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。     

梅雨前后亚洲季风区平均散度风环流和水汽输送的研究

分析了１９８３年江淮流域梅雨期及其前后亚洲季风区平均散度风环流和垂直积分水汽输送的辐散分量。在入梅前强水汽辐合中心位于华南，出梅后则位于印度季风区，梅雨期这两地区都存在着强水汽辐合中心，并且强水汽辐合区和中国东部雨带的位置变化及热源分布有着密切关系。同时指出，江谁流域梅雨与大尺度辐散环流的变化、热源的配置及水汽辐合的位置有着较好的对应关系。     

亚洲季风建立及其中期振荡的高空环流特征

本文主要通过对100毫巴散度场、高度场和垂直积分的水汽输送场的分析,着重讨论了1979年整个亚洲季风区季风建立及其振荡中的高空环流特征。发现南亚高压周围不同部位的高空辐散场的建立导致了这些地区夏季风的建立,且南亚高压脊线中部和东部散度场具有不同的分布特点。从而使得印度季风有与长江流域梅雨同时开始而与华南雨季降水反位相的特点.在东亚,南亚高压外围东风对于夏季风的向北推进具有很好的指示性。在印度,伊朗高压(南亚高压环流的一部分)外围的东北气流对于印度季风的爆发具有指示性。100毫巴若干地区高度场和亚洲季风区域大范围水汽输送场的40—50天振荡清楚地显示出向东向北移动,而20—30天的散度场的振荡在印度季风爆发后则有系统地向西移动.      

南海夏季风持续异常的特征及其对全球环流的影响II.数值试验

利用T42L9全球大气环流谱模式进行数值试验 ,以揭示南海夏季风强度异常的特征及其影响。控制试验结果表明 ,该模式不仅能够很好地模拟出气候平均的西风带槽脊和高低空气流分布以及它们的季节性变化 ,而且对于与亚洲季风有关的各个主要系统 ,如南亚高压、副高进退及越赤道气流等都有较强的模拟能力。在亚洲季风区及热带太平洋这一大范围区域的大气内部热源异常强迫下 ,模式显示出了南海夏季风持续异常的特征、北半球热带外环流的响应以及亚洲季风区降水异常分布。南海夏季风长时间强度异常所引起的大气内部热源异常 ,一方面通过三维垂直环流的异常联结着南海夏季风对北半球热带内外环流的影响 ,另一方面它又通过持续异常期的波列传播 ,即能量的传播 ,不仅影响我国长江流域降水 ,还会逐渐影响到北半球中高纬环流结构。这样西风带环流形势将会发生相应的变化和调整 ,南海夏季风持续异常影响到了北半球大气环流和天气气候的变化。     

南海夏季风持续异常的特征及其对全球环流的影响Ⅱ.数值试验

利用T42L9全球大气环流谱模式进行数值试验,以揭示南海夏季风强度异常的特征及其影响.控制试验结果表明,该模式不仅能够很好地模拟出气候平均的西风带槽脊和高低空气流分布以及它们的季节性变化,而且对于与亚洲季风有关的各个主要系统,如南亚高压、副高进退及越赤道气流等都有较强的模拟能力.在亚洲季风区及热带太平洋这一大范围区域的大气内部热源异常强迫下,模式显示出了南海夏季风持续异常的特征、北半球热带外环流的响应以及亚洲季风区降水异常分布.南海夏季风长时间强度异常所引起的大气内部热源异常,一方面通过三维垂直环流的异常联结着南海夏季风对北半球热带内外环流的影响,另一方面它又通过持续异常期的波列传播,即能量的传播,不仅影响我国长江流域降水,还会逐渐影响到北半球中高纬环流结构.这样西风带环流形势将会发生相应的变化和调整,南海夏季风持续异常影响到了北半球大气环流和天气气候的变化.     

全球稳定增温1.5℃下北非夏季风降水的响应及其机理

利用第六次国际耦合模式比较计划(CMIP6)模式模拟结果,研究了21世纪末全球稳定增温1.5 ℃下北非夏季风降水的变化及机理。结果表明,全球稳定增温1.5 ℃较1985—2014年北非夏季风降水将增加0.26 mm/d,区域降水敏感度为4.8 ％/℃,且季风区北部降水增幅大于南部。基于水汽收支诊断发现热力项对季风区总降水增加作用明显,动力项对降水空间变化起重要作用。进一步分析当地水汽条件及相应环流场发现:在热力上,相对于1985—2014年,稳定增温1.5 ℃加强了北非地区表面温度及低层水汽输送,有利于当地维持更高的大气可降水量。在动力上,稳定增温1.5 ℃下显著的撒哈拉沙漠增温加大了海陆温度梯度,增强了对流层低层季风环流,同时非洲东风急流北移,使得季风区北部低层气流辐合加强,而高层热带东风急流减弱会导致季风区高层辐散运动减弱。总的来说,热力项增加了整个季风区降水,而动力项增强了季风区北部降水,减弱南部降水,主导了降水变化的空间格局。     

云南极端干旱和多雨年5月异常环流的合成特征

应用1961 2010年NCEP/NCAR全球逐月再分析资料,对云南4次极端干旱年(下称干旱年)5月大气环流与4次5月降水偏多年(多雨年)大气环流进行合成对比分析,结果表明,两者从高纬到低纬都存在显著差异。干旱年500 hPa欧亚中高纬为两槽一脊,对应距平场呈"-+-"分布,西风带季节性北移晩;海平面气压场上亚洲为大范围负距平,影响云南的冷空气偏弱。而多雨年则相反,欧亚中高纬为两脊一槽,对应距平场呈"+-+"分布,西风带季节性北移早,乌拉尔山至里海的低槽引导冷空气入侵中国,海平面气压场上高原东部为正距平中心,影响云南的冷空气偏强。干旱年低纬地区环流差异表现为低层西太平洋副热带高压(下称西太副高)偏强、偏西,赤道西风向东、向北推进受阻,孟加拉湾、中南半岛的夏季风偏弱,爆发偏晚;而多雨年的环流形势则相反,西太副高偏弱、偏东,索马里越赤道气流和赤道西风偏强,孟加拉湾、中南半岛的夏季风偏强,爆发偏早;高层南亚高压反气旋环流多雨年比干旱年西伸更明显,范围更大、更强。与多雨年云南上空为异常上升运动不同,干旱年北半球低纬为大范围深厚的异常下沉运动,云南仍为Hadley经圈环流的下沉支控制。对水汽分析表明,多雨年西太副高偏东,云南以西南季风水汽输送为主,水汽通量辐合较常年偏强,水汽含量比多年平均增加,干湿季转换早;而干旱年西太副高偏西、偏南,云南以西风带水汽输送为主,对应异常的水汽通量辐散,水汽含量较常年减少,干湿季转换迟。亚洲夏季风强度指数WYI与5月降水有显著的正相关,并与5月极端降水有较好的对应关系。     

云南极端干旱和多雨年5月异常环流的合成特征北大核心CSCD

应用1961 2010年NCEP/NCAR全球逐月再分析资料,对云南4次极端干旱年(下称干旱年)5月大气环流与4次5月降水偏多年(多雨年)大气环流进行合成对比分析,结果表明,两者从高纬到低纬都存在显著差异。干旱年500 hPa欧亚中高纬为两槽一脊,对应距平场呈"-+-"分布,西风带季节性北移晩;海平面气压场上亚洲为大范围负距平,影响云南的冷空气偏弱。而多雨年则相反,欧亚中高纬为两脊一槽,对应距平场呈"+-+"分布,西风带季节性北移早,乌拉尔山至里海的低槽引导冷空气入侵中国,海平面气压场上高原东部为正距平中心,影响云南的冷空气偏强。干旱年低纬地区环流差异表现为低层西太平洋副热带高压(下称西太副高)偏强、偏西,赤道西风向东、向北推进受阻,孟加拉湾、中南半岛的夏季风偏弱,爆发偏晚;而多雨年的环流形势则相反,西太副高偏弱、偏东,索马里越赤道气流和赤道西风偏强,孟加拉湾、中南半岛的夏季风偏强,爆发偏早;高层南亚高压反气旋环流多雨年比干旱年西伸更明显,范围更大、更强。与多雨年云南上空为异常上升运动不同,干旱年北半球低纬为大范围深厚的异常下沉运动,云南仍为Hadley经圈环流的下沉支控制。对水汽分析表明,多雨年西太副高偏东,云南以西南季风水汽输送为主,水汽通量辐合较常年偏强,水汽含量比多年平均增加,干湿季转换早;而干旱年西太副高偏西、偏南,云南以西风带水汽输送为主,对应异常的水汽通量辐散,水汽含量较常年减少,干湿季转换迟。亚洲夏季风强度指数WYI与5月降水有显著的正相关,并与5月极端降水有较好的对应关系。     

南亚海陆热力差异及其对热带季风区环流的影响

利用NCEP/NCAR再分析资料,分析了亚洲热带季风区海陆分布所造成的热力差异,以及空间非均匀加热对热带季风区环流特别是初夏过渡季节环流的影响.在大尺度环流背景下,次大陆地形对亚洲热带地区环流的影响主要表现在对低层环流的热力作用,其中感热加热对冬、春季环流的影响明显,对秋季环流的作用相对较小.中南半岛和印度半岛之间的热力差异及其对环流的影响受到青藏高原的调配作用.在初夏过渡季节,高原热力强迫作用于低纬低层环流,使低纬约90 (E以东出现南风加强、以西出现北风加强,从而增强了中南半岛上空的潜热加热,减弱了其低层的感热加热,印度半岛地区还加强了低层的感热加热.多尺度、各种性质的加热共同作用于低纬大气,形成了亚洲热带地区独有的环流特征.     

气候平均状况下亚洲夏季风的季节内演变过程

根据1979—1995年美国NOAA的向外长波辐射逐日资料,用功率谱分析和带通滤波方法,对气候平均状况下亚洲夏季风的季节内演变过程进行分析,归纳得到亚洲季风区各个子系统季节内变化的8个关键阶段。利用1979—1999年NCEP/NCAR的大气环流再分析资料及中国气象局降水资料CMAP,对每个关键阶段亚洲夏季风的环流和降水的时空演变特征进行分析,得到亚洲季风区环流和降水季节内变化的物理图像。研究表明,在不同的季节内演变阶段,亚洲夏季风各个子系统成员的环流系统的变化特征可以将亚洲夏季风系统的季节内演变过程较好地描述出来。     

1—3月欧亚大陆热力变化及其与中国降水的关系

利用1979—2011年NCEP/NCAR再分析资料、我国160个站降水和气温资料,分析欧亚大陆热力变化特征,其在冬季和春季的气候变率最明显,且南北区域呈反相差异。在此基础上, 探讨1—3月欧亚大陆热力差异与中国降水异常的关系,欧亚大陆正 (负) 热力差异年,1—3月华南、西南至河套西部地区降水偏多 (少) 明显,后期夏季多雨带位于长江中下游地区 (华南地区)。大气环流异常特征显示:1—3月欧亚大陆南北热力差异与同期北极涛动 (AO)、东亚大槽、东亚高空急流等大尺度大气环流,以及后期东亚高空急流、南亚高压、低层季风风系异常的密切相关是欧亚大陆热力变化与中国降水联系的可能途径。     

青藏高原和亚洲夏季风动力学研究的新进展

亚洲夏季风环流受海陆和伊朗高原—青藏高原大地形的热力作用调控.亚洲季风所释放的巨大潜热又对大气环流形成反馈.这种相互反馈过程十分复杂,揭示其物理过程对理解气候变化格局的形成和变化以及提高天气预报及气候预测的准确率十分重要.夏季北半球副热带对流层上层环流的主要特征是存在庞大的南亚高压(SAH)以及强大的对流层上层温度暖中心(UTTM).本文介绍了温度—加热垂直梯度(T-Q_Z)理论的发展,并用以揭示SAH和UTTM的形成机制.指出沿副热带欧亚大陆东部的季风对流潜热加热及其中西部的表面感热加热和高层长波辐射冷却是导致SAH和UTTM在南亚上空发展的原因.文中还介绍了Gill模型用于上部对流层研究的局限性及解决的办法.     

Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses

NCEP/GFS analysis is used to investigate the scale dependence and the interplay between the terms of the atmospheric water budget over West Africa using a dedicated decomposition methodology. The focus is on a 2-month period within the active monsoon period of 2006. Results show that the dominant scales of seasonal mean precipitation and moisture flux divergence over West Africa during the monsoon period are large scales (greater than 1,400 km) except over topography, where mean values of small scales (smaller than 900 km) are strong. Correlations between moisture flux divergences in monsoon and African Easterly Jet layers and precipitation indicate that precipitation is strongly correlated to moisture flux divergence via both large-scale and small-scale processes, but the correlation signal is quite different depending on the region and vertical layer considered. The analysis of the scales associated with the rainfall and the local evaporation over 3 different regions shows that positive correlation exists over the ocean between precipitation and evaporation especially at large scale. Over the continent south of the Sahel, the correlation is negative and driven by large scale. Over the northern part of Sahel, positive correlation is found, only at small scales during the active monsoon period. Lag correlation reveals that the maximum evaporation over the Sahel occurs 1–3 days after the maximum precipitation with maximum contribution from small-scale processes during the first day. This study shows that NCEP/GFS reproduces well the known atmospheric water budget features. It also reveals a new scale dependence of the relative role of each term of the atmospheric water budget. This indicates that such scale decomposition approach is helpful to clarify the functioning of the water cycle embedded in the monsoon system.     

青藏高原季风对我国西北干旱区气候的影响

基于ERA-Interim逐月再分析资料及同期高原和我国西北干旱区观测站温度与降水资料,分析高原季风与西北干旱区气候的关系,对比高原典型强弱季风年平均大尺度环流和水汽输送条件的差异,探讨高原强弱季风年西北干旱区气候差异形成的原因。研究结果表明:高原季风与我国西北干旱区气候相关关系显著;高原强弱季风年对应的大尺度环流和垂直环流系统存在明显差异;水汽条件和抬升条件好坏与降水量多寡配合得较好,即强高原季风年,高原北部边缘水汽条件和抬升条件更有利于降水,而弱高原季风年,高原北部边缘水汽条件和抬升条件偏差,对应降水量较常年同期偏少。     

The effects of land-surface characteristics on the East Asian summer monsoon

 The Community Climate Model version 2 (CCM2) of the National Center for Atmospheric Research (NCAR) was used to investigate the effects of the land-surface characteristics on the East Asian summer monsoon. Four numerical experiments were performed in this study. They include the control run, the biosphere–atmosphere transfer scheme (BATS) run, the heavy snow run, and the light snow run. The results show that CCM2 can reasonably simulate many characteristics of the East Asian summer monsoon, such as the 850-hPa southwesterlies, 200-hPa easterlies, high precipitation rate, two monsoon subsystems, the low-level subtropical high, and the upper level South Asian anticyclone. Nevertheless, the model still exhibits some systematic errors, including oversimulation of the temperature over the Eurasian continent, which in turn intensifies the monsoon circulations. In the BATS run, the model can significantly relieve the temperature bias over the continent in spring and early summer. However, the effect of BATS decreases in the summer due to excessive incoming solar radiation. The Eurasian continent is still occupied by an oversimulated thermal low in summer. In the heavy snow case, the high albedo of snow and larger soil moisture suppress the warming rate of the surface and atmosphere in the early summer and hence the cooler troposphere results in a weaker monsoon circulation. Moreover, anomalous cyclonic flows are found in the leeside of Tibetan Plateau (i.e. the southwest vortex in China) in the heavy snow case. This may shed a light on the precipitation anomalies (floods) over Yangtze River Valley (Central China) and eastern Asia due to intensified baroclinic disturbances. Received: 8 September 1999 / Accepted: 5 June 2000     

末次间冰期亚洲中部干旱区干湿变化的模拟研究

利用国际古气候模拟对比计划第四阶段的多模式结果,分析了末次间冰期亚洲中部干旱区的干湿变化及机制。多模式集合平均结果表明,末次间冰期亚洲中部干旱区年降水减少0.7％,其中中亚地区的年降水减少2.8％,新疆地区年降水增加1.8％。水汽收支方程表明,末次间冰期中亚地区在雨季（冬春季）的降水变化主要与垂直动力项有关,新疆地区在雨季（夏季）的降水变化主要与垂直动力与热力项有关。此外,基于Penman-Montieth方法计算的亚洲中部干旱区的干旱指数在末次间冰期减小约10.2％,表明末次间冰期亚洲中部干旱区气候明显变干且存在旱区扩张的现象,这主要受到潜在蒸散变化的调控。潜在蒸散的增加进一步受到有效能量增加与地面风速增大的调控。本研究从模拟的角度揭示了末次间冰期亚洲中部干旱区干湿变化的可能特征及机制,在一定程度上有助于理解旱区气候在增暖情景下对轨道参数的响应特征。     

A Numerical Study on the Winter Monsoon and Cold Surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.     

Recent Progress in the Impact of the Tibetan Plateau on Climate in China

Studies of the impacts of the Tibetan Plateau （TP） on climate in China in the last four years are reviewed. It is reported that temperature and precipitation over the TP have increased during recent decades. From satellite data analysis, it is demonstrated that most of the precipitation over the TP is from deep convection clouds. Moreover, the huge TP mechanical forcing and extraordinary elevated thermal forcing impose remarkable impacts upon local circulation and global climate. In winter and spring, stream flow is deflected by a large obstacle and appears as an asymmetric dipole, making East Asia much colder than mid Asia in winter and forming persistent rainfall in late winter and early spring over South China. In late spring, TP heating contributes to the establishment and intensification of the South Asian high and the abrupt seasonal transition of the surrounding circulations. In summer, TP heating in conjunction with the TP air pump cause the deviating stream field to resemble a cyclonic spiral, converging towards and rising over the TP. Therefore, the prominent Asian monsoon climate over East Asia and the dry climate over mid Asia in summer are forced by both TP local forcing and Eurasian continental forcing.
Due to the longer memory of snow and soil moisture, the TP thermal status both in summer and in late winter and spring can influence the variation of Eastern Asian summer rainfall. A combined index using both snow cover over the TP and the ENSO index in winter shows a better seasonal forecast.
On the other hand, strong sensible heating over the Tibetan Plateau in spring contributes significantly to anchor the earliest Asian monsoon being over the eastern Bay of Bengal （BOB） and the western Indochina peninsula. Qualitative prediction of the BOB monsoon onset was attempted by using the sign of meridional temperature gradient in March in the upper troposphere, or at 400 hPa over the TP. It is also demonstrated by a numerical experiment and theoretical study that the heating over the TP lea     

Moisture Origins and Transport Processes for the 2020 Yangtze River Valley Record-Breaking Mei-yu Rainfall※

The summer of 2020 recorded a record-breaking flood due to excessive mei-yu rain falling over the Yangtze River Valley (YRV). Using the Lagrangian model FLEXPART, this paper investigates moisture sources and transport processes behind this extreme event. Based on climate data from 1979 to 2019, the air-particle (an infinitesimally small air parcel) trajectories reaching the YRV show sectors that correspond to five main moisture sources: the Indian monsoon region (IND, 27.5％ of the total rainfall), the local evaporation (27.4％), the Western Pacific Ocean (WPO, 21.3％), the Eurasian continent (8.5％) and Northeast Asia (4.4％). In the 2020 mei-yu season, moisture from all source regions was above normal except that from Northeast Asia. A record-breaking moisture source from the IND and WPO dominated this extreme mei-yu flood in 2020, which was 1.5 and 1.6 times greater than the climate mean, respectively. This study reveals a significant relationship between the moisture source with three moisture transport processes, i.e., trajectory density, moisture content, and moisture uptake of air-particles. A broad anomalous anticyclonic circulation over the Indo-Northwestern Pacific (Indo-NWP) provides a favorable environment to enhance the moisture transport from the IND and WPO into the YRV. In the 2020 mei-yu season, a record-breaking Indo-NWP anomalous anticyclonic circulation contributed to a higher trajectory density as well as higher moisture content and moisture uptake of air-particles from the IND and WPO regions. This collectively resulted in unprecedented moisture transport from source origins, thus contributing to the mei-yu flood over the YRV in 2020.     

South Asian high and Asian-Pacific-American climate teleconnection

Growing evidence indicates that the Asian monsoon plays an important role in affecting the weather and climate outside of Asia. However, this active role of the monsoon has not been demonstrated as thoroughly as has the variability of the monsoon caused by various impacting factors such as sea surface temperature and land surface. This study investigates the relationship between the Asian monsoon and the climate anomalies in the Asian-Pacific-American （APA） sector. A hypothesis is tested that the variability of the upper-tropospheric South Asian high （SAH）, which is closely associated with the overall heating of the large-scale Asian monsoon, is linked to changes in the subtropical western Pacific high （SWPH）, the midPacific trough, and the Mexican high. The changes in these circulation systems cause variability in surface temperature and precipitation in the APA region. A stronger SAH is accompanied by a stronger and more extensive SWPH. The enlargement of the SWPH weakens the mid-Pacific trough. As a result, the southern portion of the Mexican high becomes stronger. These changes are associated with changes in atmospheric teleconnections, precipitation, and surface temperature throughout the APA region. When the SAH is stronger, precipitation increases in southern Asia, decreases over the Pacific Ocean, and increases over the Central America. Precipitation also increases over Australia and central Africa and decreases in the Mediterranean region. While the signals in surface temperature are weak over the tropical land portion, they are apparent in the mid latitudes and over the eastern Pacific Ocean.