Simulation of the east asian summer monsoon using a variable resolution atmospheric GCM

The East Asia summer monsoon (EASM) is simulated with a variable resolution global atmospheric general circulation model (GCM) developed at the Laboratoire de Météorologie Dynamique, France. The version used has a local zoom centered on China. This study validates the model's capability in reproducing the fundamental features of the EASM. The monsoon behaviors over East Asia revealed by the ECMWF reanalysis data are also addressed systematically, providing as observational evidence. The mean state of the EASM is generally portrayed well in the model, including the large-scale monsoon airflows, the monsoonal meridional circulation, the cross-equatorial low-level jets, the monsoon trough in the South China Sea, the surface cold high in Australia, and the upper-level northeasterly return flow. While the performance of simulating large-scale monsoonal climate is encouraging, the model's main deficiency lies in the rainfall. The marked rainbelt observed along the Yangtze River Valley is missed in the simulation. This is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western Pacific subtropical high, which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies. The excessively westward extension of the high, together with the distorted Indian low, also makes the contribution of the tropical southwest monsoon to the moisture convergence over the Yangtze River Valley too weak in the model. The insufficient plateau heating and the resulting weak land-sea thermal contrast are responsible for the weakly reproduced monsoon. It is the deficiency of the model in handling the low-level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating. Comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves, in a general manner, the EASM simulation.     

RELATIONSHIP BETWEEN THE SEASONAL TRANSITION OF EAST ASIAN MONSOON CIRCULATION AND ASIAN-PACIFIC THERMAL FIELD AND POSSIBLE MECHANISMS

The NCEP/NCAR reanalysis, CMAP rainfall and Hadley Centre sea surface temperature (SST) datasets are used to investigate the relationship between the seasonal transition of East Asian monsoon and Asian-Pacific thermal contrast, together with the possible causes. Based on the 250 hPa air temperature over two selected key areas, the Asian-Pacific thermal difference (APTD) index is calculated. Results show that the APTD index is highly consistent with the Asian-Pacific Oscillation (APO) index defined by Zhao et al., in terms of different key areas in different seasons. Moreover, the time point of the seasonal transition of the Asian-Pacific thermal contrast can be well determined by the APTD index, indicative of seasonal variation in East Asian atmospheric circulation from winter to summer. The transition characteristic of the circulation can be summarized as follows. The continental cold high at lower tropospheric level moves eastward to the East China Sea and decreases rapidly in intensity, while the low-level northerlies turn to southerlies. At middle tropospheric level, the East Asia major trough is reduced and moves eastward. Furthermore, the subtropical high strengthens and appears near Philippines. The South Asia high shifts from the east of Philippines to the west of Indochina Peninsula, and the prevailing southerlies change into northerlies in upper troposphere. Meanwhile, both the westerly and easterly jets both jump to the north. The seasonal transition of atmospheric circulation is closely related to the thermal contrast, and the possible mechanism can be concluded as follows. Under the background of the APTD seasonal transition, the southerly wind appears firstly at lower troposphere, which triggers the ascending motion via changing vertical shear of meridional winds. The resultant latent heating accelerates the transition of heating pattern from winter to summer. The summer heating pattern can further promote the adjustment of circulation, which favors the formation and strengthening of the low-level southerly and upper-level northerly winds. As a result, the meridional circulation of the East Asian subtropical monsoon is established through a positive feedback between the circulation and thermal fields. Moreover, the time point of this seasonal transition has a significant positive correlation with the SST anomalies over the tropical central-eastern Pacific Ocean, providing a basis for the short-term climate prediction.     

The response of the East Asian summer monsoon to strong tropical volcanic eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon （EASM） and EASM rainfall.Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years.The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years.In response to strong tropical volcanic eruptions,the meridional air temperature gradient in the upper troposphere is enhanced,which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream （EASWJ）.At the same time,the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened.The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.     

欧亚大陆春季融雪量与东亚夏季风的可能联系

通过对观测资料的分析, 本文研究了春季欧亚大陆融雪量与东亚夏季风的关系, 并初步讨论了其可能联系机制。研究表明, 春季融雪量EOF (Empirical Orthogonal Function) 第一模态表现出年代际变化特征, 这与东亚夏季风和中国夏季降水的年代际转型具有非常好的一致性。而EOF第二模态与东亚夏季风在年际尺度上具有同位相变化关系, 当春季融雪量在东西伯利亚和巴尔喀什湖附近异常偏多时, 后期在东亚地区容易出现由高纬至低纬的“负—正—负”经向波列结构。融雪量异常偏少时, 情况则相反。文中初步分析了春季融雪量异常与后期夏季东亚地区大气环流出现经向波列结构的可能联系机制, 指出东西伯利亚以及巴尔喀什湖附近异常偏多的春季融雪量能够在该地区促使位势高度场表现为正异常, 随着时间的演变, 巴尔喀什湖附近地区的高压向东移动发展, 东西伯利亚地区的高压一部分向低纬移动, 可能造成夏季东亚地区的经向波列结构, 进而对东亚的天气和气候产生影响。     

Simulation of East Asian Summer Monsoon with IAP CGCM

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Diversity of the Coupling Wheels in the East Asian Summer Monsoon on the Interannual Time Scale: Challenge of Summer Rainfall Forecasting in China

Two types of three-dimensional circulation of the East Asian summer monsoon(EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979–2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high(SAH) anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upperlevel SAH via the meridional flow over Southeast Asia. These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years, respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel's interannual variability, illustrating the challenges in forecasting summer rainfall over China.     

Responses of East Asian summer monsoon to historical SST and atmospheric forcing during 1950–2000

The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR) community atmospheric model version 3 (CAM3) and the geophysical fluid dynamics laboratory (GFDL) atmospheric model version 2.1 (AM2.1) are analyzed. These simulations are forced separately with different historical forcing, namely tropical sea surface temperature (SSTs), global SSTs, greenhouse gases plus aerosols, and a combination of global SSTs and greenhouse gases plus aerosols. This study focuses on the relative roles of these individual forcings in causing the observed monsoon and rainfall changes over East Asia during 1950–2000. The simulations from both models show that the SST forcing, primarily from the Tropics, is able to induce most of the observed weakening of the EASM circulation, while the greenhouse gas plus (direct) aerosol forcing increases the land-sea thermal contrast and thus enhances the EASM circulation. The results suggest that the recent warming in the Tropics, especially the warming associated with the tropical interdecadal variability centered over the central and eastern Pacific, is a primary cause for the weakening of the EASM since the late 1970s. However, a realistic simulation of the relatively small-scale rainfall change pattern over East China remains a challenge for the global models.     

大气热源年代际异常与20世纪70年代末期东亚夏季风年代际减弱

利用1958-2001年ERA-40再分析资料计算大气热源,统计分析了亚洲季风区及其邻近海域大气热源年代际变异的典型模态;利用线性斜压干模式,模拟了夏季大气对大气热源年代际异常的响应,揭示了大气热源年代际异常与1970s末期东亚夏季风年代际减弱的关系。结果表明:近50 a来亚洲及其邻近海域夏季整层大气热源变异主要表现为年代际变化特征,其年代际位相转换发生在1970s中后期,这与东亚夏季风年代际减弱的时间一致;菲律宾附近海域和中国西南地区是与东亚夏季风年代际减弱有直接联系的两个热源异常关键区;东亚夏季风年代际减弱最直接地表现为这两个关键区热源异常的共同作用,而赤道中东太平洋、赤道印度洋大气热源增强则通过大气遥响应机制影响菲律宾附近海域低层大气环流异常对东亚夏季风变异起相反的贡献。     

THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols(dust, sea salt,sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon(EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM.The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25° N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced,especially in North and Southwest China where the value of moisture flux divergence increases.     

The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies※

The record-breaking mei-yu in the Yangtze-Huaihe River valley (YHRV) in 2020 was characterized by an early onset, a delayed retreat, a long duration, a wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation (QBWO) during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu, a northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly, during the mei-yu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the upper-level East Asian westerly jet, and the low-level southwesterly jet, experienced periodic oscillations linked with the QBWO. Most notably, the repeated establishment of a large southerly center, with relatively stable latitude, led to moisture convergence and ascent which was observed to develop repeatedly. This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes. Moreover, two blocking highs were present in the middle to high latitudes over Eurasia, and a trough along the East Asian coast was also active, which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths. The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV. The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH, which was observed to be favorable for excessive rainfall over YHRV.     

RELATIONSHIP BETWEEN WINTERTIME THERMAL CONTRAST OVER THE ASIAN CONTINENT AND THE CLIMATE IN CHINA

Recent studies indicated that except for the land-sea thermal contrast,there also existed the land-land thermal contrast.The composite analysis and t-test method are used to further study the local thermal contrast variation over the Asian continent,and to discuss the association of seasonal variation of land thermal state with circulation over East Asia,the early summer and summer monsoon activity,and the precipitation anomaly in China in the decadal scale.Results show that the positive meridional temperature anomaly transports downward from upper tropospheric layers in middle-high latitudes north of 25°N in the positive years.In the zonal direction,the Tibetan Plateau heating in the successive spring acts as a force to influence the atmosphere,leading to the rapid temperature warming over eastern Chinese continent,which could increase the land-sea thermal contrast with the negative SSTA.Accordingly,the monsoon activity in early summer over East Asian establishes earlier and the summer monsoon intensity becomes stronger.The early summer precipitation is more-than-normal over the Yangtze River,and the summer precipitation is more-than-normal over the north China and the southwest China.The situation is contrary in the negative years.     

东亚-太平洋型季节内演变和维持机理研究

利用850hPa的纬向风异常建立一个逐候东亚-太平洋(East Asian Pacific,EAP)型指数,研究其季节内演变特征,发现东亚-太平洋型经向波列是东亚夏季风季节内变化的主要模态.其演变过程为:扰动首先出现在北太平洋中部,并通过正压不稳定过程从基本气流中获得能量而发展,在高层罗斯贝波能量向南频散,激发热带对流异常和赤道罗斯贝波,并相互锁相,因赤道罗斯贝波受β效应影响而共同向西移动.热带对流和环流异常在菲律宾附近达到最强,此时在东亚沿岸出现经向三极型波列,此后中低纬度异常继续向西北方向移动,使降水异常在长江流域能维持较长时间.东亚-太平洋型在东亚发展和维持有以下原因:首先,菲律宾暖水上空的对流和低层环流之间存在正反馈;其次,由于海陆热力差异导致暖大陆和冷海洋之间存在特殊的纬向温度梯度和北风垂直切变,东亚-太平洋型在经向上有向北倾斜的斜压结构,能通过斜压能量转换从平均有效位能中获得能量,同时,也能从经向温度梯度的平均有效位能中获得能量.     

Influence of soil moisture in eastern China on the East Asian summer monsoon

The sensitivity of the East Asian summer monsoon to soil moisture anomalies over China was investigated based on ensembles of seasonal simulations(March–September) using the NCEP GCM coupled with the Simplified Simple Biosphere Model(NCEP GCM/SSi B). After a control experiment with free-running soil moisture, two ensembles were performed in which the soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China(YRNC) was double and half that in the control, with the maximum less than the field capacity. The simulation results showed significant sensitivity of the East Asian summer monsoon to wet soil in YRNC. The wetter soil was associated with increased surface latent heat flux and reduced surface sensible heat flux. In turn, these changes resulted in a wetter and colder local land surface and reduced land–sea temperature gradients, corresponding to a weakened East Asian monsoon circulation in an anomalous anticyclone over southeastern China, and a strengthened East Asian trough southward over Northeast China. Consequently, less precipitation appeared over southeastern China and North China and more rainfall over Northeast China. The weakened monsoon circulation and strengthened East Asian trough was accompanied by the convergence of abnormal northerly and southerly flow over the Yangtze River valley, resulting in more rainfall in this region.In the drier soil experiments, less precipitation appeared over YRNC. The East Asian monsoon circulation seems to show little sensitivity to dry soil anomalies in NCEP GCM/SSi B.     

Variations of the summer Somali and Australia cross-equatorial flows and the implications for the Asian summer monsoon

The temporal variations during 1948-2010 and vertical structures of the summer Somali and Australia cross-equatorial flows(CEFs) and the implications for the Asian summer monsoon were explored in this study.The strongest southerly and northerly CEFs exist at 925 hPa and 150 hPa level,respectively.The low-level Somali(LLS) CEFs were significantly connected with the rainfall in most regions of India(especially the monsoon regions),except in a small area in southwest India.In comparison to the climatology,the lowlevel Australia(LLA) CEFs exhibited stronger variations at interannual time scale and are more closely connected to the East Asian summer monsoon circulation than to the LLS CEFs.The East Asian summer monsoon circulation anomalies related to stronger LLA CEFs were associated with less water vapor content and less rainfall in the region between the middle Yellow River and Yangtze River and with more water vapor and more rainfall in southern China.The sea-surface temperature anomalies east of Australia related to summer LLA CEFs emerge in spring and persist into summer,with implications for the seasonal prediction of summer rainfall in East Asia.The connection between the LLA CEFs and East Asian summer monsoon rainfall may be partly due to its linkage with El Nino-Southern Oscillation.In addition,both the LLA and LLS CEFs exhibited interdecadal shifts in the late 1970s and the late 1990s,consistent with the phase shifts of Pacific Decadal Oscillation(PDO).     

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia, The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However,the opposite interdecadal variation was found in the rainfall anomaly in North China and South China.The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.     

华西秋雨起止与秋冬季节大气环流转换

根据1961—2010年平均的逐候NCEP/NCAR再分析资料、1979—2008年平均的逐候CMAP降水资料以及1961—2010年逐候平均的中国553个台站降水资料,讨论了华西秋雨起止日期与秋冬季大气环流转换特征的关系。结果表明,华西地区降水年变化表现为明显的夏、秋双峰特征,8月4—8日(第44候)为双峰间的低谷,10月8—12日(第57候)以后降水降至年平均以下。由此,将华西秋雨建立和结束日期分别确定为8月9—13日(第45候)和10月8—12日(第57候)。华西秋雨的建立对应于东亚夏季风开始向冬季风转变,其标志性环流调整特征是江南地区的西南风转为东南风。东亚经向海平面气压梯度在8月9—13日(第45候)由南高北低转为南低北高,造成850 hPa江南地区的西南风转为东南风,该东南风与来自孟加拉湾的热带西南季风交汇于华西地区,形成风向和水汽的辐合,使得华西地区的降水在夏峰之后再次增强,华西秋雨由此建立。华西秋雨的结束则对应于孟加拉湾热带西南季风结束和东亚冬季风完全建立,其标志性环流调整特征是孟加拉湾地区的西南风转为东北风。随着东亚纬向海平面气压梯度由北向南依次发生东高西低向东低西高的转变,东亚冬季风也逐步向南推进,9月8—12日(第51候)东北冬季风到达江南地区,10月8—12日(第57候)进一步推进到南海地区,此时来自孟加拉湾的热带西南季风消失,造成华西地区完全受大陆冷高压控制,东亚季风经圈环流也转为冬季型哈得来环流,东亚冬季风完全建立,华西秋雨也随之结束。因此,华西秋雨起止可能与东亚夏季风、南亚夏季风向冬季风的转变时间不同步有关,东亚季风与南亚季风的共同作用使得华西秋雨成为亚洲夏季风在中国大陆上的最后一个雨季。     

20CR再分析资料在东亚夏季风区的质量评估

本文利用NCEP1和ERA40再分析资料, 并结合观测资料, 对最新公布的一套再分析资料——20CR再分析资料在东亚夏季风区的质量进行了综合评估。本文主要是从气候态、年际变率、年代际变化三个方面, 来评估20CR再分析资料在东亚夏季风区的质量。结果表明, 在气候平均态上, 20CR再分析资料基本合理再现了东亚夏季风区的高低层环流场(包括南亚高压、副热带西风急流、近地层风场)以及经向环流圈特征。但相较于NCEP1和ERA40, 20CR所刻画的南亚高压偏强, 西风急流偏北, 对流层中上层温度系统性偏高。在年际变率方面, 除了NCEP1在1967年之前存在偏差, 使其结果和ERA40、20CR不同之外, 三套再分析资料刻画的东亚夏季风变率在其它时段高度一致。三套资料在以纬向风为基础的东亚夏季风指数上的一致性, 高于以经向风为基础的东亚夏季风指数, 其中以低层纬向风为基础的东亚夏季风指数的一致性最高。20CR再分析资料可以较好地再现与东亚夏季风相联系的地表气温和降水年际变化特征, 其刻画的地表气温正相关中心位置偏西、强度最强, 且在河套平原地区有一个弱的负相关中心, 而其描述的降水在孟加拉湾和长江流域较之另外两套再分析资料更接近观测结果, 在热带地区和海上却反之。在年代际变化方面, 20CR再分析资料未能合理再现东亚夏季风年代际减弱的现象, 这也体现在不能合理再现青藏高原下游年代际变冷和“南涝北旱”降水型上, 这主要是因为20CR再分析资料所刻画的东亚地区对流层中上层年代际变冷信号偏弱所致。而在百年时间尺度上, 20CR再分析资料所刻画的东亚夏季风变化与观测较为一致;20CR再分析资料可以合理再现出东亚夏季风区1920年代前的显著冷期和1990年代之后的迅速增暖期, 但对1920～1950年代相对暖期和1950～1980年代相对冷期的再现能力较差。     

ECHAM4/OPYC3海气耦合模式对东亚季风年循环及其未来变化的模拟

德国马普研究所海气耦合摸式ECHAM4/OPYC3对东亚地区2 m温度年循环的模拟尽管有一些偏差,但还是相当成功的.其模拟的东亚夏季风偏弱,而冬季风偏强,此偏差可能与2 m温度以及西太平洋副热带高压模拟偏差有关.该模式模拟的东亚季风区夏季降水量偏弱,这与上述夏季风环流的模拟结果是一致的.该模式较好地抓住了华北地区经向环流和降水量的年循环特征.利用最新的温室气体和SO2排放方案,即政府间气候变化委员会(IPCC)排放方案特别报告(SRES)的A2和B2方案,通过该模式111年的积分结果讨论了东亚季风气候在21世纪后30年中的变化,其主要结果为:全球变暖导致夏季海陆温差增大和冬季海陆温差减弱,进而使东亚季风环流在夏季加强,冬季减弱.长江流域和华北地区的夏季降水量显著增强,而后者的增强更为显著,使得东亚季风区的夏季多雨区向北延伸;东亚季风区9月份的降水量在两个方案中都显著增加,说明在全球变暖条件下东亚季风区的多雨季节将延迟一个月.     

Impacts of Coastal SST Variability on the East Asian Summer Monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions （EACRSST） on the East Asian summer monsoon （EASM） have been examined using a regional climate model （PδRCM9） in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM （IEASM） differences.
The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast （LSTC） which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.     

亚洲夏季风30～60天季节内振荡对中国东部地区持续性极端降水的影响

本文基于1979～2015年中国台站观测的格点化高分辨率降水和NCEP II大气再分析逐日资料,探讨了亚洲季风区夏季30～60天大气季节内振荡（ISO）与长江中下游地区持续性降水异常的关系,重点揭示了南亚和东亚子季风区ISO的相互作用及二者协同引起长江中下游持续性极端降水的物理机制。合成分析表明,南亚和东亚ISO是通过高层辐散环流发生相互作用。在ISO位相1～3（5～7）,异常活跃（抑制）对流从赤道印度洋北传至孟加拉湾—印度次大陆区域,其伴随的高层异常辐散（辐合）环流通过补偿效应,引起南海—热带西北太平洋的异常高层辐合（辐散）,加强了局地的异常下沉（上升）运动,有利于南海—西北太平洋的异常抑制（活跃）对流发展并维持。南海—西北太平洋的异常抑制（活跃）对流伴随着显著的斜压散度,并进一步激发出一个连接南海和长江中下游的经向垂直环流圈,引起长江中下游强烈的异常上升（下沉）运动和低层水汽辐合（辐散）,使得降水持续性偏多（少）,极端降水的发生概率持续地偏高（低）,有利于（不利于）形成持续性极端降水事件。研究还表明,亚洲季风区ISO的强度存在显著的年际变化,并对长江中下游持续性极端降水的发生频次和持续时间具有调制作用。在ISO偏强（弱）年,长江中下游持续性极端降水的发生频次较高（低）,且持续时间较长（短）。