Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s

It is well known that on the interannual timescale, the westward extension of the western North Pacific subtropical high(WNPSH) results in enhanced rainfall over the Yangtze River basin(YRB) in summer, and vice versa. This study identifies that this correspondence experiences a decadal change in the late 1970 s. That is, the WNPSH significantly affects YRB precipitation(YRBP) after the late 1970 s(P2) but not before the late 1970 s(P1). It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2. On the other hand, after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1, the WNPSH can still significantly affect YRB rainfall, suggesting that the WNPSH variability is not the only factor that affects the WNPSH–YRBP relationship. Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH–YRBP relationship. In P2, the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter, and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced. As a result, the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.     

Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer

The contribution of tropical cyclones(TCs)to the East Asia–Pacific(EAP)teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to2018.The results showed that the TCs over the western North Pacific(WNP)correspond to a strengthened EAP pattern:During the summers of strong convection over the tropical WNP,TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere,an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere,and a more northward shift of the East Asian westerly jet in the upper troposphere.Further analyses indicated that two types of TCs with distinctly different tracks,i.e.,westward-moving TCs and northward-moving TCs,both favor the EAP pattern.The present results imply that TCs over the WNP,as extreme weather,can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.     

夏季逐月东亚高空西风急流经向偏移与热带中、东太平洋海温的关联

Previous studies have shown that meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) dominates interannual variability of the EAJS in the summer months. This study investigates the tropical Pacific sea surface temperature (SST) anomalies associated with meridional displacement of the monthly EAJS during the summer. The meridional displacement of the EAJS in June is significantly associated with the tropical central Pacific SST anomaly in the winter of previous years, while displacements in July and August are related to tropical eastern Pacific SST anomalies in the late spring and concurrent summer. The EAJS tends to shift southward in the following June (July and August) corresponding to a warm SST anomaly in the central (eastern) Pacific in the winter (late spring-summer). The westerly anomaly south of the Asian jet stream is a result of tropical central Pacific warm SST anomaly-related warming in the tropical troposphere, which is proposed as a possible reason for southward displacement of the EAJS in June. The late spring-summer warm SST anomaly in the tropical eastern Pacific, however, may be linked to southward displacement of the EAJS in July and August through a meridional teleconnection over the western North Pacific (WNP) and East Asia.     

Different Configurations of Interannual Variability of the Western North Pacific Subtropical High and East Asian Westerly Jet in Summer

This study investigates the circulation and precipitation anomalies associated with different configurations of the western North Pacific subtropical high(WNPSH) and the East Asian westerly jet(EAJ) in summer on interannual timescales. The in-phase configuration of the WNPSH and EAJ is characterized by the westward(eastward) extension of the WNPSH and the southward(northward) shift of the EAJ, which is consistent with the general correspondence between their variations. The out-of-phase configuration includes the residual cases. We find that the in-phase configuration manifests itself as a typical meridional teleconnection. For instance, there is an anticyclonic(cyclonic) anomaly over the tropical western North Pacific and a cyclonic(anticyclonic) anomaly over the mid-latitudes of East Asia in the lower troposphere. These circulation anomalies are more conducive to rainfall anomalies over the Yangtze River basin and south Japan than are the individual WNPSH or EAJ. By contrast, for the out-of-phase configuration, the mid-latitude cyclonic(anticyclonic) anomaly is absent, and the lower-tropospheric circulation anomalies feature an anticyclonic(cyclonic)anomaly with a large meridional extension. Correspondingly, significant rainfall anomalies move northward to North China and the northern Korean Peninsula. Further results indicate that the out-of-phase configuration is associated with the developing phase of ENSO, with strong and significant sea surface temperature(SST) anomalies in the tropical central and eastern Pacific which occur simultaneously during summer and persist into the following winter. This is sharply different from the in-phase configuration, for which the tropical SSTs are not a necessity.     

The ENSO’s Effect on Eastern China Rainfall in the Following Early Summer

ENSO’s effect on the rainfall in eastern China in the following early summer is investigated by using station precipitation data and the ERA-40 reanalysis data from 1958 to 2002. In June, after the El Nino peak, the precipitation is significantly enhanced in the Yangtze River valley while suppressed in the Huaihe River-Yellow River valleys. This relationship between ENSO and the rainfall in eastern China is established possibly through two teleconnections: One is related to the western North Pacific (WNP) ...     

1998年夏季长江流域大气季节内振荡的结构演变及其对降水的影响

利用中国逐日降水格点资料和NCAR/NCEP再分析资料,对1998年发生在我国东部长江中下游流域的夏季持续性强降水过程中显著的大气季节内振荡(ISO)的三维结构演变等活动特征进行了分析。1998年夏季长江及江南地区的异常强降水对应着该地区强的ISO活动。利用位相合成方法,对长江流域两个典型的季节内循环周期的ISO降水、850 hPa水平风场以及水汽和垂直速度等循环过程的时空分布特征进行了诊断分析。在低频环流场上,对流层低层的低频气旋和反气旋环流表现出交替在热带西北太平洋增强并向西偏北方向移动发展的特征,当异常气旋环流移动到长江流域上空时,长江流域正好位于气旋环流西南侧的东北风异常和西北太平洋上向西移动的反气旋环流西北侧的西南风异常环流汇合处的下方,引起该地区强降水的发生。在强降水阶段的ISO的垂直结构上,上升运动和水汽表现出从华南到长江流域自南向北移动的特征,强烈的垂直上升运动以及来自南方充足的水汽为增强长江流域地区的降水起到了重要作用。     

Upper- and Lower-tropospheric Circulation Anomalies Associated with Interannual Variation of Pakistan Rainfall during Summer

This study investigated the large-scale circulation anomalies, in both the upper and lower troposphere, associated with the interannual variation of rainfall in Pakistan during summer, using the station observation data in this country and circulation data of the NCEP?NCAR reanalysis from 1981 to 2017. Results showed that the upper- and lower-tropospheric circulation anomalies associated with monthly rainfall variability exhibit similar features from June to August, so analyses were performed on June?August circulation and Pakistan rainfall data. The analyzed results indicated that summer rainfall in Pakistan is enhanced when there is an anticyclonic anomaly to the northwest of Pakistan in the upper troposphere and easterly anomalies along the southern foothills of the Himalayas in the lower troposphere, and vice versa. These upper- and lower-tropospheric circulation anomalies were found to be related, but show unique features. The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern along the Asian westerly jet, while the lower-tropospheric easterly anomalies are related to the cyclonic anomaly to the south of Pakistan, i.e., intensified South Asian monsoon trough. The results presented here suggest that the interannual variability of summer rainfall in Pakistan is a combined result of upper- and lower-tropospheric circulation anomalies, and of extratropical and tropical circulation anomalies.     

Predictability of the western North Pacific summer climate demonstrated by the coupled models of ENSEMBLES

The Asian monsoon system, including the western North Pacific (WNP), East Asian, and Indian monsoons, dominates the climate of the Asia-Indian Ocean-Pacific region, and plays a significant role in the global hydrological and energy cycles. The prediction of monsoons and associated climate features is a major challenge in seasonal time scale climate forecast. In this study, a comprehensive assessment of the interannual predictability of the WNP summer climate has been performed using the 1-month lead retrospective forecasts (hindcasts) of five state-of-the-art coupled models from ENSEMBLES for the period of 1960–2005. Spatial distribution of the temporal correlation coefficients shows that the interannual variation of precipitation is well predicted around the Maritime Continent and east of the Philippines. The high skills for the lower-tropospheric circulation and sea surface temperature (SST) spread over almost the whole WNP. These results indicate that the models in general successfully predict the interannual variation of the WNP summer climate. Two typical indices, the WNP summer precipitation index and the WNP lower-tropospheric circulation index (WNPMI), have been used to quantify the forecast skill. The correlation coefficient between five models’ multi-model ensemble (MME) mean prediction and observations for the WNP summer precipitation index reaches 0.66 during 1979–2005 while it is 0.68 for the WNPMI during 1960–2005. The WNPMI-regressed anomalies of lower-tropospheric winds, SSTs and precipitation are similar between observations and MME. Further analysis suggests that prediction reliability of the WNP summer climate mainly arises from the atmosphere–ocean interaction over the tropical Indian and the tropical Pacific Ocean, implying that continuing improvement in the representation of the air–sea interaction over these regions in CGCMs is a key for long-lead seasonal forecast over the WNP and East Asia. On the other hand, the prediction of the WNP summer climate anomalies exhibits a remarkable spread resulted from uncertainty in initial conditions. The summer anomalies related to the prediction spread, including the lower-tropospheric circulation, SST and precipitation anomalies, show a Pacific-Japan or East Asia-Pacific pattern in the meridional direction over the WNP. Our further investigations suggest that the WNPMI prediction spread arises mainly from the internal dynamics in air–sea interaction over the WNP and Indian Ocean, since the local relationships among the anomalous SST, circulation, and precipitation associated with the spread are similar to those associated with the interannual variation of the WNPMI in both observations and MME. However, the magnitudes of these anomalies related to the spread are weaker, ranging from one third to a half of those anomalies associated with the interannual variation of the WNPMI in MME over the tropical Indian Ocean and subtropical WNP. These results further support that the improvement in the representation of the air–sea interaction over the tropical Indian Ocean and subtropical WNP in CGCMs is a key for reducing the prediction spread and for improving the long-lead seasonal forecast over the WNP and East Asia.     

Impacts of the leading modes of tropical Indian Ocean sea surface temperature anomaly on sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer

The two leading modes of the interannual variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) anomaly are the Indian Ocean basin mode (IOBM) and the Indian Ocean dipole mode (IODM) from March to August. In this paper, the relationship between the TIO SST anomaly and the sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer is investigated by using correlation analysis and composite analysis based on multi-source observation data from 1979 to 2013, together with numerical simulations from an atmospheric general circulation model. The results indicate that the impacts of the IOBM on the circulation and rainfall over East Asia vary remarkably from spring to summer. The anomalous anticyclone over the tropical Northwest Pacific induced by the warm IOBM is closely linked with the Pacific–Japan or East Asia–Pacific teleconnection pattern, which persists from March to August. In the upper troposphere over East Asia, the warm phase of the IOBM generates a significant anticyclonic response from March to May. In June and July, however, the circulation response is characterized by enhanced subtropical westerly flow. A distinct anomalous cyclone is found in August. Overall, the IOBM can exert significant influence on the western North Pacific subtropical high, the South Asian high, and the East Asian jet, which collectively modulate the precipitation anomaly over East Asia. In contrast, the effects of the IODM on the climate anomaly over East Asia are relatively weak in boreal spring and summer. Therefore, studying the impacts of the TIO SST anomaly on the climate anomaly in East Asia should take full account of the different sub-seasonal response during boreal spring and summer.     

中国东部夏季季节内降水异常及其伴随的热带和中高纬度大气环流演变特征

利用1981—2020年夏季(5—8月)CPC(Climate Prediction Center)逐日降水资料、NCEP/NCAR逐日再分析资料以及NOAA的向外长波辐射资料,通过经验正交函数(EOF)分解、超前滞后合成等方法,分析了中国东部夏季季节内降水异常的主要模态(即南方型和江淮型降水异常)及其伴随的热带和中高...     

The interannual variation in monthly temperature over Northeast China during summer

The interannual variations of summer surface air temperature over Northeast China （NEC） were investigated through a month-to-month analysis from May to August. The results suggested that the warmer temperature over NEC is related to a local positive 500-hPa geopotential height anomaly for all four months. However, the teleconnection patterns of atmospheric circulation anomalies associated with the monthly surface air temperature over NEC behave as a distinguished subseasonal variation, although the local positive height anomaly is common from month to month. In May and June, the teleconnection pattern is characterized by a wave train in the upper and middle troposphere from the Indian Peninsula to NEC. This wave train is stronger in June than in May, possibly due to the positive feedback between the wave train and the South Asian rainfall anomaly in June, when the South Asian summer monsoon has been established. In July and August, however, the teleconnection pattern associated with the NEC temperature anomalies is characterized by an East Asia/Pacific （EAP） or Pacific/Japan （PJ） pattern, with the existence of precipitation anomalies over the Philippine Sea and the South China Sea. This pattern is much clearer in July corresponding to the stronger convection over the Philippine Sea compared to that in August.     

Subseasonal and Synoptic Variabilities of Precipitation over the Yangtze River Basin in the Summer of 2020※

Summer precipitation over the Yangtze River basin (YRB) in 2020 experienced a strong subseasonal and synoptic fluctuation in addition to contributing to an exceptionally large seasonal mean precipitation. The cause of this higher-frequency fluctuation is examined based on observational analyses. Apart from the continuous northward movement of the climatological mei-yu rainband, the mei-yu rainbelt in the summer of 2020 experienced multiple northward and southward swings. The cause of the swings was attributed to the subseasonal variability of southerly winds to the south and northeasterly winds to the north of the YRB. In addition, synoptic-scale variability, characterized by the eastward propagation of low-level cyclonic vorticity and precipitation anomalies, was also commonplace in the summer of 2020. While the strengthening of both the subseasonal and synoptic variabilities in the summer of 2020 was attributed to the increase of the background mean moisture, the synoptic variability was greatly affected by the subseasonal rainfall variability. As a result, both the synoptic-scale and subseasonal variabilities contributed to the north-south swings of the rainbelt. The large-scale modulations by both the seasonal mean and subseasonal anomalies provide insight regarding the optimization of issuing accurate, extended-range forecasts of extreme weather events.     

2015年夏季东亚和南亚降水异常与热带太平洋海温异常的联系及机理

2015年我国东部夏季降水呈现南北反位相的空间分布,河套地区降水异常偏少、长江中下游地区降水异常偏多,同期印度中部地区降水负异常,上述三个区域2015年夏季降水距平百分率绝对值极大值均超过55%。东亚和南亚地区2015年夏季降水异常的形成机理主要是由于该年夏季处于El Niňo事件的发展位相,菲律宾群岛及邻近区域反气旋环流异常,江淮地区至日本列岛气旋式环流异常,对流层低层位势高度异常场和整层水汽异常输送场亦存在相一致的空间分布,表现为负位相的EAP(East Asian-Pacific)/PJ(Pacific-Japan)型遥相关,有利于河套地区降水偏少和长江流域降水偏多。热带太平洋海温异常引起热带地区Walker环流负异常,热带西太平洋地区上空受异常下沉气流控制,热带印度洋区域对流层盛行东风异常,减弱了印度夏季风,并造成了印度中部地区夏季降水偏少。另一方面,印度上空对流层低层受异常反气旋控制,该异常反气旋北侧的西风异常沿着青藏高原南麓向东运动,增强了与EAP/PJ型遥相关相联系的异常水汽输送,有利于维持和增强河套地区降水负异常和长江中下游地区降水正异常。     

广东6月持续性暴雨期间的大气环流异常

利用1979—2011年共33年广东86个观测站日降水和全球大气多要素日平均资料,分析广东前汛期降水异常(包括暴雨和无雨)的环流特征。结果表明,广东6月持续性暴雨和持续性无雨期间大气存在显著的经向遥相关波列,其中,持续性暴雨过程波列更完整,非持续性暴雨(或非持续性无雨)则波列不显著,而4、5月的持续性暴雨或持续性无雨过程都没有波列出现。在经向波列存在的情况下,对流层中高层大气西风带环流经向度增大、槽脊发展增强,中高纬度这种持续稳定的环流形势,有利于冷空气和高空槽影响华南;在高层200 hPa,华南处于偏西风和西南风异常之间的气流辐散区域,有利于高层辐散;对流层中低层西太平洋副热带高压偏强、西脊点偏西,华南上升、南海下沉的垂直异常经圈环流建立;同时对流层低层来源于印度和孟加拉湾北部以及热带太平洋的水汽输送明显加强,从而为持续性暴雨过程提供有利的环流背景以及暴雨区所需的动力和水汽条件,可见经向波列通过对流层高、中、低层大气环流异常影响持续性暴雨。在没有经向波列的情况下,当500 hPa华南地区有西风槽活动、850 hPa南海北部西风偏强,广东局地动力上升条件和水汽输送条件达到一定程度,则只能出现非持续性暴雨。因此,经向波列可为区分持续暴雨与非持续暴雨预报提供参考。与广东降水持续异常相关的经向波列受中高纬度罗斯贝波、热带对流以及中低纬度太平洋地区大气异常等多方面的共同影响。     

中国东部夏季分区降水对海温异常响应特征的研究

用刘征宇最新发展的广义平衡反馈方法结合经验正交分析和旋转经验正交分析法,研究中国东部6个分区夏季降水异常对各海盆海温异常(SSTA)模态的响应特征,探讨了东北区降水异常对SSTA的响应机制。结果表明:各分区降水异常对SSTA的响应特征有明显差异。东北区降水异常对热带太平洋SSTA的回应显著,当SSTA表现为类似El Niño模时,该区夏季降水增加;江淮区降水异常同时受到中纬度以及热带SSTA的强迫,对热带太平洋SSTA的类似La Niña Modoki模、北大西洋SSTA三极型模等回应显著;西南地区的降水异常主要与中纬度SSTA有密切关联。对流层高度场对热带太平洋SSTA类似El Niño模的直接回应在热带太平洋上空为显著正异常。通过东亚—太平洋型遥相关波列使东北地区上空高度场出现负异常,并在其西侧形成负异常中心,对流层低层为气旋性环流异常。这一环流回应与东北地区夏季降水偏多时的环流特征相近。     

我国东部梅雨期降水的年际和年代际变化特征及其与大气环流和海温的关系

基于我国160站59年(1951～2009年)的月降水观测资料、美国气象环境预报中心和国家大气研究中心(NCEP/NCAR)提供的再分析资料和Hadley中心的海表温度(Sea Surface Temperature,简称SST)资料,对我国东部(100°E以东,15°N～40°N)梅雨期(6月和7月)降水的时空变化特...     

2014年夏季长江流域降水季节内振荡的环流分析

基于台站降水观测数据和MERRA-2再分析资料,分析了2014年夏季我国长江流域降水的季节内振荡特征,并从位涡角度重点研究了与之相关的环流演变。结果表明:2014年夏季长江流域降水季节内变率以10~20d的准双周振荡为主。在降水准双周振荡的极端湿位相,受对流层高层随中纬度波列东传的正异常位涡和南亚高压东侧西南向传播的正异常位涡的共同影响,南亚高压呈“马鞍型”分布,在长江流域形成高空辐散环流;在对流层中低层,当中纬度波列的异常气旋向东南传播至长江流域以北时,西太平洋异常反气旋延伸至中国东南沿海,二者共同导致长江流域低空水汽辐合加强;在高、低层环流的共同作用下,长江流域持续性降水显著偏多,形成准双周振荡的极端湿位相;同时,长江以北高空位涡正异常导致其下方冷空气下沉,触发长江流域异常上升运动和南海地区异常下沉运动,该经向垂直环流圈的形成有利于长江流域正异常降水的维持。反之则形成极端干相位。      

Weakening of Interannual Variability in the Summer East Asian Upper-tropospheric Westerly Jet since the Mid-1990s

In this study,we found that the intensity of interannual variability in the summer upper-tropospheric zonal wind has significantly weakened over Northeast Asia and the subtropical western North Pacific(WNP) since the mid-1990s,concurrent with the previously documented decrease of the westerly jet over North China and Northwest China.Corresponding to this weakening of zonal wind variability,the meridional displacement of the East Asian westerly jet(EAJ) manifested as the leading mode of zonal wind variability over the WNP and East Asia(WNP-EA) before the mid-1990s but not afterward.The energetics of the anomalous pattern associated with the meridional displacement of the EAJ suggests that barotropic energy conversion,from basic flow to anomalous patterns,has led to the weakening of the variability in the EAJ meridional displacement and to a change in the leading dominant mode since the mid-1990s.The barotropic energy conversion efficiently maintained the anomalies associated with the variability in the EAJ meridional displacement during 1979-1993 but acted to dampen the anomalies during 1994-2008.A further investigation of the energetics suggests that the difference in the patterns of the circulation anomaly associated with either the first leading mode or the meridional displacement of the EAJ,i.e.,a southwest-northeast tilted pattern during 1979-1993 and a zonally oriented pattern during 1994-2008,has contributed greatly to the change in barotropic energy conversion.     

华南前汛期降水的年代际异常特征及其成因

利用1981—2013年中国160站逐月降水资料、NCEP/NCAR逐月再分析资料及NOAA海表温度资料,研究了华南前汛期降水年代际异常的时空特征及其可能成因。结果表明:1)华南前汛期降水在1992前后发生由异常偏少转为偏多的显著年代际转折,最显著异常中心位于广西东北部和广东北部。2)1990年代初发生的对流层高层南冷北暖(40°N附近为界)、对流层下暖上冷的年代际转折,使得高低层环流场均出现了有利于北方干、湿冷空气和孟加拉湾、西太平洋暖湿水汽在华南区域交汇并辐合上升的形势,造成华南前汛期降水发生偏少转偏多的显著年代际转折。年代际转折的前后两个时段中,位于热带的孟加拉湾槽、东亚沿岸EAP遥相关型波列中的西太平洋副高、阿拉斯加湾附近的脊,以及中纬度贝加尔湖以西以南脊的强度或位置均具有显著差异,故这些环流系统的年代际异常是华南前汛期降水年代际异常的重要原因。3)南太平洋关键区海温在1990年代初开始呈现增暖趋势,在偏暖(偏冷)时期,华南低空受异常气旋(异常反气旋)环流控制,对流层上层西风急流偏弱偏南(偏强偏北),造成华南地区降水异常偏多(偏少)。     

夏季青藏高原对流层温度与西北太平洋副热带地区降水的关系

利用1979—2016年CMAP(CPC Merged Analysis of Precipitation)和GPCP(Global Precipitation Climatology Project)的降水数据以及ERA-Interim再分析资料,通过统计方法研究了夏季青藏高原地区对流层中上层温度年际变率与同期西北太平洋副热带地区降水的关系及其相关的物理过程。结果表明,在年际变化尺度上,夏季高原对流层温度与同期西北太平洋副热带地区降水存在显著的正相关,即当高原对流层温度偏高时,西北太平洋副热带区域的降水偏多,反之亦然。分析研究指出,当夏季高原对流层温度偏高时,高原上空南亚高压显著增强并且向东扩展至日本地区,高原北部对流层出现异常的上升运动,这一异常上升气流随着高度增加逐渐北偏,并在中高纬度地区沿着异常西风气流向东扩展至日本地区,随后向南下沉至日本南部;受该异常下沉运动影响,日本南部对流层低层出现异常反气旋,其东侧的异常北风与西北太平洋低层的异常气旋、反气旋环流存在紧密联系。西北太平洋地区这种异常环流特征为西北太平洋副热带区域的降水提供了有利的动力和水汽条件,从而使该区域降水增多。