Simulation of the East Asian Subtropical Westerly Jet Stream with GFDL AGCM (AM2.1)

The present study validated the capability of the AM2.1,a model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL),in reproducing the fundamental features of the East Asian Subtropical Westerly Jet Stream (EASWJ).The main behaviors of the EASWJ are also investigated through the reanalysis of observational NCEP/NCAR data.The mean state of the EASWJ,including its intensity,location,structure,and seasonal evolution is generally well-portrayed in the model.Compared with the observation,the model tends to reproduce a weaker jet center.And,during summer,the simulated jet center is northward-situated.Results also demonstrate the model captures the variability of EASWJ during summer well.The results of the empirical orthogonal function (EOF) applied on the zonal wind at 200 hPa (U200) over East Asia for both the observation and simulation indicate an inter-decadal shift around the late 1970s.The correlation coefficient between the corresponding principle components is as great as 0.42 with significance at the 99% confidence level.

Responses of the East Asian Jet Stream to the North Pacific Subtropical Front in Spring

This study concerns atmospheric responses to the North Pacific subtropical front （NPSTF） in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream （EAJS）. Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS.

WINTERTIME MIDDLE EAST SUBTROPICAL WESTERLY JET STREAM INTERANNUAL VARIATION CHARACTERISTICS AND ITS POSSIBLE PHYSICAL FACTORS

Using National Centers for Environmental Prediction/Department of Energy(NCEP/DOE) monthly reanalysis data and an extended reconstruction of the sea surface temperature data provided by National Oceanic and Atmospheric Administration, the basic characteristics of the interannual variation in the wintertime Middle East subtropical westerly jet stream(MEJ) and its possible physical factors are studied. The results show that the climatological mean MEJ axis extends southwestward-northeastward and that its center lies in the northwest part of the Arabian Peninsula. The south-north shift of the MEJ axis and its intensity show obvious interannual variations that are closely related to the ElNio-Southern Oscillation(ENSO) and the mid-high latitude atmospheric circulation. The zonal symmetric response of the Asian jet to the ENSO-related tropical convective forcing causes the MEJ axis shift, and the Arctic Oscillation(AO)causes the middle-western MEJ axis shift. Due to the influences of both the zonal symmetric response of the Asian jet to the ENSO-related tropical convective forcing and the dynamical role of the AO, an east-west out-of-phase MEJ axis shift is observed. Furthermore, the zonal asymmetric response to the ENSO-related tropical convective forcing can lead to an anomalous Mediterranean convergence(MC) in the high troposphere. The MC anomaly excites a zonal wave train along the Afro-Asian jet, which causes the middle-western MEJ axis shift. Under the effects of both the zonal symmetric response to the ENSO-related tropical convective forcing and the wave train along the Afro-Asian jet excited by the MC anomaly, an east-west in-phase MEJ axis shift pattern is expressed. Finally, the AO affects the MEJ intensity, whereas the East Atlantic(EA) teleconnection influences the middle-western MEJ intensity. Under the dynamical roles of the AO and EA, the change in the MEJ intensity is demonstrated.

The Effect of the Subtropical Jet on the Rainfall over Southern China in January 2008

ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over south- ern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.

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.

Impacts of Two Types of Northward Jumps of the East Asian Upper-tropospheric Jet Stream in Midsummer on Rainfall in Eastern China

The East Asian upper-tropospheric jet stream （EAJS） typically jumps north of 45~N in midsummer. These annual northward jumps are mostly classified into two dominant types： the first type corresponds to the enhanced westerly to the north of the EAJS＇s axis （type A）, while the second type is related to the weakened westerly within the EAJS＇s axis （type B）. In this study, the impacts of these two types of northward jumps on rainfall in eastern China are investigated. Our results show that rainfall significantly increases in northern Northeast China and decreases in the Yellow River-Huaihe River valleys, as well as in North China, during the type A jump. As a result of the type B jump, rainfall is enhanced in North China and suppressed in the Yangtze River valley. The changes in rainfall in eastern China during these two types of northward jumps are mainly caused by the northward shifts of the ascending air flow that is directly related to the EAJS. Concurrent with the type A （B） jump, the EAJS-related ascending branch moves from the Yangtze-Huai River valley to northern Northeast （North） China when the EAJS＇s axis jumps from 40~N to 55~N （50~N）. Meanwhile, the type A jump also strengthens the Northeast Asian low in the lower troposphere, leading to more moisture transport to northern Northeast China. The type B jump, however, induces a northwestward extension of the lower-tropospheric western North Pacific subtropical high and more moisture transport to North China.

The Asian Subtropical Westerly Jet Stream in CRA-40, ERA5, and CFSR Reanalysis Data:Comparative Assessment

The Asian subtropical westerly jet(AWJ) exerts crucial influences on Eurasian continent weather and climate. This paper analyzes the advantages and limitations of CRA-40, which is China's first generation 40-yr(1979–2018) global atmosphere and land reanalysis product, in describing the characteristics of AWJ, compared with the ECMWF Reanalysis version 5(ERA5) and NCEP Climate Forecast System Reanalysis(CFSR). The results show a close agreement across the three reanalyses on the whole.(1) In terms of climatology, overall differences of 200-h Pa zonal wind across the three reanalyses are within ± 0.5 m s^(-1)(i.e., ± 2%). Large differences with maxima of ± 2 m s^(-1)(±5%) appear over the Iranian Plateau and south of the Tibetan Plateau in the mid–upper troposphere in winter.(2) For seasonal cycle, the position and intensity of the AWJ centers in the three reanalyses are highly consistent, with correlation coefficient over 0.98. But there are some discrepancies in the zonal shift of the western AWJ center during the transition season.(3) On the interannual timescale, intensity of all AWJ centers varies consistently among the three reanalyses, while larger differences appear in their meridional displacement, especially in the eastern AWJ center.(4)For long-term variations, the three reanalyses all present a significant northward movement of the westerly jet axis in winter, and a southward displacement over central Asia(40°–80°E) and a northward migration over East Asia(80°–110°E) in summer. Thus, this study has provided confidence that CRA-40 has comparable performance with ERA5 and CFSR in depicting the characteristics of AWJ.

PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER

Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSO year,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters and their influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta- tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that in the winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone are mainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982— 1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonal changes of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinal progression or regression of about 40 day period.There are marked differences between propagat- ing passages of low frequency modes responsible for changes of subtropical jet stream in the normal year(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude show obvious phases.In general,the one in late winter is stronger than that in early winter,strongest one occurs in February.

Dynamical role of the Rocky Mountain controlled by East Asian topographies in modulating the tropospheric westerly jet in northern winter

Large-scale mountains like Asian topographies and the Rocky Mountains have important influences on subtropical jet streams(STJs)over downstream regions in winter.The dynamical role of the Rocky Mountains in modulating STJs with and without the existence of East Asian(EA)topographies in northern winter is investigated via numerical experiments.In agreement with previous studies,the Rocky Mountains(topographic forcing),with the existence of EA topographies,can only strengthen the STJ from the east coast of North America to the western Atlantic region.The independent role of the Rocky Mountains,however,strengthens the STJ over not only the east coast of North America but also over Pacific regions.It is found that the existence of EA topographies can dramatically strengthen the EA trough,as well as a downstream ridge which,in the upstream of the Rocky Mountains,acts to partly cancel out the strengthening of the anticyclone to the north of the Rocky Mountains and the northward warm air transport in the high latitudes of Pacific regions due to the Rocky Mountains’forcing alone.Such circulation changes effectively weaken the Rocky Mountains–forced strengthening of the meridional temperature gradient in the midlatitude North Pacific,and thus the STJ there.Therefore,EA topographies are of great importance in modulating the role of the Rocky Mountains as a dynamical forcing of STJ variability.

基于标准化降水指数的辽宁省大豆需水关键期干旱大气环流成因分析

利用1971—2020年辽宁省54个气象站逐日降水量资料及逐日NCEP/NCAR(美国大气科学研究中心和美国国家环境预报中心)再分析资料,计算了辽宁省各站大豆需水关键期(开花期至结荚期)标准化降水指数,分析了大豆需水关键期典型干旱年份及其大气环流成因。结果表明:副热带西风急流位置异常引发的对流层高低空大气环流的不同配置是造成辽宁省大豆需水关键期不同气候特征干旱年的主要大气环流成因。当大豆需水关键期西风急流位置较常年偏北时易发生高温干旱,辽宁省大豆主产区一致受到较大影响,西风急流位置偏北,辽宁省位于高空急流轴南侧、高空辐散风场中心,西太平洋副热带高压位置阶段性异常偏北控制整个辽宁省,对流层风场贯穿高层至低层的下沉气流配合较差的水汽条件是发生高温干旱的主要大气环流成因;当大豆需水关键期西风急流位置偏南时易发生低温干旱,辽宁省大豆主产区中部和北部受影响较大,西风急流位置偏南,季节进程缓慢,西太平洋副热带高压位置较常年持续偏南、偏西,对流层上层垂直上升运动明显,冷空气活动频繁,但对流层中下层一致的下沉运动配合较差的水汽条件不利于降水产生,导致低温和干旱现象的同时发生。

陕西夏季旱涝与欧亚大气环流异常的关系

陕西处于青藏高原边坡地带,南北狭长,气候类型多样,地形复杂,生态条件脆弱,旱涝是夏季的主要气象灾害,因此分析其旱涝的成因对该地区防灾减灾具有重要意义。利用陕西78个气象站1961—2018年夏季(6—8月)降水资料、NCEP/NCAR高度场和风场再分析资料、西太平洋副高指数资料,通过相关、合成、多元回归分析方法,研究了陕西夏季旱涝与大气环流异常的关系。结果表明,500 hPa上乌拉尔山阻塞高压和西太平洋副高是影响陕西夏季旱涝的关键因素,当乌拉尔山阻高和西太副高偏强时,陕西夏季降水偏多,反之偏少。200 hPa西风急流位置与陕西夏季降水关系密切,急流偏南时陕西夏季降水偏多,反之偏少。西太平洋副高偏强、偏西时,陕西夏季降水偏多,反之偏少,副高脊线位置主要影响陕西夏季雨带的南北移动。

北半球夏季副热带西风急流变异及其对东亚夏季风气候异常的影响

探讨了东亚地区副热带西风急流 (EAJS)位置的年际变化特征、影响及其可能机制 .EAJS南北变动的影响主要集中在亚澳季风区和气候平均的北半球副热带西风急流轴的南北两侧 ,这与SOI或ENSO相联系的大气环流响应很不相同 ,后者的影响主要表现在中、东太平洋上 .北半球副热带西风急流存在着两个显著不同的模态 ,其中一个模态反映的是亚太尤其是东亚地区的西风急流的南北变异 ,另一个模态出现在 15 0°E～ 12 0°W的中、东太平洋上 .它们分别联系着不同的太平洋海温异常分布 ,但都能对夏季 2 0 0hPa南亚高压的强度产生影响 ,尤其是南亚高压的东部 。

2009-08-17山东特大暴雨雷达回波及地形作用分析

利用常规、非常规资料对2009年8月17—18日山东省出现的特大暴雨过程进行分析。结果表明:"8.17"特大暴雨主要出现在西风槽东移、副高南退时的副高西北侧边缘,对流有效位能(CAPE)等热力环境指数对鲁南强降水有较强的反应能力。降水分布具有典型的地形影响特征,复杂山地地形对降水有显著增幅作用和对对流系统传播的阻碍作用。存在低空急流和列车效应,强回波维持时间较长是费县出现强降水的重要原因,较强的偏南气流、湿度层深厚是费县出现特大暴雨的根本原因。地形影响气流理论很好地解释了山东"8.17"特大暴雨降水分布,费县特殊地形对特大暴雨有很大贡献。雷达资料可以弥补常规观测的不足;地形对强降雨及其分布有重要影响。

东亚高空温带急流区经向风的季节变化及其与亚洲季风的关系

利用NCEP/NCAR再分析资料研究东亚高空温带急流的位置、强度、结构和季节转换特征及其与亚洲季风的关系,发现温带急流在300hPa高度上最为明显,在风场分布上表现为全风速大值脊线延伸区和流线密集区,冬季主要活动于120°E以西的45°—60°N地区,在逐日风场上对应着急流发生频数的高值区域,并与副热带急流有清晰的分界。对比温带急流和副热带急流中的经向风强度发现,温带急流区的北风分量明显强于副热带急流中的南风分量,在温带急流的形成和季节变化过程中经向风分量起着重要作用。温带急流所在区域为对流层纬向温度梯度大值区,同时经向温度梯度也比较大,因而温带急流位于具有最大纬向温度梯度同时又有南北方向温度梯度这样一个特定的区域,从而形成了温带急流与副热带急流不同的结构特征和季节变化,而东亚地区海陆热力差异引起的温度梯度及其季节转换是引起温带急流季节变化的主要原因。此外,温带急流的强度与副热带急流位置之间具有协同变化关系,温带急流区经向风强度的季节转换时间与东亚大气环流的季节转换、亚洲夏季风爆发和江淮流域梅雨开始也有着密切关系,与中国东部地区冬季和夏季的降水之间具有显著的相关关系。从气候平均的角度来看,温带急流强度变化早于亚洲季风爆发和梅雨开始时间,因而对亚洲季风爆发和梅雨开始有预示作用。

太平洋—印度洋海温与我国东部旱涝型年代际变化的关系

对我国东部各区域的夏季降水的正交小波分解表明,其大于28年的分量可以很好地表示华北和长江中下游地区在20世纪70年代前后旱涝相反的年代际变化特征。合成分析表明,北太平洋、热带印度洋海温和东亚高空急流与我国东部夏季旱涝型的年代际变化密切相关。东亚高空急流和西太平洋副热带高压在70年代前后的年代际差异对旱涝型发生年代际变化起到重要作用;北太平洋和热带印度洋海温的年代际变化近百年来是协同一致的,二者有可能共同对旱涝型的变化产生影响。进一步分析指出,北太平洋—热带印度洋海温的变化与急流和副高的南北位置在年际和年代际尺度上都密切相关。可见,北太平洋-热带印度洋海表温度异常(SSTA)对于我国东部旱型涝的年代际变化确实具有重要的预示作用。

夏季东亚西风急流扰动异常与副热带高压关系研究

利用1979—2003年NCEP/NCAR月平均再分析资料,探讨夏季(6—8月)200 hPa东亚西风急流扰动异常与南亚高压和西太平洋副热带高压的关系。研究指出:夏季200 hPa东亚西风急流扰动动能加强(减弱),东亚西风急流位置偏南(偏北)、强度偏强(偏弱);东亚西风急流扰动动能强弱不仅与北半球西风急流强弱和沿急流的定常扰动有关,而且还与东亚地区高、中、低纬南北向的扰动波列有关,亚洲地区是北半球中纬度环球带状波列异常最大的区域。夏季200 hPa东亚西风急流扰动动能加强(减弱),南亚高压的特征为位置偏东(偏西)、强度加强(减弱);西太平洋副热带高压的特征为位置偏南(偏北)。东亚环流特别是500 hPa西太平洋副热带高压对东亚西风带扰动异常的响应由高空东亚西风急流南侧的散度场及其对流层中下层热带和副热带地区的垂直速度距平场变化完成。

2020年超强梅雨特征及其成因分析

2020年梅雨呈现出入梅早、出梅晚、梅雨期长、雨区范围广、累计雨量大、强降水过程多的特点,是一次典型的超强梅雨。分析发现,梅雨期东亚季风区多个关键大气环流系统的平均位置相对稳定,且表现出显著的准双周振荡特征。梅雨的开始和结束、主雨带的北抬和停滞、强降水过程的发生维持都与准双周振荡有很好的对应关系。梅雨期间西太平洋副热带高压共经历了6次北抬和南撤的周期性振荡,同时高、低层季风环流系统则表现出5次显著增强的过程,尤其是低空西南急流的不断加强,南风大值中心反复建立和位置的相对稳定,使得源自热带的水汽输送一次次加强,水汽辐合与上升运动反复发展,从而导致梅雨在江淮流域长时间持续,暴雨过程频频发生。另外,梅雨期欧亚中高纬环流表现为“两脊一槽”型,中高纬阻塞高压活动频繁,东亚沿岸低槽活跃,经西北路径和(或)东北路径的冷空气不断南下,与低层一次次加强的西南暖湿水汽在江淮区域频繁交汇,这是造成今年梅雨异常偏强的另一重要因素。通过对历史上梅雨的时空分布、雨涝特征及致灾程度等方面的比较可知,2020年的超强梅雨的异常程度总体弱于1954年,而强于1998年和1991年。得益于现今气候预测准确率和防灾减灾能力的明显提高,今年由此次超强梅雨而造成的江淮流域洪涝灾害影响及死亡失踪人数等均较之前明显降低。

2007年7月川东北连续3场大暴雨过程的诊断分析

利用常规观测资料及NCEP 1°×1°6 h再分析资料,对2007年7月上旬四川东北部连续出现的3场大暴雨过程的环流形势及动力结构、水汽输送和热力不稳定条件进行了诊断分析。结果表明:(1)前2场区域性大暴雨出现在副热带高压和巴尔喀什湖冷涡两个长波系统稳定少动的阻塞环流形势下,第3场局地性大暴雨发生在环流调整过程中,副热带高压快速东撤导致对流云团在东移过程中迅速减弱消亡;(2)暴雨的水汽主要来自南海,低空偏南风急流的维持为连续暴雨提供了源源不断的水汽输送和持续的能量供应,3场暴雨的中心均出现在位于低空急流出口区左侧水汽辐合中心的巴中地区;(3)造成严重洪涝灾害的前2场区域性大暴雨过程期间,从地面到高层形成了"辐合—辐散—辐合—辐散"接力式上下大气运动的动力结构,大气层结处于高能和对流不稳定状态,且有冷空气触发,大暴雨发生在能量锋区偏向暖区一侧。

太阳活动对NAO与冬季中东急流轴线位置之间关系的调制作用

利用NCEP/NCAR大气环流再分析资料和NOAA提供的太阳黑子资料,讨论了冬季中东急流年际变化特征,并探讨了太阳活动对北大西洋涛动(North Atlantic Oscillation,NAO)与冬季中东急流轴线位置之间关系的调制作用。结果表明:冬季中东急流的轴线位置表现出显著的年际变化特征,在空间上主要表现了中东急流东西两侧轴线南北移动呈反向变化(Middle East Jet Axis shift east-west Out-phase,MEJAO)型和中东急流轴线南北移动呈整体一致的变化特征(Middle East Jet Axis shift In-phase,MEJAI)型。另外,在太阳活动强时期,NAO(North Atlantic Oscillation,北大西洋涛动)的空间结构更靠近北大西洋东侧的大陆上,欧洲大陆北侧与地中海地区出现相反的SLP(Sea Level Pressure,海平面气压)异常,通过Ekman抽吸作用形成次级环流,在对流层高层地中海地区易出现辐合异常,并激发Rossby波波列,在伊朗高原上空会形成位势高度异常,从而中东急流东部轴线南北侧西风呈相反的变化。同时,在对流层高层欧洲大陆南侧形成的位势高度异常,也会使得中东急流西部轴线北侧西风出现异常。中东急流东、西部西风异常的空间结构呈反相变化,即出现了MEJAO型。而在太阳活动弱时期,NAO的空间结构主要局限在北大西洋地区,不易形成地中海辐合异常,NAO与MEJAO型的关系不密切。因此,太阳活动对NAO与MEJAO型之间的关系存在着调制作用。

北方雨季中国东部降水异常模态的环流特征及成因分析

根据1958-2011年中国东部（105°E以东）316站逐日降水资料及NCEP/NCAR逐日再分析资料,利用统计分析、物理量诊断等方法,探讨北方雨季（7月11日至8月31日）中国东部降水异常模态及同期、前期的大气环流特征。分析发现,北方雨季中国东部降水异常表现为三个相互独立的降水模态：第一模态为偏西型,当其时间系数为正（负）时,河套地区降水偏多（少）,江淮流域上游降水偏少（多）,南方大部降水偏多（少）;第二模态为北方一致型,当其时间系数为正（负）时,北方降水一致偏多（少）,长江流域降水偏少（多）;第三模态为偏东型,当其时间系数为正（负）时,东北南部至长江中游降水偏多（少）,华东沿海降水偏少（多）。研究发现,造成北方雨季三个降水异常模态的环流特征各不相同：偏西型降水主要受西亚高空副热带西风急流位置南北偏移影响;北方一致型降水主要由东亚—太平洋遥相关波列导致;偏东型降水主要与海陆气压异常对比造成的东亚夏季风变化有关。此外,三个模态与前期环流异常有密切联系。第一模态的正（负）异常由7月上旬200 h Pa来自北大西洋的异常波列造成乌拉尔山位势高度负（正）异常和巴尔喀什湖以南位势高度正（负）异常引起。第二模态的正（负）异常与前期7月上旬200 h Pa北大西洋上位势高度负（正）异常产生的沿中纬度（高纬度）路径向下游传播的波列有关。第三模态的正（负）异常由春季3月份低层蒙古上空异常的气旋（反气旋）持续至同期造成。