Predictability of the summer East Asian upper-tropospheric westerly jet in ENSEMBLES multi-model forecasts

The interannual variation of the East Asian upper-tropospheric westerly jet(EAJ) significantly affects East Asian climate in summer. Identifying its performance in model prediction may provide us another viewpoint,from the perspective of uppertropospheric circulation,to understand the predictability of summer climate anomalies in East Asia. This study presents a comprehensive assessment of year-to-year variability of the EAJ based on retrospective seasonal forecasts,initiated from1 May,in the five state-of-the-art coupled models from ENSEMBLES during 1960–2005. It is found that the coupled models show certain capability in describing the interannual meridional displacement of the EAJ,which reflects the models' performance in the first leading empirical orthogonal function(EOF) mode. This capability is mainly shown over the region south of the EAJ axis. Additionally,the models generally capture well the main features of atmospheric circulation and SST anomalies related to the interannual meridional displacement of the EAJ. Further analysis suggests that the predicted warm SST anomalies in the concurrent summer over the tropical eastern Pacific and northern Indian Ocean are the two main sources of the potential prediction skill of the southward shift of the EAJ. In contrast,the models are powerless in describing the variation over the region north of the EAJ axis,associated with the meridional displacement,and interannual intensity change of the EAJ,the second leading EOF mode,meaning it still remains a challenge to better predict the EAJ and,subsequently,summer climate in East Asia,using current coupled models.     

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.     

Intermodel Diversity in the Zonal Location of the Climatological East Asian Westerly Jet Core in Summer and Association with Rainfall over East Asia in CMIP5 Models

The East Asian westerly jet(EAJ), an important midlatitude circulation of the East Asian summer monsoon system,plays a crucial role in affecting summer rainfall over East Asia. The multimodel ensemble of current coupled models can generally capture the intensity and location of the climatological summer EAJ. However, individual models still exhibit large discrepancies. This study investigates the intermodel diversity in the longitudinal location of the simulated summer EAJ climatology in the present-day climate and its implications for rainfall over East Asia based on 20 CMIP5 models. The results show that the zonal location of the simulated EAJ core is located over either the midlatitude Asian continent or the western North Pacific(WNP) in different models. The zonal shift of the EAJ core depicts a major intermodel diversity of the simulated EAJ climatology. The westward retreat of the EAJ core is related to a warmer mid–upper tropospheric temperature in the midlatitudes, with a southwest–northeast tilt extending from Southwest Asia to Northeast Asia and the northern North Pacific, induced partially by the simulated stronger rainfall climatology over South Asia. The zonal shift of the EAJ core has some implications for the summer rainfall climatology, with stronger rainfall over the East Asian continent and weaker rainfall over the subtropical WNP in relation to the westward-located EAJ core.     

Projection of the East Asian westerly jet under six global warming targets

东亚高空急流(简称急流)对夏季东亚地区的天气和气候有着重要影响。本文利用CMIP5的历史气候模拟试验和RCP8.5路径下的未来气候变化预估试验数据,预估了急流在6个全球变暖阈值(1.5℃,2.0℃,2.5℃,3.0℃,3.5℃和4.0℃)下相对于当代气候的变化情况。结果表明东亚高空西风在1.5℃阈值下略微减弱。在2.0℃阈值下,西风在急流轴(约40°N)南侧增强,北侧减弱。这种变化趋势在2.5℃和更高的变暖阈值下愈加明显,使急流轴逐渐向南移动,但急流强度变化不大。研究表明,在急流的入口和出口区,对流层中上层大气升温速度相对较慢,导致在急流轴南(北)侧出现向南(北)的负(正)经向温度梯度,使西风在南(北)侧增强(减弱).     

夏季东亚高空西风急流气候特征分析

利用NCEP/NCAR全球再分析风场资料定义了西风急流强度指数和位置指数,然后利用EOF方法对西风急流进行了进一步的分析,分析了高空西风急流的空间分布特征,从强度和位置两方面分析了西风急流与东亚环流及其与海温的关系。分析表明： EOF第一模态反映了东亚高空急流的位置指数,第二模态反映了高空急流的强度指数。东亚高空急流与对流层大气环流包括南亚高压,西太平洋副热带高压,东亚夏季风存在着密切关系,其气候变化与热带副热带东太平洋、印度洋海温密切相关。     

东亚副热带西风急流位置异常对长江中下游夏季降水的影响

利用NCEP/NCAR 200 hPa月平均风场再分析资料,定义东亚大陆对流层上层不同经度上最大西风所在位置的平均纬度为东亚副热带西风急流轴线指数,该指数能准确反映东亚副热带西风急流位置的南北变化及其对长江中下游降水的影响,并能较好地体现东亚夏季风盛行期间对流层低层与高层的纬向风场变化特征。分析表明,该指数的时间变化具有与长江中下游夏季降水较一致的年代际变化及年际振荡特征。对东亚副热带西风急流位置异常年的大气环流差异分析表明,急流异常偏北时,南亚高压偏弱,位置偏北偏西,呈伊朗高压型;西太平洋副热带高压(下称西太副高)偏弱、位置偏东偏北;气流的辐合上升区北移至华北一带,而长江流域低层风场为辐散异常,上升气流较常年偏弱,降水偏少。急流异常偏南时,南亚高压偏强,位置偏南偏东,呈青藏高压型;西太副高偏强、位置偏西偏南;长江流域地区上空低层有较强辐合上升气流,高层有较强的气流辐散,对流旺盛,雨带在此维持,容易引发洪涝。     

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.     

Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS‘s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.     

亚洲西风急流纬向非均匀性变化成因及其对东亚夏季气候的影响

利用美国NOAA（National Oceanic and Atmospheric Administration）的CMAP（Climate Prediction Center (CPC) Merged Analysis of Precipitation）月平均降水资料、NCEP/DOE（National Centers for Environmental Prediction/Design of Experiments）II的月平均再分析资料和中国气象局国家信息中心提供的中国160站逐月降水和平均气温资料,通过定义一个亚洲急流纬向非均匀性指数（I_Aja）,分析了1979～2019年夏季亚洲西风急流纬向非均匀性的年际变化特征,揭示了夏季亚洲急流纬向非均匀性变化异常的成因及其对东亚夏季降水和气温的影响。结果表明:夏季亚洲西风急流纬向非均匀性具有显著的年际变化特征,并存在6～8年和2年左右的振荡周期。当急流纬向非均匀性典型偏强（弱）年,东亚东部地区从低纬到高纬,降水异常主要呈现出偏多—偏少—偏多（偏少—偏多—偏少）的经向分布;气温则在中国西部地区和日本北部偏高（低）,贝加尔湖地区偏低（高）。引起夏季亚洲急流纬向非均匀性异常的可能原因如下:由大气非绝热加热异常而引起的热带和中纬度地区辐合/辐散运动造成的涡度源强迫,和来自西风带中波扰动能量的注入,两者共同作用形成并维持了与急流纬向非均匀性强弱变化相联系的异常环流,从而使亚洲急流东、西段强度差异增强（减弱）,进而有利于急流纬向非均匀性异常偏强（偏弱）。而上述西风带中波扰动能量的东传可能与北大西洋海表面温度异常有关。这对于深刻理解夏季亚洲急流纬向非均匀性异常的形成机理提供了有用的线索。     

Simulations of the 100-hPa South Asian High and precipitation over East Asia with IPCC coupled GCMs

The South Asian High (SAH) and precipitation over East Asia simulated by 11 coupled GCMs associated with the forthcoming Intergovernmental Panel on Climate Change’s (IPCC) 4th Assessment Report are evaluated. The seasonal behavior of the SAH is presented for each model. Analyses of the results show that all models are able to reproduce the seasonal cycle of the SAH. Locations of the SAH center are also basically reproduced by these models. All models underestimate the intensity and the extension of coverage in summer. The anomalous SAH can be divided into east and west modes according to its longitudinal position in summer on the interannual timescale, and the composite anomalies of the observed precipitation for these two modes tend to have opposite signs over East Asia. However, only several coupled GCMs can simulate the relationship between rainfall and SAH similar to the observed one, which may be associated with the bias in simulation of the subtropical anticyclone over the West Pacific (SAWP) at 500 hPa. In fact, it is found that any coupled GCM, that can reproduce the reasonable summer mean state of SAWP and the southward (northward) withdrawal (extension) for the east (west) mode of SAH as compared to the observed, will also simulate similar rainfall anomaly patterns for the east and west SAH modes over East Asia. Further analysis indicates that the observed variations in the SAH, SAWP and rainfall are closely related to the sea surface temperature (SST) over the equatorial tropical Pacific. Particularly, some models cannot simulate the SAWP extending northward in the west mode and withdrawing southward in the east mode, which may be related to weak major El Ni?no or La Ni?na events. The abilities of the coupled GCMs to simulate the SAWP and ENSO events are associated partly with their ability to reproduce the observed relationship between SAH and the rainfall anomaly over East Asia.     

Simulations of the East Asian subtropical westerly jet by LASG/IAP AGCMs

Performances of two LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Sci- ences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics)Atmospheric General Circulation Models(AGCMs),namely GAMIL and SAMIL,in simulating the major characteristics of the East Asian subtropical westerly jet(EASWJ)in the upper troposphere are examined in this paper.The mean vertical and horizontal structures and the correspondence of the EASWJ location to the meridional temperature gradient in the upper troposphere are well simulated by two models.However,both models underestimate the EASWJ intensity in winter and summer,and are unable to simulate the bimodal distribution of the ma- jor EASWJ centers in mid-summer,relative to the observation,especially for the SAMIL model.The biases in the simulated EASWJ intensity are found to be associated with the biases of the meridional temperature gradients in the troposphere,and furthermore with the surface sensible heat flux and condensation latent heating.The models capture the major characteristics of the seasonal evolution of the diabatic heating rate averaged between 30°-45°N,and its association with the westerly jet.However,the simulated maximum diabatic heating rate in summer is located westward in comparison with the observed position,with a rela- tively strong diabatic heating intensity,especially in GAMIL.The biases in simulating the diabatic heating fields lead to the biases in simulating the temperature distribution in the upper troposphere,which may further affect the EASWJ simulations.Therefore,it is necessary to improve the simulation of the meridional temperature gradient as well as the diabatic heating field in the troposphere for the improvement of the EASWJ simulation by GAMIL and SAMIL models.     

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.     

The response of the East Asian summer monsoon to strong tropical volcanic eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon （EASM） and EASM rainfall.Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years.The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years.In response to strong tropical volcanic eruptions,the meridional air temperature gradient in the upper troposphere is enhanced,which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream （EASWJ）.At the same time,the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened.The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.     

夏季东亚高空急流与天气尺度波动的气候特征之间的联系

利用NCEP/NCAR逐日资料(1979~2003年),分析了东亚夏季高空急流和天气尺度波动气候特征之间的联系。通过分析急流与天气尺度波动活动量Q(用于描述某一时间段内天气尺度波动活动强弱程度)、热量涡动输送(V′T′)的关系,发现:在气候平均意义下,天气尺度波动活动量在6、8月较大,这意味着在这两个月中,天气尺度波动较为剧烈。这和急流的强度变化是相对应的,即强的天气尺度波动活动量对应着强的急流,弱的天气尺度波动活动量对应着弱的急流。热量涡动输送也表现出与天气尺度波动活动量类似的特征。年际变化上,夏季平均和6、7、8各月平均的热量涡动输送和急流强度都存在显著的正相关。对强、弱急流年份条件下热量涡动输送合成分析的结果表明:热量涡动输送偏南时,对应着强的急流;热量涡动输送偏北时,对应着弱的急流。以上的研究结果说明,在气候平均意义和年际变化上夏季东亚高空急流和天气尺度波动之间有着密切的联系。     

Northern East Asian low and its impact on the interannual variation of East Asian summer rainfall

The strength of the East Asian summer monsoon and associated rainfall has been linked to the western North Pacific subtropical high (WNPSH) and the lower-tropospheric low pressure system over continental East Asia (EA). In contrast to the large number of studies devoted to the WNPSH, little is known about the variability of the East Asian continental low. The present study delineates the East Asian continental low using 850-hPa geopotential height. Since the low is centered over northern EA (NEA), we refer to it as the NEA low (NEAL). We show that the intensity of the NEAL has large interannual variation, with a dominant period of 2–4 years. An enhanced NEAL exhibits a barotropic structure throughout the whole troposphere, which accelerates the summer-mean upper-tropospheric westerly jet and lower-tropospheric monsoon westerly to its south. We carefully identify the anomalous NEAL-induced rainfall anomalies by removal of the tropical heating effects. An enhanced NEAL not only increases rainfall locally in northern Northeast China, but also shifts the East Asian subtropical front northward, causing above-normal rainfall extending eastward from the Huai River valley across central-northern Japan and below-normal rainfall in South China. The northward shift of the East Asian subtropical front is attributed to the following processes without change in the WNPSH: an enhanced NEAL increases meridional pressure gradients and the monsoon westerly along the East Asian subtropical front, which in turn induces a cyclonic shear vorticity anomaly to its northern side. The associated Ekman pumping induces moisture flux convergence that shifts the East Asian subtropical front northward. In addition, the frequent occurrence of synoptic cut-off lows is found to be associated with an enhanced NEAL. Wave activity analysis indicates that the interannual intensity change of the NEAL is significantly associated with the extratropical Polar Eurasian teleconnection, in addition to the forcing of the tropical WNP heating.     

东亚夏季环流多齿轮耦合特征及其对中国夏季降水异常的影响分析

东亚夏季风成员的相互作用,构成了东亚夏季风高、低层环流的“多齿轮耦合”形态。本文利用多变量主成分分析（MV-EOF）等方法诊断分析了东亚夏季风多齿轮耦合的变化特征、耦合机制、时间稳定性、空间稳定特征及其对中国夏季降水的影响机制,并在此基础上构建了典型多齿轮耦合形态影响夏季降水的概念模型。结果表明,多齿轮耦合受到垂直温、压场的强迫和青藏高原大地形的影响,主要表现在年际变化上（周期为2～6年）。其前两个模态稳定地反映了东亚夏季风成员典型联动作用。在第一模态中,北方气旋、南亚高压和西太平洋副热带高压为主要耦合系统。其中北方气旋为正压结构,在高层通过南侧偏西气流与南亚高压耦合,南亚高压则通过中纬东部地区下沉辐散气流与西太平洋副热带高压联动。当该耦合模态增强时,有利于中国夏季降水呈自北向南“＋－＋－”分布。第二模态主要反映中高纬气旋、东亚副热带西风气流、南亚高压、西北太平洋反气旋系统和西太平洋副热带高压耦合特征。其中,中高纬气旋和西北太平洋反气旋为正压系统,两者通过其间的东南气流联动。气旋系统在高层通过南侧西风与东亚副热带西风急流和南亚高压联动。反气旋在中低层通过南侧的偏东气流影响副热带高压强度和面积。当该耦合模态增强时,中国黄河以北及河套地区降水偏多,黄河以南降水偏少。     

东亚海陆表面温度季节性增温差异对东亚夏季风强度的影响

本文通过对比几种不同的东亚夏季风强度指数,发现东亚及附近地区海陆表面温度的变化与东亚夏季风强度有密切联系。在此基础上,根据强、弱夏季风年东亚表面温度差值的逐候数据做EOF分析,结果发现:第一模态可以揭示从春到夏的季节转换,中国东部陆地增温相对较快,而西太平洋及孟加拉湾海温增温较慢,季节转换提前,有利于夏季风偏强;第二模态则反映了春季中高纬度地区增温快、中低纬增温慢的情形,有利于夏季风增强。在5月份两种模态的综合作用显示:陆地较冷、海洋较暖,夏季陆地的快速增温、海洋增温慢,有利于夏季风增强。将上述影响因素引入到改进的东亚夏季风强度指数中,修正后的指数可以反映东亚地区5月到夏季的海陆增温特点以及季节转换的早晚,并更好地描述了季风区中、高纬度的热力差异,合理地解释夏季风强度与西北太平洋副高及低空急流的关系,因此新指数能够更好地反映全国范围内夏季降水的特点。     

大气环流模式IAP AGCM4.0对东亚高空副热带西风急流的模拟及偏差原因分析

基于中国科学院大气物理所大气环流模式IAP AGCM4.0总共30年(1979~2008年)的AMIP(大气环流模式比较计划)数值模拟试验结果,评估了模式对东亚高空副热带西风急流的模拟能力,分析了模式模拟偏差的可能原因,以及不同对流参数化方案对模拟结果的影响。结果表明,IAP AGCM4.0可以较好地模拟出东亚高空副热带西风急流冬季和夏季的空间结构及其季节变化特征;与JRA-25再分析资料相比,模式模拟的急流强度总体偏弱;就急流位置而言,模式模拟的急流位置冬季略偏南,夏季则相对偏北;模式可以较好地模拟出夏季西风急流的季节内演变特征,包括夏季西风急流位置逐月北跳的特征,只是模式模拟的逐月西风急流位置仍偏北。夏季200 h Pa纬向风EOF分解结果表明,模式模拟和再分析资料的EOF第一模态空间型态较为接近,均反映了西风急流的年际变化特征,但两者的时间系数相关较小,表明模式对西风急流南北位置年际变化的模拟偏差较大。针对模式模拟的地表感热通量及对流层中上层经向温度差(MTD)的分析结果表明,模式对阿拉伯半岛东南部、阿拉伯海西北部及印度北部的地表感热通量的模拟存在偏差,影响到对流层中高层温度场、高度场的模拟,使得IAP AGCM4.0模拟的MTD强度较再分析资料相对偏弱,MTD变化最大的区域位置相对偏北,且模式模拟的MTD年际变化与再分析资料相比也有较大偏差,从而造成模式对西风急流模拟的偏差。此外,不同积云对流参数化方案也可影响对流层中上层经向温度差的模拟,进而影响模式对东亚高空副热带西风急流的模拟。     

东亚季风系统的动力过程和准定常行星波活动的研究进展

本文系统地回顾了近几年来关于东亚季风系统的动力过程与机理方面的研究,特别是关于东亚季风系统年际和年代际变异与准定常行星波活动关系的研究。最近的许多研究表明东亚夏季风系统变异的动力过程主要与东亚／太平洋型(即EAP型)遥相关有关,利用EAP型遥相关理论不仅可以说明东亚夏季风系统各成员之间内在联系的机理,而且可以揭示热带西太平洋热力和菲律宾周围对流活动影响东亚夏季风系统季节内、年际变化及其异常的经向三极子结构的动力过程;除了EAP型遥相关外,研究还表明北半球夏季从北非到东亚的对流层上层经向风异常存在一个沿急流传播的遥相关型,它对东亚夏季风系统异常的经向三极子型分布也有重要影响。并且,最近关于东亚冬季风变异与行星波活动的关系已做出许多研究,并获得很大进展。这些研究表明:北半球冬季准定常行星波传播波导在年际和年代际变化上存在着反相振荡特征,即若“极地波导”加强,则“低纬波导”将减弱,反之亦然;准定常行星波两支波导的反相振荡与北半球环状模(NAM)的年际和年代际振荡有紧密联系,而NAM的变化通过行星波活动的异常可以导致东亚冬季风的年际和年代际变化;此外,准定常行星波活动的年际变化与东亚冬季风异常之间的关系明显地受热带平流层纬向风准两年周期振荡(QBO)的调制,进一步的研究还提出了可能的机理。最后本文还指出:2005～2007年冬季东亚冬季风的异常不仅与西伯利亚高压和阿留申低压的变异有关,而且与极涡的演变和准定常行星波活动密切相关。     

The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

During June and July of 2020, the Yangtze River basin suffered from extreme mei-yu rainfall and catastrophic flooding. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts captured these tropical signals well. We explore extratropical drivers but find a large model spread among the forecast members regarding the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that was observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more mei-yu rainfall. The model forecast failed to satisfactorily reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.