部夏季降水偏多。受海温和大气环流异常等的共同影响,我国出现了冬冷、春夏热、秋冷和夏季降水"北多南少"的气候特征。

中国夏季降水的水汽通道特征及其影响因子分析

利用NCEP/NCAR的1958～1998年再分析资料研究夏季东亚季风区水汽输送特征,综合这些特征划分出夏季输送到中国大陆主要有来自低纬的三条水汽通道:西南通道、南海通道和东南通道,此外在高纬还有一条很弱的西北通道,分别体现了南亚季风、南海季风、副热带季风和中纬度西风带对中国夏季降水的影响。定义和计算了四条水汽通道强度指数来表征水汽通道的强弱,并研究其年际变化。相关分析表明四条水汽通道对我国夏季降水的影响范围分别是:西南通道是华南中部和西南边境降水的水汽来源,南海通道对华南降水有直接贡献,东南通道为长江流域降水输送水汽,西北通道则为黄河中上游及华北东部降水输送水汽。物理分析显示,水汽输送异常与大气环流异常直接相关,而与同期水汽源地的海温异常关系不密切,海洋的作用主要体现在前期大范围的海温异常分布上。

青藏高原加热对东亚地区夏季降水的影响

东亚地区降水主要集中在夏季,是亚洲夏季风系统的重要特征。本文利用NCEP再分析资料和CRU的降水资料,分析了青藏高原非绝热加热对东亚夏季降水的影响。结果表明,东亚地区夏季降水的分布形势与青藏高原非绝热加热变化有很好的相关关系。由于高原非绝热加热可在亚洲东部沿海地区强迫出类似Rossby波列的大气环流低频振荡结构,而此低频波可以影响到西太平洋副热带高压的形态和位置变化,从而使得东亚夏季降水的形势发生变化。而青藏高原非绝热加热的形态从春季到夏季有很好的持续性,春季高原加热与夏季东亚的降水形势分布也有很好的相关。本研究中采用的青藏高原非绝热加热指数可作为东亚夏季降水预测的一个指标,亚洲季风降水不仅受赤道太平洋海温的影响,青藏高原地区的非绝热加热对其也有显著的影响作用。

全新世青藏高原东部西南季风的演变

本研究报道一组新的西南季风代用指标,即泥炭中单一种属植物———木里苔草残体纤维素和泥炭混合植物残体纤维素的δ13C时间序列,所记录的青藏高原东部全新世气候变化。两记录表明,该区全新世的下限年龄约11200aBP(14C年龄约9900aBP);从约11200aBP起该区迅速进入湿暖的全新世阶段,季风活动迅速增强;在约10800～5500aBP期间,季风总体保持在强盛状态,但其间有4次突然减弱,气候变干冷;约从5500aBP起季风活动在波动中逐渐减弱,其中有4次减弱最明显。所有这8次气候的突然变化都与北大西洋浮冰事件一一对应。这种密切的相关关系表明,西南季风强度的波动可能是对全球变化,特别是对海洋热盐环流引起的地球南北方气候波动的所谓"跷跷板效应"的响应。

伴随南海夏季风爆发的大尺度大气环流演变

主要基于美国ＮＣＥＰ和ＮＣＡＲ的再分析资料（１９８０～１９９６ 年），针对南海夏季风爆发日期进行合成分析， 研究了伴随南海夏季风爆发的大尺度大气环流演变。其结果清楚地表明伴随南海夏季风爆发， 南亚和东南亚地区的对流层低层风场、对流层高层位势高度场以及大气湿度场和垂直运动场都有极显著的变化。南亚和东南亚８５０ ｈＰａ 上涡旋对的发展和活动以及５００ ｈＰａ 副高从南海地区的东撤对南海季风爆发起着重要作用。伴随南海夏季风的爆发， 在孟加拉湾到南中国海一带整层湿度和５００ ｈＰａ 垂直上升运动都出现了极明显的增加。对流层高层和对流层低层环流演变的特征也清楚表明， 南海夏季风爆发既是全球环流冬夏演变的一个部分， 又有显著的区域性特征。本文还指出南海夏季风在北部比中部和南部早建立的结论依据不足， 进而补充给出了亚洲季风爆发日期示意图。

青藏高原地形对东亚夏季风北缘影响的数值试验

使用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(LASG/IAP)发展的大气环流谱模式SAMIL(R42L9),进行了有、无青藏高原对亚洲夏季风北边缘影响的数值模拟。结果表明,青藏高原大地形对夏季风北边缘活动有重要影响。有(无)高原时,其东侧的偏南风较强(弱)、较深(浅),向北扩展偏北(南),有(不)利于引导和加强夏季风北上,使北边缘偏北(南);同时,西太平洋副热带高压偏北偏西(偏南偏东),也有(不)利于夏季风向北深入我国大陆,从而使夏季风北边缘偏北(南)。与之相对应的夏季风降水区也偏北(南)。