夏季南海低空越赤道气流变化与亚澳季风区降水异常的联系

基于1979—2014年ERA-Interim逐月风场和水汽通量资料及GPCP逐月降水率资料,采用相关分析及合成分析等方法研究了夏季南海低空越赤道气流的变化特征及其与亚澳季风区降水异常的联系。结果表明:1)夏季南海低空越赤道气流强度的年际变化特征明显,具有3~4 a的周期。2)夏季南海低空越赤道气流强度变化与热带东印度洋和海洋性大陆区域降水异常具有显著的负相关关系、与热带西太平洋降水异常存在明显的正相关关系、与我国中部地区降水异常存在较好的负相关关系。3)当夏季南海低空越赤道气流强度偏强时,850 hPa上自阿拉伯海向东一直延伸到热带西太平洋为西风异常,这种环流形势有利于热带西太平洋出现水汽辐合,使得该区域降水出现明显偏多,同时热带东印度洋低层为东风异常,受其影响,热带东印度洋和海洋性大陆区域出现水汽辐散,使得该区域降水偏少;此外,在我国东南沿海为一个气旋式风场异常,不利于来自热带海洋的水汽输送到达我国中部地区,使得该地区降水偏少;反之亦然。4)当夏季南海低空越赤道气流偏强时,东亚地区局地Hadley环流表现为异常偏弱,低空偏南越赤道气流异常在20°N附近与来自北半球的冷空气交汇上升,赤道附近及30~40°N地区出现异常下沉运动,使得热带海洋性大陆区域和我国中部地区降水减少;反之亦然。     

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。     

南海夏季风爆发早晚的越赤道气流特征

根据国家气候中心提供的南海夏季风爆发期典型偏早（1966、1972、1996、2000、2001年）和偏晚年份（1970、1973、1987、1989、1991年）,利用ECMW F再分析1-5月逐日经向风资料,计算5个通道越赤道气流和越赤道乞流总量的距平值;探讨越赤道气流与南海夏季风爆发早晚的关系与特征。为预测南海夏季风爆发早晚提供判据。     

盛夏南海低空越赤道气流变化与东亚夏季风的联系

利用1979—2014年ERA-Interim逐月的风场、海平面气压场和位势高度场等再分析资料以及中国160站降水观测资料,采用回归分析等方法分析了盛夏(7、8月)南海(South China Sea,SCS)低空越赤道气流(Cross-Equatorial Flow,CEF)的变化及其与东亚夏季风的联系,结果表明:盛夏南海低空越赤道气流(SCEF)强度指数与南海夏季风强度指数呈显著的正相关关系,与东亚副热带夏季风强度指数呈显著的负相关关系。当盛夏SCEF偏强(弱)时,亚洲热带低压及西太平洋赤道辐合带增强(减弱),西太平洋副热带高压强度减弱(增强)、东撤(西伸),南海北部和西北太平洋地区为明显的气旋式(反气旋式)环流异常,使得南海夏季风增强(减弱)和东亚副热带夏季风减弱(增强)。此外,当盛夏SCEF偏强时,由于东亚副热带夏季风减弱,我国华南地区为东北风异常,华北地区为偏南风异常,受其影响,我国华南地区为显著的水汽辐合区,华中地区为显著的水汽辐散区,使得盛夏华南地区降水增多,华中地区降水减少;反之亦然。     

南海夏季风爆发早晚的越赤道气流特征

根据国家气候中心提供的南海夏季风爆发期典型偏早(1966、1972、1996、2000、2001年)和偏晚年份(1970、1973、1987、1989、1991年),利用ECMW F再分析1~5月逐日经向风资料,计算5个通道越赤道气流和越赤道乞流总量的距平值;探讨越赤道气流与南海夏季风爆发早晚的关系与特征。为预测南海夏季风爆发早晚提供判据。     

越赤道气流的季节变化及其对南海夏季风爆发的影响

基于NCEP/NCAR资料分析了对流层越赤道气流的季节变化,指出越赤道气流中心在低层位于925hPa,在高层位于150 hPa。东半球的越赤道气流是一种典型的季风型气流,而西半球越赤道气流具有信风特征。研究结果还表明,低层的索马里和南海越赤道气流对南海夏季风的爆发有至关重要的作用,在季风爆发前2候,索马里急流有一次迅速的增强,这一增强有利于加速孟加拉湾地区西风的向东扩展,并使控制在南海上空的西太平洋副高东撤;同时,南海越赤道气流的迅速增强也推动副高北上,共同促使南海夏季风全面爆发。不仅如此,二者对季风爆发的早晚也有重要影响,当前期这两支越赤道气流建立偏早、强度偏强时,南海夏季风爆发易偏早。反之,当其建立偏晚、强度偏弱时,季风爆发易偏晚。     

索马里越赤道气流对越南降水变化的影响

基于1979-2002年CMAP全球降水资料和NCEP／NCAR再分析资料，研究了越南降水的季节和年际变化特征及其受索马里越赤道气流的影响。结果表明，越南南北方降水具有不同的季节和年际变化特征，南方降水主要集中在夏季和秋季，且夏、秋季降水的变化呈反位相关系，而北方降水则集中在夏季。夏季索马里气流与同期及后期越南南方降水有很好的相关性，同期为显著的正相关，后期为显著的负相关。相对而言，索马里越赤道气流和越南北方降水的关系不显著。另外，夏季赤道印度洋西风与越南夏秋季降水的相关与索马里气流类似，表明索马里越赤道气流可能是通过改变纬向水汽输送来影响越南南方降水。     

索马里越赤道气流与淮河流域夏季降水关系的年代际增强

基于1961~2016年淮河流域172个测站逐日降水资料,分析了淮河夏季降水的多时间尺度变率,发现其具有显著的准两年周期,1990年代以来亦呈现出明显的年代际变化。EOF分析结果显示,淮河夏季降水的异常主要表现为全流域一致偏多或偏少型（第一模态）,其方差贡献高达37%。相比于长江中下游地区,淮河夏季降水与东亚500 hPa位势高度场上的EAP遥相关型关系很弱,但和对流层低层西南水汽输送有更好的对应关系,表现为从索马里半岛至淮河流域的多个正负交替的相关波列。这一波列表明当索马里越赤道气流加强时,通过热带印度洋西风的纽带作用加强了进入淮河流域的西南暖湿气流,并在淮河上空低层形成水汽辐合,造成淮河多雨,反之当索马里越赤道气流强度弱时,淮河夏季降水偏少。索马里越赤道气流和中国台站降水的空间相关同样显示出最显著的相关区位于淮河流域。进一步的分析发现,研究时段内二者关系并非稳定维持,而是表现出显著的年代际变化,近二十年来索马里越赤道气流对淮河流域夏季降水的影响明显增强,且在季节预测上的指示意义也在增强。这一增强的可能原因是索马里越赤道气流与流域南侧的经向水汽输送和西侧的纬向水汽输送的关系均发生了年代际反转,并且这两条水汽输送带对流域夏季降水的影响发生了年代际增强。     

1950—2009年夏季菲律宾低空越赤道气流的变化特征

利用1950—2009年NECP/NCAR的月平均经向风再分析资料,根据夏季(6—8月)菲律宾低空越赤道气流的特征,定义了该支气流的强度指数IS和位置偏移指数IL,并根据这两个指数分析了菲律宾越赤道气流的时间变化特征及对应的环流特征,以及其与亚洲夏季降水的关系。结果表明,IS主要呈现弱、强、弱、强的年代际变化特征,IL主要呈现振荡、稳定、径直转向的年代际变化特征。IS与由低层澳大利亚高压北侧冷空气活跃和西太平洋副热带高压减弱所造成的经向气压梯度的变化有关,IL与澳大利亚地区和赤道低槽区高、低层的南北气压梯度有关。IS与印度尼西亚、孟加拉湾南部、热带西太平洋和澳大利亚东部的降水关系密切,IL与孟加拉湾和中国南海南部的降水关系密切。此外,IL与澳大利亚上空的垂直运动具有密切联系。     

夏季越赤道气流对南海季风及华东旱涝的影响

应用NCEP／NCAR全球再分析资料和NOAA卫星接收的HIRS－Tb12等资料，研究了东半球夏季越赤道气流的气候学特征、越赤道气流对南海季风爆发的影响、越赤道气流与高压对越赤道气流的维持和强度变化具有重要的作用；北太平洋海温场对越赤道气流的强弱也具有重要的影响；5、6月份索马里越赤道气流偏强，则南海季风爆发偏早；华东地区夏旱年越赤道气流显著偏强，雨涝年呈弱相间的变化。     

南海夏季风降水的区域差异及其突变特征

使用1950~1997年NCAR/NCEP再分析逐日降水资料,采用聚类和相关分析相结合的方法对南海夏季风降水进行了区域划分,分析了南海夏季风降水爆发前后南海降水的突变特征。结果表明:南海 105~120°E,0~20°N区域可划分为 SCS1区、SCS2区、SCS3区和SCS4区4个小区域,每个区域的降水有其各自不同的变化特征。前三个区域的降水变化不显著,不能反映南海夏季风降水爆发的突然性,变化最显著的是SCS4区,它最好地刻画了南海夏季风降水的变化特征,因此,我们选取它作为今后工作中南海夏季风降水的研究范围。突变检验表明,5月17日,南海SCS4区降水发生明显的突变,与5月15日相比,SCS4区降水场形势发生明显变化,其区域平均降水突增超过6 mm/day,标志着南海夏季风降水的爆发。     

南海夏季风垂直结构的变化特征及其对中国东部降水的影响

采用NCEP/NCAR再分析月平均资料和NOAA全球逐日降水资料,首先利用EOF方法分析了南海夏季风的垂直结构时空变化特征,然后初步探讨了南海夏季风垂直结构对中国夏季降水的影响和机制。(1)南海夏季风的垂直结构有明显的年际和年代际变化特征。EOF第一模态型主要表现为南海夏季风垂直结构的年际变化特征,为对流层低层西南风和对流层高层东北风同时增强(同时减弱)(简称“低层-高层同时增强”和“低层-高层同时减弱”)两种典型结构变化;EOF第二模态主要表现为南海夏季风垂直结构的年代际变化,为对流低层(高层)西南风(东北风)由下向上的增强到减弱变化和相反的对流层低层(高层)西南风(东北风)减弱到增强的变化(简称“低层强弱-高层强弱”和“低层弱强-高层弱强”)的两个不同年代(时段)的垂直结构变化。(2)南海夏季风垂直结构变化通过改变对流层低层、高层的环流异常变化来影响中国东部夏季降水的异常变化。南海夏季风呈“低层-高层同时增强”垂直结构时,南海低纬热带季风环流异常加强,长江流域低层辐散、高层辐合及异常下沉运动,其南侧的华南地区和北侧的东北地区是低层辐合、高层辐散和异常上升运动,导致华南降水异常偏多、...     

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.     

INTENSITY INDEX OF SOUTH CHINA SEA MONSOON AND ITS VARIATION CHARACTERISTICS

According to the basic characteristics of the activities of summer monsoon in the South China Sea,Standardized index,Is,has been designed that integrates a dynamic factor(southwesterly component) and a thermodynamic factor(OLR) for the indication of summer monsoon in the South China Sea,With the index determined for individual months of June,July and August and the entire summertime from 1975 to 1999,specific months and years are indicated that are either strong or weak in monsoon intensity,The variation is studied for the patterns and Is‘s relationship is revealed with the onset of summer monsoon and the precipitation in Guang-dong province and China.The results show that there are quasi-10 and quasi-3-4 year cycles in the interannual variation of the monsooon over the past 25 years.When it has an early(late)onset,the summer monsoon is usually strong (weak),In the strong(weak)monsoon,years,precipitation tends to be more(less)in the first raining season of the year but normal or less(normal)in the second,in the province,but it would be more(less) in northeastern China and most parts of the northern china and south of the lower reaches of the Changjiang River and less(more)in the middle and lower reaches of the river,western part of northern China and western China.     

南海夏季风强度与我国汛期降水的关系

对4种南海夏季风强度指数(1951～1998年)进行了对比分析,发现尽管它们在某些年份有差异,但在年际变化总体趋势上仍表现一致,并且由它们所确定的季风强弱年也基本相同.统计分析了南海夏季风强度与我国汛期降水的关系,结果表明,南海夏季风强(弱)年,我国夏季雨带型呈Ⅰ(Ⅲ)类分布,长江中下游地区夏季(6～7月)少雨干旱(多雨洪涝),广东省后汛期降水以偏涝(正常和偏旱)为主.南海夏季风强度指数与夏季长江中下游区降水和淮河区降水有显著的反相关,与江南区降水和华南后汛期降水有显著的正相关.我国夏季出现的严重洪涝(如1998年长江流域特大洪涝和1994年华南特大暴雨)与南海夏季风的强度异常有关.此外,分析还表明,南海夏季风活动强弱造成的北半球东亚500 hPa位势高度场的经向波列型遥相关是影响中国夏季降水的一个重要机制.     

THE CHARACTERISTICS OF LOW-FREQUENCY OSCILLATIONS OVER THE SOUTH CHINA SEA

Using 1975-1993 (with 1978 missing) data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low-frequency oscillation for the early summer. The study also reveals that the circulation for the low-frequency oscillation during the summer monsoon in the Sea is embodied as north-south fluctuations of the ITCZ and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. the work illustrates how the low-frequency oscillation in the South China Sea are superimposed with the seasonal variation of the general circulation. so that the summer inonsoon covers the establishment of the I^st, intensification of the 2^nd and 3^rd the low-frequency oscillations and decay of the 4^th oscillation.     

CONNECTION OF THE SOUTH CHINA SEA SUMMER MONSOON TO MARITIME CONTINENT CONVECTION AND ENSO

The relationship between the intensity of the South China Sea summer monsoon (SCSSM) and the Nino3.4 index and anomalous atmospheric circulation patterns associated with a strong and weak SCSSM are investigated using the NCEP/NCAR reanalysis data, Extended Reconstructed Sea Surface Temperature (ERSST) data and Climate Prediction Center Merged Analysis of Precipitation (CMAP) data. The SCSSM is significantly positively correlated with the Nino3.4 index in the succeeding northern autumn and winter. In the strong minus weak SCSSM composite, a positive East Asia-Pacific teleconnection (EAP) pattern and a negative Europe-Asian-Pacific teleconnection (EUP) pattern appear in the 500 hPa height difference field; low-level cross-equatorial flows are strengthened over the Maritime Continent (MC) region; positive (negative) precipitation anomalies occur in the South China Sea and western north Pacific (MC). A possible mechanism through which SCSSM affects ENSO is proposed. A strong (weak) SCSSM strengthens (weakens) cross-equatorial flows over the MC. The anomalous cross-equatorial flows cool (warm) the SST around the MC through enhanced (reduced) surface latent heat fluxes. The cooling (warming) further leads to suppressed (enhanced) convection over the MC, and causes the anomalous westerly (easterly) in the equatorial western Pacific, which favors the onset of El Ni?o (La Ni?a) through modulating the positive air-sea feedback process.     

A STUDY ON THE PRECIPITATION CHARACTERISTICS OVER THE SOUTH CHINA SEA BEFORE AND AFTER THE MONSOON ONSET

This paper presents a study on the temporal and spatial variations of the precipitation over the area of the South China Sea (SCS) during the monsoon onset period. The data used are from the Tropical Rainfall Measuring Mission (TRMM) observations between April and June over the nine years from 1998 to 2006. This study focuses on the central and northern part of South China Sea (110-120°E, 10-20°N). Based on the observations, the 27th pentad is selected as the occurrence time of the SCS monsoon onset. The conclusions are as follows. (1) After the monsoon onset, the specific area, defined as the ratio of the number of pixels with certain type of precipitation to the number of total pixels, extends significantly for both convective and stratiform rain, with the latter having a larger magnitude. The specific rainfall, defined as the ratio of the amount of certain type of precipitation to the total amount of precipitation, decreases for convective rain and increases for stratiform rain. (2) Results also show significant increase in heavy rain and decrease in light rain after the monsoon onset. (3) Changes are also observed in the rainfall horizontal distributions over the SCS before and after the monsoon onset, manifested by the relocation of precipitation minima for both convective and stratiform rain. (4) After the monsoon onset, the variability in characteristics of precipitation vertical structure increases significantly, leading to more latent heat release and consequently deeper convection. Meanwhile, the bright-band altitude of stratiform precipitation is also elevated.     

南半球高纬地区前期环流异常和我国华北地区夏季降水的联系

利用1951~2000年共50年中国160个站的逐月降水与地面气温资料以及美国NCEP/NCAR全球再分析资料,找出了影响华北地区夏季降水的南半球关键区——50~60°S。研究结果表明,该区前期1月份的环流异常能够影响当年夏季华北地区的降水,反映这种异常的指数IDX高时,华北夏季降水偏少;IDX偏低,华北夏季降水偏多。