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.

Numerical simulation of influence of the anomalies of theCentral-eastern Equatorial Pacific SST and Arctic seaice cover in summer on the atmospheric circulation

A series of numerical experiments have been conducted with a perpetual July, nine-level general circulation spectral model to determine the effect of variation of the Arctic sea ice cover extent and the joint effect of anomalies of both the Arctic sea ice cover and the Central-eastern Equatorial Pacific sea surface temperature on the summer general circulation. Results show that the two factors,anomalously large extent of the Arctic sea ice cover and anomalously warm sea surface temperature over the Central-eastern Equatorial Pacific Ocean, play substantially the equal role in the effect on the summer general circulation, and either of them can notably induce the atmospheric anomalies. The main dynamical processes determining the effect of the Arctic sea ice and the equatorial SST anomalies are associated with two leading teleconnection patterns, i. e. the Asia North/American and Eurasian patterns observed in atmosphere. The results presented in this paper again prove that the general circulation is fundamentally motivated by the non-uniform heating between the equator and the pole on the rotating earth.

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.

Relationship between Reduction of Summer Precipitation in North China and Atmospheric Circulation Anomalies

Based on Reanalysis datasets from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and summer rainfall datasets from China National Climate Center (NCC), by using trend analysis and composite analysis methods, the relationship between the reduction of summer precipitation in North China and northern hemispheric circulation changes was investigated. The results show that summer rainfall in North China had a significant decreasing tendency, especially true since 1965 in which an abrupt change occurred. The northern hemisphere atmospheric circulation at 500 hPa had a remarkable change after 1965, from outstanding meridional circulation to outstanding zonal circulation, leading to upper trough activity to decrease, resulting in the rainfall weather processes caused by upward motion behind trough significantly to reduce. At 500 hPa in Mongolian region, air temperature decreased, resulting in lower troposphere pressure to increase, leading to low pressure activity significantly to decrease and rainfall weather processes influencing North China to reduce. At the same time, the decreased air temperature in 500 hPa would caused the upper troposphere geopotential height to reduce, resulting in high–altitude jet southerly location, the East Asian summer monsoon to weaken, then it was difficult for water vapor transport to cross the Yangtze River valley and reach the North China region, with a southerly summer monsoon rainfall zone. The summer precipitation reduction in North China had a good correlation with the northern hemispheric circulation changes.

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.

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.

Numerical Experiments on the Impact of Spring North Pacific SSTA on NPO and Unusually Cool Summers in Northeast China

A set of numerical experiments designed to analyze the oceanic forcing in spring show that the combined forcing of cold （warm） El Ni（n）o （La Ni（n）a） phases in the Ni（n）o4 region and sea surface temperature anomalies （SSTA） in the westerly drifts region would result in abnormally enhanced NorthEast Cold Vortex （NECV） activities in early summer.In spring,the central equatorial Pacific El Ni（n）o phase and westerly drift SSTA forcing would lead to the retreat of non-adiabatic waves,inducing elliptic low-frequency anomalies of tropical air flows.This would enhance the anomalous cyclone-anticyclonecyclone-anticyclone low-frequency wave train that propagates from the tropics to the extratropics and further to the mid-high latitudes,constituting a major physical mechanism that contributes to the early summer circulation anomalies in the subtropics and in the North Pacific mid-high latitudes.The central equatorial Pacific La Ni（n）a forcing in the spring would,on the one hand,induce teleconnection anomalies of high pressure from the Sea of Okhotsk to the Sea of Japan in early summer,and on the other hand indirectly trigger a positive low-frequency East Asia-Pacific teleconnection （EAP） wave train in the lower troposphere.

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.

Relation among Summer Rainfall in South Shandong and High Pressure in South Asia and Atmospheric Circulation

[Objective] The aim was to study the relation among summer rainfall in south Shandong and high pressure in South Asia and atmospheric circulation.[Method] Taking the precipitation in south Shandong along the Yellow River and Huaihe River,using the NCEP/NCAR data and summer rainfall data in south Shandong in summer from 1961 to 2005,the characteristics of high pressure in South Asia and atmospheric circulation in drought year and flood year in summer in south Shandong Province were expounded.The mechanism of 100 hPa pressure in South Asian influencing precipitation in south Shandong Province was discussed.The interaction of different equipment,different altitude and different system of atmosphere circulation in low and high layer was expounded.[Results] The first mode of EOF decomposition of precipitation in summer in south Shandong Province explained above 63% variances and reflected universal form of precipitation.The difference of central position of the central position of height field of high pressure in South Asia in drought and flood year was small.But the wind field center was inconsistent.As the area of SAH was smaller and its eastern ridge line stretched to the Western Pacific between the middle of south Shandong and Changjiang Estuary,flood summer occurred when there was an unusual cyclone lied in the east of 90° E and south of Lake Baikal.The area of SAH was larger and its eastern ridge line stretching to the Western Pacific over Changjiang Estuary,drought summer occurred,when there was an unusual anticyclone lied in the east of 90° E and south of Lake Baikal.SAH and summer rainfall also had close relationship with Tele-connection Patterns over the Eurasia continent and EAP Tele-connection.When the height anomaly was in '+-+' form in the north of the Caspian Sea,around Lake Baikal and Kamchatka,and when the height anomaly in East Asia-West Pacific area was in '-+' form from low altitude to high altitude,there was much precipitation in summer;and conversely,it was drought in summer in south Shandong.[Conclusion] It provided the oretical basis for summer rainfall in south Shandong.

Atmospheric Diabatic Heating and Summertime Circulation in Asia-Africa Area

Utilizing data from NCEP/ NCAR reanalysis, the summertime atmospheric diabatic heating due to different physical processes is investigated over the Sahara desert, the Tibetan Plateau, and the Bay of Bengal. Atmospheric circulation systems in summer over these three areas are also studied. Thermal adaptation theory is employed to explain the relationship between the circulation and the atmospheric diabatic heating. Over the Sahara desert, heating resulting from the surface sensible heat flux dominates the near-surface layer, while radiative cooling is dominant upward from the boundary layer. There is positive vorticity in the shallow boundary layer and negative vorticity in the middle and upper troposphere. Downward motion prevails over the Sahara desert, except in the shallow near—surface layer where weak ascent exists in summer. Over the Tibetan Plateau, strong vertical diffusion resulting from intense surface sensible heat flux to the overlying atmosphere contributes most to the boundary layer heating, condensation associated with large—scale ascent is another contributor to the lower layer heating. Latent heat release accompanying deep convection is critical in offsetting longwave radiative cooling in the middle and upper troposphere. The overall diabatic heating is positive in the whole troposphere in summer, with the most intense heating located in the boundary layer. Convergence and positive vorticity occur in the shallow near—surface layer and divergence and negative vorticity exist deeply in the middle and upper troposphere. Accordingly, upward motion prevails over the Plateau in summer, with the most intense rising occurring near the ground surface. Over the Bay of Bengal, summertime latent heat release associated with deep convection exceeds longwave radiative cooling, resulting in intense heating in almost the whole troposphere. The strongest heating over the Bay of Bengal is located around 400 hPa, resulting in the most intense rising occurring between 300 hPa and 400 hPa, and producing positive vorticity in the lower troposphere and negative vorticity in the upper troposphere. It is also shown that the divergent circulation is from a heat source region to a sink region in the upper troposphere and vice versa in lower layers. Key words Atmospheric diabatic heating - Summer - Circulation This work was jointly supported by “ National Key Program for Developing Basic Sciences” G1998040904 by NSFC projects 49805003, 49635170, 49823002, and 49825504.

Summer extreme precipitation patterns and synoptic-scale circulation precursors over the Tibetan Plateau

In the context of global warming,the extreme summer precipitation over the Tibetan Plateau(TP)has changed significantly.In this study,the summer(June–August)extreme precipitation on the TP was classified into three spatial types by applying the K-means clustering method to the Third Pole Region long time-series high-resolution(1/30°)precipitation dataset(TPHi Pr,1979–2020).The characteristics of the circulation anomalies and precursors corresponding to the extreme precipitation on the TP in summer during 1979–2020 were investigated.The results showed that the summer extreme precipitation of the TP can be categorized into northwestern(NW),southeastern(SE),and southern Himalayan(HS)types based on extreme precipitation thresholds.The NW and SE types are mainly influenced by anomalous signals in the mid-to-high latitude regions upstream of them,whereas the HS type is controlled by the localized subtropical anomalous circulation.On the 8th day before the onset of the NW type,an anomalous cyclone was observed in the western Atlantic Ocean(60°W,50°N),which triggered the west-to-east quasi-latitudinal propagation of Rossby waves.On the onset day of the NW type,the upper troposphere showed positive-negative-positive geopotential height anomalies along the latitudinal 40°N from the Caspian Sea,the western part of Xinjiang,to the northeastern TP.Moisture entered from the Arabian Sea along the southeastern edge of the anomalous cyclone on the southwestern TP and converged in the northwestern TP.Compared to the NW type,the precursors of the SE type appeared at higher latitudes and were more intense.On the 8th day before the onset of the SE type,an anomalous cyclone occurred near Greenland(60°W,70°N)and excited Rossby waves propagating southeastward.On the onset day of the SE type,the upper troposphere showed negative-positive-negative geopotential height anomalies across Eurasia from the Ural Mountains and the Iranian Plateau to the northern TP in the northwest-southeast direction and entered the southeastern TP from the Bay of Bengal along the southeastern edge of the anomalous cyclone in the southern TP.On the 6th day before the onset of the HS type,the anomalously high pressure in the middle and lower layers of the low-latitude region extended westward,and a significant anticyclonic anomalous circulation occurred on the southern TP on the onset day of the HS type,enabling the delivery of moisture from the Bay of Bengal to the southern foothills of the Himalayas.Additionally,an anomalous cyclone perched in the northeastern TP at a geopotential height of 200 h Pa strengthened westerly winds in the southern TP and contributed to the maintenance of the anticyclonic system on the southern TP.

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.

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.

AN EOF ITERATION SCHEME FOR THE PREDICTION OF SUMMER RAINFALL IN CHINA IN TERMS OF PREVIOUS-PERIOD ATMOSPHERIC CIRCULATION PATTERNS

Many studies have shown the pronounced statistical relationship between seasonal averaged 500 hPa height departures in the preceding winter/spring and rainfall anomalies over China in the following summer (June—August), which suggests a possibility that the previousperiod atmospheric circulation pattern can be employed to forecast the summer rainfall

ENSO对中国沿海夏季降水影响的区域特征研究

利用降水和海表温度格点数据以及中国沿海台站降水数据,分析中国沿海夏季降水对ENSO时空响应特点,并探讨中国沿海夏季降水年代际变化原因。结果表明:1)中国沿海夏季降水受ENSO影响区域特征明显,以连云港和云澳为界可分为3个区域。渤黄海沿海夏季降水与Niño3.4指数呈负相关,东海沿海呈正相关,南海沿海相关关系不显著。2)年代际尺度上,中国沿海夏季降水与Niño3.4指数关系不稳定。渤黄海沿海夏季降水与Niño3.4指数负相关在1980年之前和2010年之后较为显著,东海沿海的正相关在20世纪80年代之后变得不显著,南海沿海的相关关系不显著。3)年代际尺度上,渤黄海沿海夏季年代际降水与同期Niño3.4指数、前一年冬季AO指数和当年春季AAO指数呈显著负相关,而东海沿海都呈显著正相关,南海沿海夏季年代际降水与当年春季北极海冰指数呈明显负相关。对于Niño3.4指数,中东太平洋偏高的海表温度能在500hPa位势高度场激发出负太平洋-日本型遥相关波列,使得渤黄海沿海夏季年代际降水减少和东海沿海增多;当前一年冬季AO和当年春季AAO为正相位时,850 hPa风场在贝加尔湖南部的异常反气旋引导中高纬地区气流南下,引起东亚夏季风减弱,造成渤黄海沿海夏季年代际降水减少;另外,西北太平洋副热带高压强度偏强且位置偏西引起上升运动增强,导致东海沿海夏季年代际降水增多;春季北极海冰年代际变化能在850hPa风场和500hPa位势高度场上,激发出导致南海沿海夏季降水年代际变化相反的大气环流形势。

EOF迭代方案恢复夏季大气环流场的试验

本文设计了一种恢复夏季大气环流场的EOF(经验正交函数)迭代方案,并利用1951～1984年中国夏季降水距平场和北半球500hPa夏季高度距平场作镢复试验,数值试验效果较好.此项工作表明:利用某些长时间序列的资料通过EOF迭代方案恢复历史时期大气环流是可行的.

影响中国夏季降水的前冬北半球中高纬大气环流年际变异主模态

本文利用观测和再分析资料,通过奇异值分解(Singular Value Decomposition,SVD)分析,发现北极涛动(Arctic Oscillation,AO)是显著影响中国夏季降水年际异常的前冬中高纬大气环流变异的主模态。AO在冬季发展成熟,在春季衰亡,在夏季发生位相反转。AO会导致华北、东北、长江中下游和华南夏季降水异常呈现三极型分布。伴随正位相的AO,在黄海至日本海上空的异常低压伴随的东北风异常引起华北和东北水汽通量异常辐散及降水减少,而西北太平洋的异常高压不仅增强其北侧的西南风水汽输送,和北部异常低压共同作用导致长江中下游水汽通量异常辐合及降水增加,而且使得华南水汽通量异常辐散,降水减少。因此,本文发现的前冬AO模态与我国夏季三极型异常降水分布的关系可为我国夏季旱涝预测提供一个重要的中高纬前期因子。

秦岭山地夏季降水的时空变化特征及其气候归因

探究秦岭山地夏季降水及其大气归因,是为了研究区域环境对全球气候变化的响应关系。根据1959—2022年陕西省秦岭山地32个气象站点数据和15个大气环流指数,运用一元线性回归法和小波变换分析法(CWT),研究了秦岭山地64年来夏季降水的时间变化特征和空间演变规律及其与大尺度环流指数的关系。结果表明,1959—2022年秦岭山地夏季降水呈现不显著的上升趋势,一元线性回归法的变化速率为10.81 mm/10 a。其中,秦岭山地南坡的商南站降水量变化率最大,为19.3 mm/10 a。秦岭山地夏季平均降水量为344.34 mm。位于秦岭山地南坡的紫阳县降水量最大,约为469.35 mm。位于秦岭山地北坡的华阴县降水量最少,约为216.51 mm。秦岭山地南坡夏季降水明显多于北坡,降水量约为107.68 mm。南坡夏季平均降水量约为377.99 mm,北坡夏季平均降水量约为270.31 mm,均未通过显著性检验。秦岭山地64年来的夏季降水量与SOI、SAODI、SWACI均有较强的正相关关系,与EASMI、SCSMI、SASMI均存在负相关关系。

1961—2020年河南省夏季极端降水异常分布特征及其与大气环流和海温的关系

利用河南省125个气象台站逐日降水量资料和ERA5再分析资料以及HadISST海表温度资料,分析了1961—2020年河南省夏季极端降水的气候特征和主模态,及其与大气环流和海温的关系。结果表明:河南省极端降水的气候态频次和强度均呈由西北向东南递增的分布,东南部地区极端降水最多,东部和北部地区次之,西部山脉地区最少。标准差分布与气候态类似,降水较多的区域也具有较大的年际差异。河南省夏季极端降水年际变化显著,但线性趋势较弱。河南省夏季极端降水EOF第1模态呈全省一致偏多的空间分布特征,欧亚型遥相关模态有利于高纬度冷空气南下,副热带高压西伸偏北有利于充沛的水汽向河南地区输送,同期海温场呈现出显著的黑潮延伸体正异常海温、La Niña模态、南印度洋海温偶极子负位相和北大西洋海温三极子正位相模态,反之亦然;第2模态为河南省北部地区偏多和南部地区偏少的反位相分布特征,乌拉尔山地区的阻塞高压和东西伯利亚地区的长波槽有利于高纬冷空气南下,副热带高压西伸偏北有利于水汽向河南地区输送,同期海温场呈现出典型的La Niña模态,但降水的极端性越强,海温异常信号也越复杂,反之亦然。

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。