夏季索马里越赤道气流垂直结构特征及其与南亚夏季风的关系

采用1950-2009年NCEP/NCAR月平均再分析风场资料,对夏季低空索马里越赤道气流的垂直结构及其与南亚夏季风的关系进行研究.结果表明:夏季索马里越赤道气流在垂直方向上从低层至高层先增强,在925 hPa高度上达到最大值后逐渐减弱.某些年份索马里越赤道气流核心可向上延伸至850 hPa高度,而某些年份则维持在925 hPa高度上.索马里越赤道气流垂直结构不同时,其对应的南亚夏季风也有所不同,这种差异主要体现在对流层低层风场的变化,以及南亚夏季风的强弱差异方面.总体来说,索马里急流核心高度延伸至850 hPa时,对应的南亚夏季风偏强;急流核心高度维持在925 hPa时,南亚夏季风偏弱.     

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).     

索马里急流和澳洲越赤道气流年际变异不同配置及其影响

使用NCEP/NCAR大气再分析资料、Hadley中心海表温度分析资料和中国160站降水观测资料,分析了夏季索马里急流与澳洲越赤道气流年际变异之间的关系及相关联的海表温度、大气环流和中国降水异常分布特征。结果表明:夏季索马里急流和澳洲越赤道气流的年际变异存在两类关系,即多数的反位相和少数的同位相关系。当夏季索马里急流和澳洲越赤道气流呈前者减弱、而后者增强的反位相变化时,热带印度洋—太平洋海气异常表现为处于发展阶段的经典的东部El Nio型,造成东亚夏季风显著减弱,中国降水呈南方偏多、北方偏少的偶极型分布;当夏季索马里急流和澳洲越赤道气流同位相增强时,海气异常表现为处于成熟阶段的中太平洋El Nio型,东亚夏季风增强,中国降水呈长江流域降水偏少、而华北和华南沿海降水显著偏多的三极型分布。     

冬季流入南印度洋的低空越赤道气流分析

本文利用1980—1987年850 hPa格点风资料和500 hPa高度场资料，分析了冬季自北半球流入南印度洋的越赤道气流的特征，指出在35°—135°E的近赤道地区冬季低空有四支源自北半球的越赤道气流，它们具有明显的年变化和年际变化；它们主要来自阿拉伯半岛—东亚地区大陆冷高压的东北季风，其强弱变化取决于东北季风的活动。     

夏季索马里越赤道气流垂直结构的变化特征及其与东亚夏季风活动的关系

采用NCEP/NCAR和ERA40再分析月平均资料,对比分析夏季垂直尺度上五层(1000 hPa、925 hPa、850 hPa、700 hPa、600 hPa)索马里越赤道气流(SMJ)的时空演变特征。指出对流层中低层SMJ存在不同的垂直结构,且具有年际、年代际变化特征。夏季SMJ垂直结构主要呈现出全区一致、上下反相两种分布型(“A”型和“B”型),细分为一致增强型(“A+”型)、一致减弱型(“A-”型)、上弱下强型(“B+”型)和上强下弱型(“B-”型)。进一步分析指出,SMJ垂直结构与东亚夏季风活动关系密切,SMJ“A+”(“A-”)型垂直结构对应东亚夏季风偏强(弱),我国内蒙古东部、华北地区降水偏多(少);夏季SMJ“B+”(“B-”)型垂直结构对应东亚夏季风偏弱(强),我国江南、日本地区降水偏少(多)。     

越赤道气流对副高脊线北抬至25°N的影响

副高脊线北抬至25°N的时间早晚是福建前汛期结束和开始进入夏季的重要环流背景。应用850hPa月平均风场、500hPa环流场和西太平洋副热带高压脊线北抬至25°N日期及福建省25个代表站(县)6—7月的降水为基本分析资料,首先标定副高北抬至25°N的标准与年例,其次采用合成分析法揭示异常年例6月850hPa风场的基本特征,进而探讨了索马里越赤道气流强度变化对副高北抬至25°N的影响关系,最后对2005年进行诊断。其主要结果有:(1)6月索马里越赤道气流强劲(不够明显),较常年偏强(偏弱),有利于副高北抬至25°N提早(推迟);(2)5—6月索马里越赤道气流强度与500hPa东亚至西太平洋中纬度区域的高度场呈现正相关关系,该区域高度场高(低)有(不)利于副高主体北抬,为副高北抬25°N时间提早(推迟)提供有利环流背景;(3)索马里越赤道气流强度为副高北抬至25°N提供了一个较强的预报预测信号;诊断2005年副高北抬至25°N提早,实况与诊断相符。     

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.     

The Summer Monsoon Onset over the Tropical Eastern Indian Ocean： The Earliest Onset Process of the Asian Summer Monsoon

The onset process of the tropical eastern Indian Ocean (TEIO) summer monsoon (TEIOSM) and its relationship with the cross-equatorial flows are investigated via climatological analysis. Climatologically, results indicate that the earliest onset process of the Asian summer monsoon occurs over the TEIO at pentad 22 (April 15–20). Unlike the abrupt onset of the South China Sea (SCS) summer monsoon, the TEIOSM onset process displays a stepwise advance. Moreover, a close relationship between the TEIOSM development and the northward push of the cross-equatorial flows over 80–90E is revealed. A difference vorticity center, together with the counterpart over the southern Indian Ocean, constitutes a pair of difference cyclonic vortices, which strengthens the southwesterly wind over the TEIO and the northerly wind to the west of the Indian Peninsula from the end of March to late May. Therefore, the occurrence of the southwesterly wind over the TEIO is earlier than its counterpart over the tropical western Indian Ocean, and the cross-equatorial flows emerge firstly over the TEIO rather than over the Somali area. The former increases in intensity during its northward propagation, which provides a precondition for the TEIOSM onset and its northward advance.     

Reappraisal of Asian Summer Monsoon Indices and the Long-Term Variation of Monsoon

The Webster and Yang monsoon index (WYI)-the zonal wind shear between 850 and 200 hPa was calculated and modified on the basis of NCEP/NCAR reanalysis data. After analyzing the circulation and divergence fields of 150-100 and 200 hPa, however, we found that the 200-hPa level could not reflect the real change of the upper-tropospheric circulation of Asian summer monsoon, especially the characteristics and variation of the tropical easterly jet which is the most important feature of the upper-tropospheric circulation. The zonal wind shear U850-U(150 100) is much larger than U850-U200, and thus it can reflect the strength of monsoon more appropriately. In addition, divergence is the largest at 150 hPa rather than 200 hPa, so 150 hPa in the upper-troposphere can reflect the coupling of the monsoon system. Therefore, WYI is redefined as DHI, i.e., IDH=U850* - U(150 100)*, which is able to characterize the variability of not only the intensity of the center of zonal wind shear in Asia, but also the monsoon system in the upper and lower troposphere. DHI is superior to WYI in featuring the long-term variation of Asian summer monsoon as it indicates there is obvious interdecadal variation in the Asian summer monsoon and the climate abrupt change occurred in 1980. The Asian summer monsoon was stronger before 1980 and it weakened after then due to the weakening of the easterly in the layer of 150-100 hPa, while easterly at 200 hPa did not weaken significantly. After the climate jump year in general, easterly in the upper troposphere weakened in Asia, indicating the weakening of summer monsoon; the land-sea pressure difference and thermal difference reduced, resulting in the weakening of monsoon; the corresponding upper divergence as well as the water vapor transport decreased in Indian Peninsula, central Indo-China Peninsula, North China, and Northeast China, indicating the weakening of summer monsoon as well. The difference between NCEP/NCAR and ERA-40 reanalysis data in studying the intensity and long-term variation of Asian summer monsoon is also compared in the end for reference.     

INFLUENCE OF THE INTERANNUAL VARIATION OF CROSS-EQUATORIAL 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.     

南海周边越赤道气流的多时间尺度变化特征及其与环流和降水的联系

利用NCAR/NCEP-1再分析资料、NOAA的OLR资料以及GPCP降水资料等,通过功率谱分析、超前滞后回归等方法,对夏季南海周边105 °E、125 °E以及150 °E三支越赤道气流进行了多尺度特征分析,重点探讨三支越赤道气流季节内振荡与热带大气环流异常及南海周边降水的联系。结果表明,在季节内时间尺度上,105 °E与125 °E越赤道气流均具有10~20 d以及30~60 d低频振荡显著周期,而150 °E越赤道气流则以10~20 d周期为主。在年际尺度上,105 °E、125 °E、150 °E越赤道气流分别具有2~4年、2~3年、2~6年振荡周期。无论是季内还是年际变化,皆以105 °E与125 °E这两支越赤道气流之间关系较密切。南亚-南海-西太平洋地区对流层低层10~20 d振荡的气旋(对流加强)和反气旋(对流减弱)的环流活动变化,决定着105 °E及125 °E越赤道气流的10~20 d振荡的演变。这两支越赤道气流之30~60 d振荡所伴随的异常变化与热带夏季季节内振荡(BSISO)的演变过程非常相似,而150 °E越赤道气流之30~60 d振荡所伴随的异常低频环流则与南半球热带辐合带关系密切。105 °E及125 °E越赤道气流的季节内振荡及年际异常均与南海周边降水异常密切相关。      

THE IMPACT OF PRECEDING ATMOSPHERIC CIRCULATION AND SST VARIATION ON FLOOD SEASON RAINFALL IN YUNNAN

Spatial and temporal distribution characteristics and scale range of two significant areas were obtained by analyzing the relationship among summer rainfall in Yunnan province, height field and SST field (40°S – 40°N, 30 °E – 70°W) across the North Hemisphere at 200 hPa, 500 hPa and 850 hPa for Jan. to May and correlation, and field wave structure. Remote key regions among summer rainfall in Yunnan province, height field and SST field (40°S – 40°N, 30°E – 70°W) across the North Hemisphere at 200 hPa, 500 hPa and 850 hPa were studied through further analyzing of the circulation system and its climate / weather significance. The result shows that the forecast has dependable physical basis when height and SST fields were viewed as predictors and physical models of impacts on rainy season precipitation in Yunnan are preliminarily concluded.     

近百年中国东部夏季降水年代际变化特征及其原因

本文利用测站降水观测资料分析过去一百多年中国东部华北、长江流域以及华南夏季降水的年代际变化特征发现,尽管这三个地区的夏季降水具有不同的年代际转折时期,但是均同时在1910年代初期、1920年代初期、1940年代中期、1960年代中期、1970年代末期以及1990年代初期发生了跃变。近一百年间不同年代际时期东部夏季降水的分布型主要以南正北负或者南负北正的偶极型为主,并且无论是偶极型分布还是三极型分布,两个相邻年代际时期中国东部降水分布型发生完全反向变化的概率较高（60%）。此外,夏季的PDO、冬季的AO以及春季的北极海冰也同时在1920年代末期、1940年代中期、1970年代末期以及1990年代中期左右发生了跃变,这几次跃变时期与中国东部三个不同地区夏季降水发生跃变的时期一致,表现出近百年来太平洋年代振荡（PDO）、北极涛动（AO）以及北极海冰这三个因子对中国东部夏季降水年代际变化的协同作用。在年代际时间尺度上,夏季的PDO与华北夏季降水显著负相关。PDO的年代际变化能够在500 hPa位势高度场中激发出太平洋—日本（PJ）型年代际遥相关波列;同时在850 hPa风场中激发出类似于影响华北夏季降水年代际变化的大气环流型,从而影响华北降水的年代际变化。冬半年的AO与长江流域夏季降水存在显著正相关关系。冬季到春季正位相的AO导致亚洲大陆南部处于湿冷状态,土壤湿度的记忆性可将这种状态延续到夏季。因此,夏季海陆热力对比减弱,东亚夏季风发生年代际减弱,相应地长江流域的降水年代际增多。春季北极海冰与华南夏季降水显著负相关,北极海冰的年代际异常能在500 hPa位势高度场中激发出与静止Rossby波异常传播相联系的欧亚—华南年代际遥相关波列,从而影响华南降水的年代际变化。     

越赤道气流与西北地区东部夏季降水的联系

利用1951-2002年NCEP／NCAR再分析月平均气候资料和西北地区东部夏季降水资料，研究了越赤道气流的年际变化及其对西北地区东部夏季降水的影响。结果表明，自1951年以来，越赤道气流总量呈增强趋势，且持续性好；索马里急流是最主要的越赤道气流，且比较稳定，它是影响西北地区东部夏季降水的重要环流因素。     

华南连续性特大致洪暴雨个例分析

利用气象常规观测资料及NCEP/NCAR再分析资料,从越赤道气流、降水时空分布以及过程开始、持续和结束环流时特征等方面对2005年6月18~24日华南连续性特大致洪暴雨过程进行了分析。结果发现,暴雨的空间分布有着双雨带特征,即南北两条雨带,北面雨带对应着切变线,南面的雨带对应着低空急流;暴雨开始前及暴雨过程中,索马里附近一直维持强盛的越赤道气流,同时在100°~160°E之间也有越赤道气流活动并有西进现象;500hPa西风槽、850hPa切变线是这次过程开始和维持的重要系统;华南上空对流层高层东风的建立是这次过程结束时重要特征;南海北部至广东上空低空急流中心风速的周期性脉动与广东强降水的日变化现象有着很好的对应关系。     

华南夏季风降水开始日的异常与前冬大气环流和海温的关系

利用1951～2004年NCEP/NCAR再分析资料及ERSST海温资料,研究了华南夏季风降水开始日期的变化特征及其与前期冬季大气环流和海温的关系。小波分析表明,夏季风降水开始日期具有明显的年际和年代际变化,年际变化以准2～3年变化为主,年代际变化周期约16年。华南夏季风降水开始偏早年在大气环流上的前兆信号表现为前期冬季乌拉尔山阻塞高压偏强、东亚大槽较深,阿留申低压偏强,冷空气活动偏强。冬季,鄂霍次克海附近的海温异常为华南夏季风降水开始迟早有物理意义的、稳定正相关前兆信号。合成分析表明,冬季鄂霍次克海附近SST正异常时,5月100 hPa青藏高压偏东偏北偏弱,异常偏西风控制华南;850 hPa环流在华南表现为东北风,华南受冷空气影响为主,华南夏季风降水开始日期偏晚。相反时,若冬季鄂霍次克海附近SST负异常,5月100 hPa青藏高压偏西偏南偏强,异常偏东风控制华南;850 hPa环流在华南表现为偏南风,华南受热带系统影响为主,华南夏季风降水开始日期偏早。并提出冬季中高纬度地区冷空气活动影响华南夏季风降水开始日异常的物理机制。     

太平洋—日本遥相关型的年代际变化特征及其成因

本文首先利用NCEP/NCAR和ERA-40再分析资料以及中国753站降水资料对太平洋—日本(Pacific-Japan,简称P-J)遥相关型在上世纪70年代末期气候突变前后的年代际变化特征进行了分析研究。结果表明,在气候突变前后,P-J遥相关型的位置发生了显著的变化,气候突变以后其位置明显向西向南偏移。这种位置的变化同样也反映在纬向风场、高度场上。研究结果还表明,气候突变前后P-J遥相关型的年代际变化与热带太平洋和印度洋海温变化有关。气候突变之前,P-J遥相关型的变化与前期热带太平洋和印度洋海温不存在显著的相关;但在气候突变之后,P-J遥相关型与前期冬春季的热带太平洋、印度洋海温之间存在大范围的显著相关区。这种P-J遥相关型与热带太平洋、印度洋海温相关关系的年代改变可能与1970年代中期以后赤道中东太平洋海温变化振幅明显增强有关。随后,本文采用一个高分辨率的大气环流模式,通过一系列的数值试验也进一步证实了1970年代末期热带太平洋和印度洋海温的年代际变化确实可致使P-J遥相关型位置发生相应的改变。     

The Leading Mode of Indian Ocean SST and Its Impacts on Asian Summer Monsoon

The Indian Ocean （IO） sea surface temperature （SST） was analyzed by using empirical orthogonal function （EOF）, and the leading mode of Indian Ocean （LMIO） SST was extracted. The major spatial and temporal characters of LMIO were discussed, and the relationships between LMIO with Indian summer monsoon （ISM） and with China summer rainfalls （CSR） were investigated, then the impacts of LMIO on Asian summer monsoon （ASM） circulation were explored. Some notable results are obtained： The significant evolutional characters of LMIO are the consistent warming trend of almost the whole IO basin, the distinctive quasi-3- and quasi-ll-yr oscillations and remarkably interdecadal warming in 1976/1977 and 1997/1998, respectively. The LMIO impaired the lower level circulation of ISM and was closely related with the climate trend of CSR. It was associated with the weakening of South Asian high, the easterly winds south of the Tibetan Plateau, and the cross-equatorial flows over 10°-20°N, 40°-110°E at the upper level; with the strengthening of Somali cross-equatorial jet but the weakening of the circulation of ISM in the sector of India, the strengthening of south wind over the middle and lower reaches of Yangtze River and South China but the weakening of southwesterly winds over North China at lower level and with the increasing of surface pressure over the Asian Continent. Changes in the moisture flux transports integrated vertically over the whole troposphere associated with LMIO are similar to those in the lower level circulation. To sum up, the significant SST increasing trend of IO basin was one of the important causes for weakening of the ASM circulation and the southwards shifting of China summer rainband.     

中国东北暖季气温变化特征及其与海温和大尺度环流的关系

利用1980~2014年CRU TS3.24月平均气温数据和NCEP/NCAR再分析资料,分析了中国东北暖季（5~9月）气温的时空变化特征及其相应的大气环流状况。结果表明:中国东北暖季气温主要表现为全区一致型和南北反位相型两个模态,二者总解释方差高达86%。全区一致型具有明显的年代际变化特征,并在1990年代中期发生了显著的年代际突变,而南北反位相型具有明显的年际和年代际变化特征。全区一致型增暖对应着中国东北地区上空500 hPa位势高度的正异常和850 hPa的反气旋环流异常。当500 hPa位势高度南北反相时,对应于中国东北暖季气温的南北反位相型。进一步分析表明中国东北暖季气温的全区一致型及其1990年代中期的年代际突变与日本海及黑潮延伸区的海温异常及太平洋年代际振荡和大西洋多年代际振荡指数紧密相关。菲律宾以东的西太平洋、北太平洋中部、我国东南沿海、靠近北美东北部的北大西洋等海域的海温异常对中国东北暖季气温全区一致型的出现具有一定的预测作用。而南北反位相变化型与黑潮延伸区的海温异常关系显著,与大尺度指数的相关普遍不明显。在1990年代中期突变前,南北反位相型受到ENSO事件的影响,之后影响不显著。     

南海夏季风季节内振荡的年际变化研究

利用NCEP/NCAR风场、 垂直速度再分析资料和NOAA的逐日向外长波辐射 (简称OLR) 资料, 运用Butterworth带通滤波方法, 对南海夏季风季节内振荡 (简称ISO) 的特征和年际变化情况进行了初步研究。气候平均状况下ISO在亚洲季风区存在5个活动中心, 其中包括南海南部地区 (5°N～15°N, 110°E～120°E)。气候平均状况下南海ISO在5～9月有三次比较明显的活跃过程。以850 hPa风在西南方向上的投影 (Vsw) 经30～60天带通滤波后的绝对值在南海区域的夏季平均定义了当年南海夏季风ISO活动指数, 以此确定了南海夏季风ISO活跃年和不活跃年, 并由此提取了两者在逐日序列和水平分布场特征上的差异。通过对南海夏季风ISO活跃年与不活跃年ISO信号差别的研究发现: 在南海夏季风ISO活动较活跃年份的一个完整周期的8个位相合成中, 低频对流中心从热带印度洋西部东传到达南海后, 受Walker环流和西太平洋副热带高压底层东风气流阻挡等其它动力和热力因素的影响转向东北传。春季南海地区较强的Walker环流和对流活动有可能作为南海夏季风ISO活跃的前期信号。