FGOALS-s2 Simulation of Upper-level Jet Streams over East Asia: Mean State Bias and Synoptic-scale Transient Eddy Activity

Upper-level jet streams over East Asia simulated by the LASG/IAP coupled climate system model FGOALS-s2 were assessed, and the mean state bias explained in terms of synoptic-scale transient eddy activ- ity （STEA）. The results showed that the spatial distribution of the seasonal mean jet stream was reproduced well by the model, except that following a weaker meridional temperature gradient （MTG）, the intensity of the jet stream was weaker than in National Centers for Environment Prediction （NCEP）/Department of Energy Atmospheric Model Inter-comparison Project II reanalysis data （NCEP2）. Based on daily mean data, the jet core number was counted to identify the geographical border between the East Asian Sub- tropical Jet （EASJ） and the East Asian Polar-front Jet （EAPJ）. The border is located over the Tibetan Plateau according to NCEP2 data, but was not evident in FGOALS-s2 simulations. The seasonal cycles of the jet streams were found to be reasonably reproduced, except that they shifted northward relative to reanalysis data in boreal summer owing to the northward shift of negative MTGs. To identify the reasons for mean state bias, the dynamical and thermal forcings of STEA on mean flow were examined with a focus on boreal winter. The dynamical and thermal forcings were estimated by extended Eliassen-Palm flux （E） and transient heat flux, respectively. The results showed that the failure to reproduce the tripolar-pattern of the divergence of E over the jet regions led to an unsuccessful separation of the EASJ and EAPJ, while dynamical forcing contributed less to the weaker EASJ. In contrast, the weaker transient heat flux partly explained the weaker EASJ over the ocean.

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.

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.

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.

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.

Relationship between Meridional Displacement of the Monthly East Asian Jet Stream in the Summer and Sea Surface Temperature in the Tropical Central and Eastern Pacific

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.

Stream-coordinate structure of oceanic jets based on merged altimeter data

The jet structure of the Southern Ocean front south of Australia is studied in stream-coordinate with a new altimeter product—Absolute Dynamic Topography (ADT) from AVISO. The accuracy of the ADT data is validated with the mooring data from a two-year subantarctic-front experiment. It is demonstrated that the ADT is consistent with in-situ measurements and captures the meso-scale activity of the Antarctic Circumpolar Current (ACC). Stream-coordinate analysis of ADT surface geostrophic flows finds that ACC jets exhibit large spatio-temporal variability and do not correspond to particular streamfunction values. In the circumpolar scope ACC jets display a transient fragmented pattern controlled by topographic features. The poleward shift of jet in streamfunction space, as revealed by a streamwise correlation method, indicates the presence of meridional fluxes of zonal momentum. Such cross-stream eddy fluxes concentrate the broad ACC baroclinic flow into narrow jets. Combined with a recent discovery of gravest empirical mode (GEM) in the thermohaline fields, the study clarifies the interrelationship among front, jet and streamfunction in the Southern Ocean.

CHARACTERISTICS OF MIDDLE EAST JET STREAM DURING SEASONAL TRANSITION AND ITS RELATION WITH INDIAN SUMMER MONSOON ONSET

By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream(MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67 th pentad to the 24 th pentad the following year; when MEJS is weak, it is at 45°N from the 38 th pentad to the 44 th pentad. The first Empirical Orthogonal Function(EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31 st pentad is the spring-summer transition of MEJS, and the 54th-61 st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference(SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.

Branches of the Summer Asian Lower-Level Jet Stream and Its Influence on the Rain Belt in China

Based on the analysis of the Asian lower-level jet stream, this paper indicates that having shifted to the Bay of Bengal, this large-scale lower-level jet(LLJ) develops into two branches: the northern branch (NB) which is a strong southwest flow moving into the inland of China along the southeast side of the Qinghai-Xizang Plateau and then moving eastward to Japan along the north side of the subtropical high, and the southern branch (SB) which continues to be a west flow and travels into the West Pacific across the Indo-China Peninsula along the south side of the monsoon trough. Above the two branches are two synoptic-scale transient tubular monsoon circulation systems, the northern branch being a subtropical monsoon stream tube (SMST) and the southern branch a tropical monsoon stream tube (TMST). Their ascending branches, corresponding to a subtropical monsoon rain belt and a tropical monsoon rain belt respectively, bear considerable influence on the weather over China.

Associations of Jet Streams with Tornado Outbreaks in the North America

Jet streams, a current of fast winds located about seven miles up near the tropopause, are major weather driving factors in the mid-latitudes. Using the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data resources for the period 1979 to 1988, we defined the latitudinal distribution of daily wind maxima by month, and analyzed latitudinal distribution of the highest daily wind speed at the selected longitude in the 19th week of the year. We showed the typical diving pattern of jet stream over Central America in spring. We found that latitudinal distribution of daily wind maxima was concurrently oscillated with latitudinal distribution of tornado outbreaks in April, May and June. KZ filter smoothed latitudinal distribution of highest daily wind speed on tornado days showed a substantial increase in number of daily wind maxima over tornado alley region, compared to that on non-tornado-days.

The effects of the intertropical convergence zone on the easterly jet stream during Northern summer

In this paper, a strong 1TCZ process and an 1TCZ - absent process during FGGE in 1979 were selected for comparison to explore how they were subject to the influence of the evolution of the upper easterly jets.

Jet Stream as a Major Factor of Tornados in USA

Using the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data resources for the period 1979 to 1988, we defined the spatial distribution and temporal trends of jet stream by month, and analyzed geographic distribution of the smoothed hourly wind speed of jet stream in May, after applied Kolmogorov-Zurbenko Adaptive (KZA) filter. We showed the impressive synchronousness between hourly tornado risks and diurnal distribution of top 3 maximum daily wind speed of jet stream over USA. We observed that latitudinal distribution of jet stream concurrently oscillated with latitudinal distribution of tornado outbreaks. KZA filter smoothed spatial distribution of jet stream on tornado days showed a substantial increase in presence of jet stream over tornado alley region, compared to that on non-tornado-days.

2021年11月7—9日辽宁省极端雨雪天气分析

选用常规气象观测资料、人工加密观测资料和NCEP/NCAR格点资料,对2021年11月7—9日辽宁省极端雨雪天气进行诊断分析。结果表明:东北冷涡和地面气旋为此次极端雨雪天气的主要影响系统,对流层中低层辐合、高层辐散以及强的冷暖气流交汇,是产生极端雨雪天气的主要原因;高低空急流耦合和高空辐散抽吸作用,配合东北冷涡动力抬升作用,暖湿低空急流沿冷垫爬升,进一步加强了上升运动。暖湿急流为此次极端雨雪天气提供了充沛的水汽条件,暖湿急流的增强对应水汽辐合作用增强,配合低层冷垫和东北冷涡的动力抬升作用,对降雪有明显增幅作用;地面辐合线沿地形分布,触发了此次极端强降雪天气。地形阻挡导致地面冷空气堆积形成冷垫,是极端降雪天气发生的重要原因。温度层结差异是鞍山、岫岩雨雪相态差异的主要原因。具有冻结层特征的低层冷垫,为鞍山极端暴雪提供了有利的温度层结条件;融化层厚度和地面温度是岫岩出现冻雨的关键因素。

北半球夏季急流遥相关:进展与展望

急流遥相关是沿急流波导传播的准静止行星尺度波列,在大气环流的维持和演变中有重要作用,其异常活动常伴随极端天气气候事件的发生。研究急流遥相关在不同时间尺度的变异特征和机理不仅可以加深对大气环流变化规律的理解,还可以为相应时间尺度上的天气预报、气候预测乃至气候预估提供科学依据。根据波导的性质,北半球夏季的急流波导可以分为以正压效应为主要维持机制的副热带急流波导和以斜压效应为维持机制的极锋急流波导。本文聚焦夏季急流遥相关活动最为活跃的欧亚大陆地区,在回顾急流遥相关理论的基础上,简要总结了近年来关于夏季副热带急流遥相关和极锋急流遥相关的形成条件、动力机制、气候影响和未来变化的研究进展,并展望了一些未来值得继续深入研究的问题。

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。

2021年1月与2024年1月我国东部极端寒潮发生原因及机制探讨

伴随全球变暖和有卫星观测记录以来北极秋季海冰密集度的下降,近几年,东亚地区极端降温事件频繁发生。基于北极涛动指数、北极海冰密集度及温度、位势高度和风场等大气环流数据,研究2021年1月和2024年1月我国极端寒潮的时空变化特征及其原因机制。2021年1月1日至9日,2024年1月17日至24日,我国东部地区(115°00′~120°00′E,20°00′~37°30′N)均出现极端寒潮,最低温异常分别达到-9.028℃和-7.574℃,降温中心分别集中在蒙古高原南部和秦岭北部。2021年1月我国东部地区平均气温降至-3.8℃,达到21世纪以来的次低温纪录。本文基于美国国家环境预报中心(National Centers for Environmental Prediction,NCEP)的再分析资料,得出西风带的异常减弱致使西风带对高纬度冷空气的阻拦作用减弱。2021年、2024年寒潮爆发时位于西伯利亚、太平洋西北部的高压中心和华北地区低压中心均出现显著增强,导致位于西伯利亚的冷空气的南移,致使我国东部地区出现寒潮现象。2024年1月极端寒潮现象显示于气温经验正交分解(Empirical Orthogonal Function,EOF)的第一模态(29.33%),而EOF分解的第二模态(14.92%)显示2021年出现降温现象。回归分析表明北极涛动指数和北极海冰的下降与1月我国东部地区气温之间存在较强的关系,北极涛动负相位及北极海冰的减少也是促使北极冷空气南下的重要因素。异常强劲的北风、减弱的西风急流及对应高压中心的移动共同作用导致了极地寒流入侵我国大陆地区,是造成2021年1月和2024年1月我国大陆地区极端天气事件的主要原因之一。

清远一次极端龙舟水的中尺度系统演变分析

利用多源观测数据和ERA5再分析资料,对2022年6月20到21日发生在清远的一次极端特大暴雨过程进行分析。结果表明:(1)该次强降水是在200 hPa分流辐散的背景下,由低涡切变配合其前侧西南急流缓慢东移经过清远造成的。(2)强降水发生前大气层结已处于极不稳定状态,低层来自孟加拉湾和南海的水汽不断在清远市上空辐合,为暴雨的发生提供充足的水汽条件和不稳定能量。(3)强降水的触发机制主要是前期广西降水造成的冷池与低空急流带来的暖湿气团形成的温度锋区和地面辐合线;地形的抬升作用和狭管效应,对降水起到增幅作用。(4)该次强降水主要有2个低质心的热带海洋型中尺度对流系统先后影响发生。

Surface and Upper-Level Features Associated with Wintertime Cold Surge Outbreaks in South Korea

The surface and upper-level features associated with a sharp drop ofwintertime daily temperature over South Korea is investigated in this study. This sharp drop indaily temperature is called a cold surge and is one of the most hazardous weather phenomena in EastAsian winters. An upper-level baroclinic wave of 60° wavelength propagating eastward at a phasespeed of 12° longitude per day across the continent of northern China from the west of Lake Baikaltoward the eastern coast of China causes the outbreak of cold air over South Korea. The coolingassociated with the upper-level baroclinic jvave is found at all altitudes under the geopotentialheight-fall center near the tropopause. The development in the ridge seems to derive the earlyevolution of the eastward-propagating sinusoidal wave, whereas the trough is connected directly withthe tropospheric temperature-drop. An enhancement of the wintertime East Asian jet stream after theoutbreak of a cold surge is a response to the steep temperature gradient associated with thedeveloping baroclinic wave.

THE STUDY OF STORM RAINFALL CAUSED BY INTERACTION BETWEEN THE NON-ZONAL HIGH LEVEL JET STREAK AND TYPHOON IN THE DISTANCE

In this paper, statistics were analyzed concerning correlation between the storm rainfall far from typhoon and non-zonal upper-level jet stream. The results show that the jet stream at 200 hPa is constantly SW (90.2 %) during the period in which storm rainfall occurs. Rainfall area lies in the right rear regions of the jet axes. While the storm intensifies, the jet tends to be stronger and turn non-zonal. With the MM4 model, nu-merical simulation and diagnosis were carried out for Typhoon No.9711 (Winnie) on August 19 to 20, 1997. The distant storm rainfall is tightly correlative to the jet and low-level typhoon trough. The divergence field of jet is related to the v component. The upper level can cause the allobaric wind convergence at low level. This is the result of the form of low-level typhoon trough and the strength of the storm. By scale analysis, it is found that there is a branch of middle scale transverse inverse circulation in the right entrance regions behind the jet below the 300-hPa level, which is very important to the maintenance and strengthening of storm rainfall. This branch of inverse circulation is relative to the reinforcement of jet’s non-zonal characteristics. From the field of mesoscale divergence field and non-zonal wind field, we know that the stronger symmetry caused by transverse circulation in the two sides of the jet, rainfall抯 feedback and reinforcement of jet抯 non-zonal characteristics had lead to positive feedback mechanism that was favorable of storm rainfall抯 strengthening.