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 low- level 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 associ- ated 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 tempera- ture 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 E1 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）.

Water Vapor Transport and Cross-Equatorial Flow over the Asian-Australia Monsoon Region Simulated by CMIP5 Climate Models

The summer mean water vapor transport （WVT） and cross-equatorial flow （CEF） over the Asian- Australian monsoon region simulated by 22 coupled atmospheric-oceanic general circulation models （AOGCMs） from the World Climate Research Programme＇s Coupled Model Intercomparison Project Phase 5 （CMIP5） were evaluated. Based on climatology of the twentieth-century simulations, most of models have a reason- ably realistic representation of summer monsoon WVT characterized by southeast water vapor conveyor belt over the South Indian Ocean and southwest belt from the Arabian Sea to the East Asian. The correlation coefficients between NCEP reanalysis and simulations of BCC-CSMI-1, BNU-ESM, CanESM2, FGOALS-s2, MIROC4h and MPI-ESM-LR are up to 0.8. The simulated CEF depicted by the meridional wind along the equator includes the Somali jet and eastern CEF in low atmosphere and the reverse circulation in upper atmosphere, which were generally consistent with NCEP reanalysis. Multi-model ensemble means （MME） can reproduce more reasonable climatological features in spatial distribution both of WVT and CEF. Ten models with more reasonable WVT simulations were selected for future projection studies, including BCC- CSMI-1, BNU-ESM, CanESM2, CCSM4, FGOALS-s2, FIO-ESM, GFDL-ESM2G, MRIOCS, MPI-ESM-LR and NorESM-1M. Analysis based on the future projection experiments in RCP （Representative Concentra- tion Pathway） 2.6, RCP4.5, RCP6 and RCP8.5 show that the global warming forced by different RCP scenarios will results in enhanced WVT over the Indian area and the west Pacific and weaken WVT in the low latitudes of tropical Indian Ocean.

NUMERICAL SIMULATION OF SSTA IMPACTS UPON THE INTERDECADAL VARIATION OF THE CROSS-EQUATORIAL FLOWS IN EASTERN HEMISPHERE

Impacts of regional sea surface temperature(SST)anomalies on the interdecadal variation of the cross-equatorial flows(CEFs)in Eastern Hemisphere are studied using numerical simulations with a global atmospheric circulation model(NCAR CAM3)driven with 1950-2000 monthly SSTs in different marine areas(the globe,extratropics,tropics,tropical Indian Ocean-Pacific,and tropical Pacific)and ERA-40reanalysis data.Results show that all simulations,except the one driven with extratropical SSTs,can simulate the interdecadal strengthening of CEFs around Somali,120oE,and 150oE that occurred in the midand late-1970s.Among those simulated CEFs,the interdecadal variability in Somali and its interdecadal relationship with the East Asian summer monsoon are in better agreement with the observations,suggesting that changes in the SSTs of tropical oceans,especially the tropical Pacific,play a crucial role in the interdecadal variability of CEFs in Somali.The interdecadal change of CEFs in Somali is highly associated with the interdecadal variation of tropical Pacific SST.As the interdecadal warmer(colder)SST happens in the tropical Pacific,a"sandwich"pattern of SST anomalies,i.e."+,-,+"("-,+,-"),will occur in the eastern tropical Pacific from north to south with a pair of anomalous anticyclone(cyclone)at the lower troposphere;the pair links to another pair of anomalous cyclone(anticyclone)in the tropical Indian Ocean through an atmospheric bridge,and thus strengthens(weakens)the CEFs in Somali.

Vertical Structure of Interannual Variability in Cross-Equatorial Flows over the Maritime Continent and Indian Ocean in Boreal Summer

This study investigates the vertical structure of variability in the cross-equatorial flows(CEFs)over the Maritime Continent and Indian Ocean in boreal summer,based on three reanalysis datasets:ERA-Interim,JRA-55 and NCEP-2.The results show a high consistency in the interannual variability among the reanalysis datasets,especially between ERAInterim and JRA-55,while great uncertainty exists in the decadal or long-term changes of CEFs.Further analyses on the interannual variability in CEFs indicate that there is a significant negative relationship between the upper-and lower-level CEFs over the Maritime Continent—that is,the northerlies at the upper level and southerlies at the lower level are both enhanced or weakened.This seesaw pattern is also significantly related to the CEFs over the Indian Ocean at the upper level and lower level(i.e.,the Somali jet).This close relationship between the upper-and lower-level CEFs and between the Maritime Continent and Indian Ocean is manifested as the leading mode of equatorial meridional winds in the vertical-zonal section over the Maritime Continent and Indian Ocean.Finally,it is found that ENSO is closely related to the vertical leading mode,and to all the CEFs at the upper and lower levels over the Maritime Continent and Indian Ocean.

INTERANNUAL VARIATION CHARACTERISTICS OF EAST HEMISPHERIC CROSS-EQUATORIAL FLOW AND ITS CONCURRENT RELATIONSHIPS WITH TEMPERATURE AND RAINFALL IN CHINA

The NCEP/NCAR global reanalysis data were used to analyze the interannual variation characteristics of the cross-equatorial flow (CEF) and its concurrent relationships with temperature and rainfall in China. The results indicated that CEF changes more in summer than in winter. As the main flow channel in summer, the Somali CEF changes in a way that does not markedly influence the changes in the CEF total except for winter. The summer CEF total has two sudden increases and one sudden decrease during the last century while the winter total has just one decrease. Long-period data show that the correlation between CEF and summer rainfall in China is not very significant, but is different before and after the 1970s, which is due to CEF's close links with the East Asia summer monsoon. Winter CEF's correlation with concurrent winter temperature in northern and southern China varies with the relationship between CEF and sea-level pressure in different areas.

INFLUENCE OF THE INTERANNUAL VARIATION OF CROSSEQUATORIAL FLOW ON TROPICAL CYCLOGENESIS OVER THE WESTERN NORTH PACIFIC

The influence of the interannual variation of cross-equatorial flow(CEF) on tropical cyclogenesis over the western North Pacific(WNP) is examined in this paper by using the tropical cyclone(TC) best track data from the Joint Typhoon Warning Center and the JRA-25 reanalysis dataset. The results showed that the number of TCs forming to the east of 140°E over the southeastern part of the western North Pacific(WNP) is in highly positive correlation with the variation of the CEF near 125° E and 150° E, i.e., the number of tropical cyclogeneses increases when the cross-equatorial flows are strong. Composite analyses showed that during the years of strong CEF, the variations of OLR, vertical wind shear between 200-850 h Pa, 850 h Pa relative vorticity and 200 h Pa divergence are favorable for tropical cyclogenesis to the east of 140°E over the tropical WNP, and vice versa. Moreover, it is also discussed from the view of barotropic energy conversion that during the years of strong CEF, an eastward-extended monsoon trough leads to the rapid growth of eddy kinetic energy over the eastern part of WNP, which is favorable for tropical cyclogenesis;but during the years of weak CEF, the monsoon trough is located westward in the western part of the WNP, consistent with the growth area of eddy kinetic energy. As a result, there are fewer TC geneses over the eastern part of WNP.Besides, the abrupt strengthening of a close-by CEF 2-4 days before tropical cyclogenesis may be the one of its triggers.

东半球越赤道气流的年际变化特征及其与我国同期气温、降水的相关性

利用NCEP/NCAR全球再分析资料分析了东半球冬、夏两季越赤道气流的年际变化特征,以及与中国160个站点同期气温、降水的相关性。分析表明:越赤道气流的年际变化夏季强于冬季,索马里急流并不显著影响到总量的变化,而冬季最强通道与总量变化关系密切。夏季越赤道气流总量在1950年代后期和1970年代中期有两次剧增,并与冬季一样都在1994～1995年有一次剧减。东半球夏季越赤道气流与中国同期降水的相关性很小,但在1970年代突变前后的相关性明显不同,原因与东亚夏季风的关系密切。冬季越赤道气流与中国同期南北的气温的相关随着越赤道气流与不同地区海平面气压的相关性的不同而不同。

北半球冬季风时期越赤道气流的初步分析

利用１９８０－１９８６年格点风资料，分析了各年北半球冬季风期间（１２－２月）东半球对流层低层及高空的越赤道气流通道。在该地区冬季低空具有气候意义的通道是１０５°Ｌ１２５°Ｅ、４５°Ｌ８０°Ｅ及１５０°Ｅ５条，其中以１００°一１３０“Ｅ为主要通道。高空则主要集中在８０°一１２０°Ｅ区间。在１９８３一１９８４、１９８４一１９８５年两个冬季，４５°Ｅ处出现较强的低空北风越赤道气流，这与高纬度大西洋东部上空持续的强阻塞形势有关。这支强越赤道气流与南印度洋及南太平洋多热带风暴也有联系。由平均经圈环流分析指出，冬季风期间在８０°一１２５°Ｅ存在着一个较完整的冬季风环流圈。

澳洲北部强/弱越赤道气流对应的环流配置

采用1980～2004年的NCEP/NCAR再分析资料,挑选5～8月中澳洲北部越赤道气流的极强和极弱年作为典型样本,探讨了强/弱越赤道气流对应的环流系统的配置特征。结果表明:强越赤道气流通常对应北半球辐合带维持较强,或南半球澳高维持较强,亦或澳洲冷空气活动频繁发生的环流形势,同期西太平洋副高通常偏东或偏北;弱越赤道气流通常对应西太平洋副高的偏西或偏南,或者是西太平洋副高-辐合带共同演进过程的多次发生。

西南季风潮与2004年5月我国南方暴雨

以2004年5月初及5月中旬我国华南等地两次较大暴雨过程为例,分析了西南季风潮与我国前汛期降水的关系.初步结论指出：西南季风潮的爆发与我国华南降水,特别是大暴雨的形成关系极为密切,而这次西南季风潮的爆发又与来自南半球的越赤道气流直接有关.同时指出,这次西南季风潮的爆发主要与来自85°～95°E孟加拉湾地区所在经度的越赤道气流有关,它们是印度洋“半球间宏观系统”的一个部分.而南海季风潮仅仅是西南季风潮的一种特例,在这两次重大降水过程中没有南海季风潮的爆发和影响.

低空越赤道气流的研究进展

越赤道气流是南、北半球物质、能量交换的主要通道,其强弱对夏季风爆发、台风的发生及中国降水等有重要影响。简要回顾了低空越赤道气流强弱突变对亚洲夏季风爆发早晚、西太平洋副热带高压脊线北抬位置、西北太平洋地区台风生成多寡及中国夏季降水影响的研究进展,并简要讨论了越赤道气流研究中存在的问题及未来的研究前景。

ONSET AND RETREAT DATES OF THE SOUTH CHINA SEA SUMMER MONSOON AND THEIR RELATIONSHIPS WITH THE MONSOON INTENSITY IN THE CONTEXT OF CLIMATE WARMING

Global gridded daily mean data from the NCEP/NCAR Reanalysis(1948-2012) are used to obtain the onset date,retreat date and duration time series of the South China Sea summer monsoon(SCSSM) for the past 65 years.The summer monsoon onset(retreat) date is defined as the time when the mean zonal wind at 850 hPa shifts steadily from easterly(westerly) to westerly(easterly) and the pseudo-equivalent potential temperature at the same level remains steady at greater than 335 K(less than 335 K) in the South China Sea area[110-120°E(10-20°N)].The clockwise vortex of the equatorial Indian Ocean region,together with the cross-equatorial flow and the subtropical high,plays a decisive role in the burst of the SCSSM.The onset date of the SCSSM is closely related to its intensity.With late(early) onset of the summer monsoon,its intensity is relatively strong(weak),and the zonal wind undergoes an early(late) abrupt change in the upper troposphere.Climate warming significantly affects the onset and retreat dates of the SCSSM and its intensity.With climate warming,the number of early-onset(-retreat) years of the SCSSM is clearly greater(less),and the SCSSM is clearly weakened.

Modulation of Low-Latitude West Wind on Abnormal Track and Intensity of Tropical Cyclone Nargis (2008) in the Bay of Bengal

Tropical cyclone （TC） Nargis （2008） made landfall in Myanmar on 02 May 2008, bringing a storm surge, major flooding, and resulting in a significant death toll. TC Nargis （2008） displayed abnormal features, including rare eastward motion in its late stage, rapid intensification before landing. Using reanalysis data and a numerical model, we investigated how a low-latitude westerly wind modulated TC Nargis’ （2008） track and provided favorable atmospheric conditions for its rapid intensification. More importantly, we found a possible counterbalance effect of flows from the two hemispheres on the TC track in the Bay of Bengal. Our analysis indicates that a strong westerly wind burst across the Bay of Bengal, resulting in TC Nargis’ （2008） eastward movement after its recurvature. This sudden enhancement of westerly wind was mainly due to the rapidly intensified mid-level cross-equatorial flow. Our results show that a high-pressure system in the Southern Hemisphere induced this strong, mid-level, cross-equatorial flow. During the rapid intensification period of TC Nargis （2008）, this strong and broad westerly wind also transported a large amount of water vapor to TC Nargis （2008）. Sufficient water vapor gave rise to continuously high and increased mid-level relative humidity, which was favorable to TC Nargis’ （2008） intensification. Condensation of water vapor increased the energy supply, which eventuated the intensification of TC Nargis （2008） to a category 4 on the Saffir-Simpson scale.