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.

A Climatological Perspective on Extratropical Synoptic-Scale Transient Eddy Activity Response to Western Pacific Tropical Cyclones

An observational study focusing on the contribution of tropical cyclones(TCs)that form over the western North Pacific(WNP)to the synoptic-scale transient eddy activity(STEA)over the North Pacific during the boreal autumn and early winter in the period 1979–2019 is presented in this paper.Statistical results show that WNP TCs entering the midlatitudinal North Pacific provide significant positive effects on the pentad mean strength of STEA,which is primarily concentrated over the Kuroshio/Oyashio Extensions(KOE)and regions from east of Japan to 160°W in the lower and midto-upper troposphere,respectively.TC intensity is highly indicative of the subsequent STEA with a correlation coefficient of 0.37/0.33/0.45 at 300 hPa/500 hPa/850 hPa exceeding the 99%confidence level for the period 1979–2019.The strength of STEA in the upper troposphere associated with TCs presents a more significant linear growth with TC intensity than that at the mid-to-lower levels after the cyclones enter the KOE region,suggesting that the impact of TCs on STEA gradually increases with height.Further analyses reveal that the contribution of TCs accounts for 4%–6%of the total STEA change over the KOE region during the late autumn and early winter.In addition,the influence of TCs on STEA experienced an interdecadal decrease from the early 2000 s through the early 2010 s.

Diagnostic Comparison of Wintertime East Asian Subtropical Jet and Polar-Front Jet: Large-Scale Characteristics and Transient Eddy Activities

Diagnostic comparison of the East Asian subtropical jet （EASJ） and polar-front jet （EAPJ） in winter season is carried out by using the ERA-40 dataset. The large-scale circulation characteristics and synopticscale transient eddy activities （STEAs） associated with the EASJ and EAPJ are examined. The results show that the EASJ and EAPJ in the upper-level monthly mean data have no clear geographical border, while the distribution of the numbers of jet cores from the daily data exhibits a distinct boundary at the latitudes of the northern Tibetan Plateau. The two areas with large numbers of jet cores correspond to the EASJ and EAPJ regions. The analysis of STEAs over the East Asian region shows a spatial match of STEAs with the EASJ and EAPJ in winter： the strong EASJ is located within the weak southern branch of the STEA while the relatively weak EAPJ appears within the active northern branch of the STEA, indicating that the EAPJ is the jet coexisting with the STEA. Further analysis shows two anomalous modes of the winter EAPJ： the anomalous anticyclonic/cyclonic circulation and the weakened/strengthened local westerly wind. The large-scale circulation anomalies in the Northern Hemisphere related to the first mode are concentrated in the Eurasian mid to high latitudes, and are also influenced by the anomalous circulation in the upstream area. When the local westerly wind over the EAPJ region is weakened/strengthened, the westerly jet in the eastern part of the EASJ and that in the western Pacific region show opposite variations. The corresponding anomalous atmospheric circulation demonstrates the Eurasian （EU） pattern. The EAPJ anomalies are also closely linked with the STEA anomalies over East Asia. The anomalies in the northern branch of the STEA propagate as a wave train along its axis into the East Asian coastal waters, and then migrate eastward to the oceanic region. However, the ones near the southern branch are trapped over the eastern part of East Asia and its coastal waters at 200 hPa.

The East Asian Upper-Tropospheric Jet Streams and Associated Transient Eddy Activities Simulated by a Climate System Model BCC_CSM1.1

In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_ CSMI.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of the East Asian subtropical jet （EASJ） and the East Asian polar-front jet （EAPJ）. Associated stationary wave and synoptic-scale transient eddy activities （STEA） are also examined. The results show that the climatological positions of the westerly jet streams are well captured by BCC_CSMI.1, but with slight intensity biases. Statistics from the 6-h model outputs reveal that the jet core number （JCN） of ESPJ is significantly underestimated. Examination of the simulated seasonal evolution of the westerly jet stream indicates that the model has produced a westward movement of the EASJ core in May, one month earlier than that in the reanalysis. Analysis of stationary wave activities shows that the overestimated meridional wind component may have caused considerable enhancement of meridional momentum and heat transport. The stationary Rossby wave represented by the wave activity flux at the southern flank of the Tibetan Plateau is favorable to the growth of asymmetric zonal wind and the multiple-center pattern of JCN. Unlike the stationary wave heat flux transport, the model tends to systematically generate weaker transient heat flux over East Asia. Further analysis of STEA exhibits a general consistent pattern between the simulation and the reanalysis, while the intensity of the northern STEA branch associated with the EAPJ is greatly reduced. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may contribute to the biases of the simulated upper-tropospheric jet streams, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation, which is not yet fully and accurately resolved in the current BCC-CSMI.1.

北太平洋冬季海-气耦合的主模态及其与瞬变扰动异常的联系

利用45年的ECMWF再分析资料,使用SVD方法研究了冬季北太平洋地区表层海温(SST)异常与大气环流异常间的主要耦合模态,探讨了大尺度海-气耦合型与天气尺度瞬变扰动的相互关系。分析结果表明,中纬度北太平洋地区冬季存在两种主要的海-气耦合型,第1种耦合型反映了与ENSO紧密相关的中纬度北太平洋冬季海温异常分布型以及大气的PNA型,第2种耦合型SST异常集中在东亚沿海以及中纬度北太平洋海流区,相应的大气场则为暖(冷)SSTA上空东西向带状区域内位势高度偏高(低),明显独立于ENSO型。进一步的合成分析表明,在第1种耦合型SST正(负)异常年里,冬季阿留申低压主体位置偏西南(东北),从东北亚到北美西海岸的西北—东南向带状区域内是低层大气温度正(负)异常区和高层西风负(正)异常区,西风负(正)异常中心位于西风急流出口处的北太平洋中东部,而西风急流主体区的风速变化很小。在第2种耦合型东亚沿海至中纬度北太平洋海流区SST偏暖(冷)时,阿留申低压整体偏弱(强),SST暖(冷)异常上空的大气温度偏暖(冷),高层西风急流区西风偏弱(强)。两种耦合型均显示出在北太平洋中纬度地区大气和海洋的异常相关中心有很好的空间对应性。在两种耦合型下,中纬度北太平洋冬季的大气斜压性也发生截然不同的改变,引起中纬度天气尺度瞬变扰动活动异常。瞬变扰动异常的动力强迫作用对北太平洋西风异常的形成存在正反馈作用,而其热力作用则试图破坏与两种海-气耦合模态相关的大气温度异常型。

冬季东亚副热带急流与温带急流的比较分析：大尺度特征和瞬变扰动活动

使用ERA40再分析的月资料和逐日资料,从大尺度特征和瞬变扰动活动两个角度对冬季东亚副热带急流(EASJ)和东亚温带急流(EAPJ)进行了比较分析。结果表明,使用月资料分析的EASJ与EAPJ在高层风场上没有清晰的地理分界区,而使用逐日资料计算得到的冬季逐日急流发生数则以高原北部上空所处的纬度带为分界岭,存在两个急流中心集中区,分别对应于EASJ和EAPJ区域。通过分析东亚上空天气尺度瞬变扰动活动(STEA)表明,与强盛的冬季EASJ相伴随的是较弱的南支STEA,而与较弱的EAPJ相伴随的北支STEA却十分活跃,显示出EAPJ是与瞬变活动相伴而存的急流。进一步的诊断分析揭示了冬季东亚温带急流的两种主要异常模态,一种是EAPJ区域反气旋性/气旋性异常环流型,另一种是局地西风的减弱/增加。与第1种异常模态相关的北半球大尺度环流异常主要集中在欧亚中高纬地区,其形成受中高纬大气环流以及东亚上游大气环流异常的共同影响。当EAPJ局地西风减弱/增加时,EASJ东段至西太平洋上空的西风急流呈现出与之相反的变化型,北半球大气环流异常表现为大气遥相关的欧亚(EU)型。冬季EAPJ的两种异常模态还与东亚上空STEA异常密切联系,其中在北支STEA区域出现的瞬变异常以波列的形式沿STEA北支轴线传播到达东亚沿海上空,然后东传入洋面上空,而对于靠近STEA南支轴线的异常扰动活动则只能存在于东亚东部及其沿海上空的200 hPa层上。

冬季南极涛动对欧亚大陆地表气温的影响

基于1962~2011年NCEP/NCAR逐日再分析资料,本文研究了北半球冬季南极涛动(Antarctic Oscillation,AAO)对欧亚大陆地表气温的影响及其物理过程。线性回归结果表明,在扣除冬季ENSO信号后,1月AAO与2月欧亚大陆地表气温呈显著正相关关系。当1月AAO处于正位相时,东南太平洋出现显著的位势高度正异常,这对应着南半球副热带高压的增强。该高度正异常在空间上从南半球向北扩展至北半球东太平洋地区,在时间上可一直持续到2月,它在南北半球相互作用过程中起重要作用。2月,北半球东太平洋高度正异常随高度向北倾斜,在对流层上层位于美国西南侧,该位置对应着北大西洋风暴轴入口处。进一步的相关分析表明,美国西南侧的高度正异常与地中海西北侧的瞬变波活动显著正相关,进而对应着斯堪的纳维亚半岛地区的高度负异常的形成。该负异常通过向下游频散波能量,引起贝加尔湖西侧高度正异常,形成典型的负位相斯堪的纳维亚环流型。该环流型对应60°N附近的西风异常,抑制了北侧冷空气南下,进而引起欧亚大陆地表气温正异常,反之亦然。

东北冷涡持续活动的分析研究

本文首先分析了东北冷涡的持续性活动特征，然后讨论了东亚大气１０～２０天低频振荡及瞬变扰动对东北冷涡持续活动的影响。结果表明，准双周振荡在我国东北地区十分活跃，从时间连续的低频天气图上发现，该地区附近周期性循环出现的低频气旋同东北冷涡的形成和发展关系密切，并且其传播路径也较有规律。另外，本文还通过Ｅ矢量分析以及月平均准地转位涡的收支计算，研究了东亚大气瞬变扰动对形成时间平均冷涡的影响，得出，在时间平均东亚阻高／东北冷涡偶极子系统控制的区域附近有瞬变扰动动能向时间平均气流动能转换，同时这里也存在有明显的Ｅ矢量辐合，导致平均西风减弱，而有利于经向环流的发展。时间平均气流的位涡平流不利于阻高／冷涡偶极子系统的存在，且使偶极子系统高、低压中心的经向距离拉大，而瞬变扰动的位涡输送则有利于冷涡高位涡值和阻高低位涡值在原地维持。

东亚—北太平洋大气环流和瞬变扰动的年代际变化

用NCEP/NCAR再分析资料,分析了东亚—北太平洋大气环流、大气斜压性和天气尺度瞬变扰动活动的年代际变化特征。东亚—北太平洋地区低层大气温度和比湿及高层的西风急流都存在明显的年代际变化。西风年代际异常与大气斜压性年代际异常之间有密切联系。夏季北太平洋北部到西伯利亚高原上空的天气尺度瞬变扰动活动年代际增强,但夏季低层瞬变热力和瞬变水汽强迫的年代际异常对北太平洋中纬度低层平均温度和比湿年代际异常的作用并不显著。冬季,西太平洋西风急流年代际增强与急流下方大气斜压性年代际增加相一致,使该地区瞬变扰动年代际增强,增强的瞬变扰动将更多能量传送给西风气流,巩固了西风急流的年代际异常。年代际尺度上冬季异常的瞬变热力强迫和瞬变水汽强迫对低层大气温度和比湿异常有显著的削弱作用。

中纬度瞬变涡旋活动对东亚夏季平均环流和降水的影响

首先利用CFSR再分析数据,分析了东亚夏季平均环流结构及瞬变涡旋活动特征,再通过WRF模式设计控制性试验和敏感性试验分别模拟受到/不受到来自北边界中纬度瞬变涡旋活动影响的东亚夏季环流和降水,通过两组试验对比揭示了瞬变涡旋活动对东亚夏季平均环流和降水的贡献。结果表明,中纬度瞬变涡旋活动可以通过系统性的输送动量、热量、水汽和涡度来改变背景场,从而影响夏季平均环流和降水。当中纬度瞬变涡旋活动大幅度减弱时,其产生的向极动量、热量和水汽输送也显著减弱。一方面向北热量和水汽输送的减少使东亚东部大陆上低层的平均温度和水汽增加,为降水提供了环流不稳定条件和水汽条件,另一方面瞬变涡旋的动力输送变化在环流场上形成相当正压结构响应,整个中国东部地区受强西南风控制,东亚夏季风增强并推进到更北的区域。这两个因素的共同作用使我国北部地区和华南地区产生了水汽辐合和对流增强,导致该地区降水的增强。

阿留申低压低频变化及其相关的瞬变动力学过程分析

利用NCEP/NCAR 1979—2013年的再分析资料,研究了冬季阿留申低压低频变化的环流特征,探讨了阿留申低压低频变化形成和维持的相关天气尺度瞬变强迫机制。冬季阿留申低压的低频变化在850 h Pa环流场上表现为北太平洋海盆区一个异常气旋/异常反气旋在局地强弱变化的过程,即阿留申低压在低频尺度上先异常增强/减弱随后逐渐恢复正常态的演变过程;850 h Pa上大气温度低频变化表现为低频冷中心在西北太平洋建立并逐渐东移的过程。对天气尺度瞬变扰动活动及其强迫的异常进行分析表明,北太平洋海盆区上空的瞬变动力强迫在阿留申低压异常增强的时期为负异常,有利于阿留申低压低频变化异常空间型的形成和维持。由瞬变热力强迫异常引起的温度倾向异常场表现为北太平洋中部以40°N为界南正—北负的空间分布,其南部正异常在一定程度上抑制和削弱了低频冷中心向南的扩张。

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.

夏季印度洋海温偶极子对冬季东亚高空急流的影响研究

基于1979—2019年ERA5、NCEP/NCAR再分析资料及多套海温资料,分析夏季印度洋偶极子模态对东亚高空急流的影响,并对热力动力机制展开探讨。结果表明,当夏季印度洋偶极子为正(负)位相,即夏季印度洋西部出现暖(冷)异常,东部出现冷(暖)异常时,冬季的东亚副热带急流增强(减弱),极锋急流强度减弱(增强)。伴随着印度洋偶极子位于正位相,增强(减弱)的经向温度梯度,加强(减少)的低层大气斜压性和较强(较弱)的瞬变涡动动能,易导致冬季的副热带急流增强(极锋急流减弱)。进一步利用多元线性回归方法定量区分热力和动力因子的相对贡献,夏季印度洋偶极子主要通过热力作用影响两支急流。比较两个动力因子发现,极锋急流的减弱主要以大气斜压性为主,而副热带急流的加强则以涡动动能为主。

Response of extreme precipitation to increasing extratropical cyclonic vortex frequency

Since the 2000 s,extratropical extremes have been more frequent,which are closely related to anomalies of planetary-scale and synoptic-scale systems.This study focuses on a key synoptic system,the extratropical cyclonic vortex(ECV)over land,to investigate its relations with extreme precipitation.It was found that ECVs have been more active post-2000,which has induced more extreme precipitation,and such variation is projected to persist along with increasing temperature within 1.5℃of global warming.An enhanced quasi-stationary vortex(QSV)primarily contributes to the ECV,rather than inactive synoptic-scale transient eddies(STEs).Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient.However,such conditions are helpful to widening the westerly jet belt,favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave,and the result favors an increasing frequency and persistence of QSVs,contributing to extreme precipitation.

近60年新疆降水趋势与波动机制分析

用小波多尺度分析和回归等方法研究了近60年新疆降水记录的波动与趋势特征,结果发现21世纪头十年降水亦然保持增加趋势,但春、夏降水已呈弱的非线性下降趋势,秋、冬季节降水增加趋势明显.这可能与气候暖化的季节不均匀性有关.年降水量的振荡与北大西洋涛动(NAO)关系密切,二者约80%的方差都集中在年际尺度和多年代尺度成分两个带上,年际变化之间呈显著负相关关系.NAO的多年代尺度成分位相超前降水10年左右,而大西洋海温的多年代尺度涛动与之相差5年,后者与降水的关联更为密切."极差"分析表明,对应于NAO的极端正或负位相事件,下游的降水和水汽环境明显不同.在NAO极端负位相年,中亚和新疆降水都明显增加,来自欧洲南部的水汽输送和中纬度瞬变涡动水汽输送均偏强,中亚大气可降水量上升,冷空气路径偏西,瞬变天气系统易得到西南水汽输送带的配合,水汽辐合增强,有利于新疆和中亚的降水.反之,西风带水汽输送偏弱,天气过程偏少,降水量偏小,易发生干旱.因此,NAO的异常是新疆和中亚降水年际波动的主要原因之一.