阿留申低压与东亚冬季风关系的年代际变化

东亚冬季风系统是北半球冬季最活跃的环流系统,通过与西伯利亚高压、阿留申低压、东亚大槽和东亚西风急流等系统之间的相互作用,对东亚地区的气候产生重要影响。本文主要关注阿留申低压与东亚冬季风关系的年代际变化特征,利用经验正交函数分析方法,提取阿留申低压的强度变化、南北移动、西北-东南传播型和东西移动模态,并研究这4种模态与冬季风的北部型、南部型模态关系。结果表明阿留申低压的强度变化与2种东亚冬季风型的关系都不显著。阿留申低压的南北移动与南部型冬季风的相关性较强且具有年代际变化特征。1995年后南北涛动中心受ENSO(El Niño-Southern Oscillation)影响向东移动,阿留申低压和西伯利亚高压的配置不利于中国东南部气压梯度的形成,导致阿留申低压的南北移动与南部型冬季风的相关性降低。另外,1975年后北大西洋涛动在中西伯利亚地区激发正位势高度异常,扩大传播型模态影响范围,给中高纬地区带来东北风异常,有利于北部型冬季风的形成,此时传播型模态与北部型冬季风显著相关。     

多模式预报系统对东亚冬季风预测性能的评估

基于哥白尼气候变化服务中心（C3S）提供的五个最先进的季节预报系统输出的1993~2016年回报数据,结合ERA5再分析资料和GPCP降水资料,对其预测东亚冬季风的性能进行评估。结果表明:C3S多模式预报系统能很好地预测东亚冬季风气候态的主要特征,包括西伯利亚高压、阿留申低压、东亚大槽、东亚高空急流及东亚地表气温和降水;SEAS5、GloSea5、MF-Sys7、GCFS2等多个模式均对东亚冬季风指数显示出了预测技巧,同时可以很好地预测与东亚冬季风相关的区域大气环流、地表气温及降水异常;SPSv3模式表现出与观测相反且位置偏西的大气环流、地表气温及降水异常,使得该模式对东亚冬季风指数表现出负技巧。      

2018/2019年冬季浙江罕见持续阴雨天气的环流异常与海温强迫

利用NCEP/NCAR全球再分析资料、地面观测资料和自动站降水资料,分析了2018/2019年冬季浙江罕见连续阴雨寡照天气过程中冬季风环流和南支槽等环流异常,并从西风带波动、海温强迫等方面研究了局地环流异常的成因。结果表明:2018/2019年冬季连阴雨事件中雨日、日照破历史记录,雨量较常年同期明显偏多。主要的环流异常为西北太平洋异常反气旋（WNPAC）明显偏北,同时阿留申低压和西伯利亚高压亦偏北,东亚地区40°N以南有强的偏南风异常,冬季风偏弱;南支槽较常年偏强,保证了浙江上空有持续的水汽和扰动输送。对流层中层存在沿欧洲向东亚—西太平洋传播的波动能量,波能在东亚地区一直向南传播至20°N以南,可能导致WNPAC明显北抬和南支槽加强。ENSO是WNPAC的重要强迫源,ENSO暖位相使得海洋性大陆出现异常对流冷却,而浙江上空对流加强,ENSO对南支槽活动强度亦有明显的制约作用。中国近海海温偏高是WNPAC和阿留申低压明显偏北的重要影响因素。2018/2019年冬季局地环流异常可能由ENSO和中国近海海温协同强迫所致。     

GRAPES_GFS在东亚地区中期预报技巧极端下降现象的初步研究

利用2016年4月1日—2018年12月31日GRAPES_GFS模式的业务预报数据,将GRAPES_GFS模式在东亚地区144小时预报500 hPa高度场的距平相关系数小于0.4,均方根误差大于60 gpm的事件定义为模式在东亚地区的中期预报技巧极端下降事件,据此标准筛选出29个个例进行了研究。结果表明,GRAPES_GFS在东亚地区中期预报技巧极端下降事件的高发期主要在春秋季,春季和秋季分别占比31.03%、41.38%,预报技巧极端下降事件通常表现在对于东亚大槽、西伯利亚槽脊、副热带高压的预报失败,四个季节合成的模式偏差均与该季节影响东亚地区主要天气系统的预报偏差大有关系。进一步通过典型个例的研究表明,东亚地区中期预报技巧极端下降事件的误差来源在不同季节表现出不同特征。冬夏季的预报偏差来源于高纬极区,多与模式在极区存在较大预报误差关联;春秋季的预报偏差主要来源于上游地区,与模式在东亚上游预报误差向下游的传播有关,未能合理预报台风活动也是预报偏差来源之一。      

对流层上层副热带西风急流与东亚冬季风的关系

利用NCEP/NCAR月平均再分析资料,研究冬季对流层上层西风急流的时空变化特征,提出表征急流强度和位置变化的指数,进而探讨西风急流与东亚冬季风的关系。结果表明:冬季西风急流强度指数体现了西太平洋与高纬大陆的热力对比,较好地反映了西伯利亚高压与阿留申低压的强度变化,可作为表征冬季风强弱变化的一个定量指标,急流增强(减弱)对应西伯利亚冷高压和阿留申低压加强(减弱),东亚冬季风偏强(弱)。急流强度指数与不同高度冬季风子系统的显著相关表明,东亚冬季风活动异常不只是对流层中低层的现象,而在整个对流层都有明显反映,低层的西伯利亚高压和阿留申低压、中层的欧亚脊、东亚大槽及西太平洋副热带高压与高层的西风急流是同相变化的。在此基础上还比较了急流强度指数和北极涛动指数(AO)与东亚冬季风的关系,急流强度变化体现了欧亚大陆与西太平洋的热力差异,而AO则主要反映极地与中纬度环状模的反相变化,所以急流强度变化与东亚冬季风的关系更为密切。     

ENSO与北半球冬季大气环流异常年代际关系的数值模拟

利用NCAR/CCM3大气环流模式,模拟不同海温背景场强迫下,北半球冬季大气环流对ENSO事件的响应。结果表明,模式能够模拟出不同海温背景场下,北半球冬季大气环流对ENSO事件的不同响应,模拟结果与诊断结果基本一致。在20世纪70年代末之后的平均海温背景场强迫下,El Nio对阿留申低压的加强作用比70年代末之前更明显,而ENSO暖位相对蒙古高压的作用则由70年代末之前的轻微减弱变为明显加强。虽然70年代末以后ENSO事件的强度比70年代末之前有所增大,但ENSO对东亚大槽和东亚冬季风的减弱作用反而不再那么明显;同时在高空风场中,由ENSO激发出的PNA型异常环流在70年代末以后明显加强。     

亚洲和南半球大气冷(热)源对亚洲冬季风影响的数值试验

利用NCAR CAM3.1模式及NCEP/NCAR(version 1)再分析资料计算了几种现实大气热源分布情况,讨论了亚洲各地区和南半球上空冬季1月大气冷(热)源对东亚冬季风环流系统和印度冬季风环流系统形成的影响.结果表明:(1)冬季1月东亚地区和澳大利亚上空大气冷(热)源与东亚冬季风环流关系密切,南半球澳大利亚附近的非绝热加热可以激发出澳大利亚北部的热低压系统,东亚大陆东部的大气冷源可以使东亚大陆低空出现冷高压,基本上模拟出东亚季风系统冬季主要环流成员;(2)亚洲地区西部及其对应的南半球印度洋非绝热加热与印度冬季风环流关系密切,同样对东亚冬季风也有一定的影响,特别是亚洲大陆西部副热带地区的非绝热加热可以加强冬季南海的越赤道气流并能调整阿留申低压的位置.     

贵州冬季冻雨的大尺度环流特征及海温异常的影响

利用1981-2013年贵州冬季冻雨资料、NCEP/NCAR再分析资料和NOAA海温资料等,分析了影响贵州冬季冻雨日数的大尺度环流系统,讨论了海温异常对冬季冻雨日数的可能影响。基于赤道中东太平洋海温异常建立冬季冻雨日数的预测模型,并检验了模型的预测能力。研究表明:贵州冬季冻雨日数偏多(少)年,大气环流异常呈现出西伯利亚高压偏强(弱)、东亚地区海陆气压差偏大(小)的强(弱)东亚冬季风环流特征,同期印缅槽偏强(弱)、东亚副热带急流偏强(弱)。贵州冬季冻雨日数多寡与ENSO事件存在密切联系,秋、冬季赤道中东太平洋冷(暖)海温发展有利于冬季冻雨日数偏多(少)。前期秋季赤道太平洋Ni?o区海温异常是显著影响贵州冬季冻雨日数的年际预报信号,对贵州冬季冻雨日数的多寡具有较好的预测指示意义。     

东亚冬季风气候变异和机理以及平流层过程的影响

本文综述了近几年来关于东亚冬季风变异特征和机理方面的研究,特别对平流层过程对东亚冬季风和气候异常的可能影响作了回顾和进一步分析.东亚冬季风的变异除了季风强弱变化外,还有东亚冬季风的路径变化;研究表明,前者往往对应全国气温一致的变化,而后者可以引起我国气温的南北反相振荡,并导致东亚冬季风变异存在南北两个子系统.此外,进入本世纪后,东亚冬季风的建立推迟,并且东亚冬季风在盛期明显减弱,但冬季风活动在早春比以往要更为活跃,这些变化与冬季气温南北反相变化也有密切的联系.进一步的分析揭示出东亚气温的南北反相变化是东亚冬季风变异的主要模态之一,而且它与平流层极涡强度密切相关.当异常的平流层极涡向下传播时,可以引起对流层低层北极涛动(AO)的异常以及西伯利亚高压的异常,并在东亚地区出现南北反相的温度变化.有关东亚冬季风变异的成因研究表明,上世纪70年代中后期以后,热带厄尔尼诺—南方涛动(ENSO)的影响变弱,而中高纬的北太平洋涛动(NPO)和乌拉尔地区阻塞强度的影响显著增强,相关研究还揭示了这些变化的原因.此外,东亚冬季风在1987年以后的持续减弱主要与准定常行星波活动年代际变化有关,行星波活动通过波流相互作用可以影响AO以及西伯利亚高压和阿留申低压,从而导致冬季风异常.最后,本文还讨论了太阳活动11年周期变化对东亚冬季气候异常的可能影响和过程.     

BP-CCA方法用于中国冬季温度和降水的可预报性研究和降尺度季节预测

使用变形的典型相关分析(BP-CCA)方法,基于交叉检验的结果建立了东亚冬季500 hPa大尺度环流和中国冬季温度、降水的最优BP-CCA降尺度预测模型,并在此基础上进行了中国冬季温度和降水的可预报性研究,表明用东亚冬季500hPa高度场降维后的大尺度环流来解释中国冬季温度,平均距平相关系数(ACC)为0.7左右,最高可达0.9,解释中国冬季降水的平均ACC为0.3左右,最高可达0.7,温度的可预报性远高于降水,且二者的可预报水平存在明显的区域差异。可预报性研究表明东亚冬季500 hPa大尺度环流异常与中国冬季温度、降水异常有密切的联系,BP-CCA方法可以很好地揭示大尺度环流与温度、降水的内在联系,并且物理意义清晰。在东亚大尺度环流系统中,东亚大槽和西太平洋副热带高压是影响中国冬季温度、降水异常的重要系统。进一步利用国家气候中心海气耦合模式(CGCM/NCC)回报和预测的500 hPa环流场和BP-CCA方法对温度和降水进行降尺度预测应用,对温度和降水的预测效果明显高于模式直接输出的结果,而且对温度预测的改善高于对降水的改善。对模式预测的环流进行EOF(经验正交函数)分析,表明BP-CCA方法对降尺度要素预报的可预报性来源于CGCM/NCC对500 hPa高度场主要大尺度特征的模拟能力较好。     

Potential impacts of East Asian winter monsoon on climate variability and predictability in the Australian summer monsoon region

This study explores potential impacts of the East Asian winter monsoon (EAWM) on summer climate variability and predictability in the Australia–Asian region through Australia–Asia (A-A) monsoon interactions. Observational analysis is conducted for the period of 1959 to 2001 using ERA-40 wind reanalysis and Climate Research Unit rainfall and surface temperature monthly datasets. Statistically significant correlations are established between the Australian summer monsoon and its rainfall variations with cross-equatorial flows penetrating from South China Sea region and northerly flow in the EAWM. The underlying mechanism for such connections is the response of the position and intensity of Hardley circulation to strong/weak EAWM. A strong EAWM is associated with an enhanced cross-equatorial flow crossing the maritime continent and a strengthened Australia summer monsoon westerlies which affect rainfall and temperature variations in northern and eastern part of the Australian continent. Furthermore, partial correlation analysis, which largely excludes El Niño-Southern Oscillation (ENSO) effects, suggests that these connections are the inherent features in the monsoon system. This is further supported by analyzing a global model experiment using persistent sea surface temperatures (SSTs) which, without any SST interannual variations, shows similar patterns as in the observational analysis. Furthermore, such interaction could potentially affect climate predictability in the region, as shown by some statistically significant lag correlations at monthly time scale. Such results are attributed to the impacts of EAWM on regional SST variations and its linkage to surface conditions in the Eurasian continent. Finally, such impacts under global warmed climate are discussed by analyzing ten IPCC AR4 models and results suggest they still exist in the warmed climate even though the EAWM tends to be weaker.     

Predictability of the East Asian winter monsoon indices by the coupled models of ENSEMBLES

The seasonal predictability of various East Asian winter monsoon （EAWM） indices was investigated in this study based on the retrospective forecasts of the five state-of-the-art coupled models from ENSEMBLES for a 46-year period of 19612006.It was found that the ENSEMBLES models predict five out of the 21 EAWM indices well,with temporal correlation coefficients ranging from 0.54 to 0.61.These five indices are defined by the averaged lower-tropospheric winds over the low latitudes （south of 30°N）.Further analyses indicated that the predictability of these five indices originates from their intimate relationship with ENSO.A cross-validated prediction,which took the preceding （November） observed Nifo3.4 index as a predictor,gives a prediction skill almost identical to that shown by the model.On the other hand,the models present rather low predictability for the other indices and for surface air temperature in East Asia.In addition,the models fail to reproduce the relationship between the indices of different categories,implying that they cannot capture the tropicalextratropical interaction related to EAWM variability.Together,these results suggest that reliable prediction of the EAWM indices and East Asian air temperature remains a challenge.     

Prediction of primary climate variability modes at the Beijing Climate Center

Climate variability modes, usually known as primary climate phenomena, are well recognized as the most important predictability sources in subseasonal–interannual climate prediction. This paper begins by reviewing the research and development carried out, and the recent progress made, at the Beijing Climate Center (BCC) in predicting some primary climate variability modes. These include the El Niño–Southern Oscillation (ENSO), Madden–Julian Oscillation (MJO), and Arctic Oscillation (AO), on global scales, as well as the sea surface temperature (SST) modes in the Indian Ocean and North Atlantic, western Pacific subtropical high (WPSH), and the East Asian winter and summer monsoons (EAWM and EASM, respectively), on regional scales. Based on its latest climate and statistical models, the BCC has established a climate phenomenon prediction system (CPPS) and completed a hindcast experiment for the period 1991–2014. The performance of the CPPS in predicting such climate variability modes is systematically evaluated. The results show that skillful predictions have been made for ENSO, MJO, the Indian Ocean basin mode, the WPSH, and partly for the EASM, whereas less skillful predictions were made for the Indian Ocean Dipole (IOD) and North Atlantic SST Tripole, and no clear skill at all for the AO, subtropical IOD, and EAWM. Improvements in the prediction of these climate variability modes with low skill need to be achieved by improving the BCC’s climate models, developing physically based statistical models as well as correction methods for model predictions. Some of the monitoring/prediction products of the BCC-CPPS are also introduced in this paper.     

不同类型ENSO对东亚季风的影响和机理研究进展

近十几年来有关热带太平洋存在不同类型的增暖型的研究成为热点课题,其中主要依据海温的空间形态将厄尔尼诺与南方涛动（ENSO）分为两类:一类为传统的东太平洋型ENSO,另一类为中太平洋型ENSO。该两类ENSO的形成机制、演变过程均有不同,产生的气候影响也有差异。本文简要回顾了国内外有关不同类型ENSO及其气候影响的研究进展,特别综述了近年来关于两类ENSO事件对东亚夏季风、东亚冬季风以及东亚冬夏季风关联的影响和机理方面的主要研究进展。文中侧重讨论了年际和年代际时间尺度上ENSO事件对东亚季风的影响,并提出了今后在该领域一些需要进一步研究的科学问题。     

东亚季风近几十年来的主要变化特征

本文简要综述了关于东亚夏季风和冬季风近几十年来的主要变化特征的若干研究结果,特别是关于其年代际变化方面.夏季风及夏季气候的主要变化特征有:1970年代末之后东亚夏季风的年代际时间尺度的减弱以及相应的我国夏季降水江淮流域增多而华北减少、1992年之后我国华南夏季降水增多、1999年之后我国长江中下游夏季降水减少而淮河流域夏季降水增多、东亚夏季风和ENSO之间的年际变化相关性存在不稳定性.而关于东亚冬季风与冬季气候的主要变化特征有:1980年代中期之后东亚冬季风及其年际变率减弱、1970年代中期之后冬季风和ENSO的年际变化相关性较弱、近年来的北极秋季海冰减少对北半球冬季积雪增多有显著贡献、东北冬季积雪在1980年代中期以后增多.与上述变化有关的极端气候和物候都发生了多方面的变化.     

TBO的原因－异常东亚冬季风与ENSO循环的相互作用

基于对 ＮＣＥＰ／ ＮＣＡＲ再分析资料以及其他资料（ＯＬＲ，降水和气温等）的分析研究，结果表明东亚和西北太平洋地区的对流层环流和气候变化都有明显的准两年振荡（ＴＢＯ）特征。同时，异常东亚冬季风可以影响次年夏季的大气环流和气候变化，特别是在东亚地区；而异常东亚冬季风和ＥＮＳＯ循环间又有明显相互作用：持续的强（弱）东亚冬季风通过海─气相互作用可以激发 Ｅｌ Ｎｉ　ｏ（Ｌａ Ｎｉ　ａ）， Ｅｌ Ｎｉ　ｏ（Ｌａ Ｎｉ　ａ）反过来又可通过遥相关或遥响应而导致东亚冬季风偏弱（强）。强或弱的冬季风和ＥＮＳＯ循环是相互衔接在一起的，因此可以认为异常东亚冬季风与ＥＮＳＯ循环的相互作用是ＴＢＯ对流层准两年振荡）的基本原因。     

Multi-timescale variation of East Asian winter monsoon intensity and its relation with sea surface temperature during last millennium based on ECHO-G simulation

Based on the simulation results derived from ECHO-G global coupled climate model, several East Asian winter monsoon (EAWM) indices are compared in order to choose the most suitable one for signaling the intensity of winter monsoon in the last millennium. The index I_shi, which is defined with normalized sea level pressure difference between sea and land in mid and low latitudes, is selected to describe the winter monsoon intensity variation owing to its better capability for reflecting the variation of winter monsoon subsystems, such as the continental high pressure, Aleutian low, East Asian major trough, westerly jet stream, and surface air temperature than the other indices examined. Wavelet analysis on index I_shi shows that the EAWM intensity is characterized by multi-timescale variation with inter-annual, decadal, inter-decadal and inter-centennial oscillations on the background of a slight descending trend. Correlation analysis between the EAWM index and sea surface temperature (SST) at various timescales reveals that the SST in mid-latitudes might provide the background of the EAWM strength changes above decadal timescales, and a negative-feedback process lasting for about two years is found between the EAWM intensity and the SST in the eastern equatorial Pacific. According to the correlation, the El Nino occurrence in the second-half of the year leads to weaker EAWM than normal in the following winter and the weakened EAWM corresponds to lower SST in eastern equatorial Pacific after about half a year, which will then strengthen the EAWM intensity in the next winter. It is a stable feedback process and its mechanism is discussed.     

BCC_CSM模式夏季关键区海温回报评估

利用国家气候中心气候系统模式(Beijing Climate Center Climate System Model, BCC_CSM)的汛期回报试验数据集, 评估了夏季中低纬度海表面温度(Sea Surface Temperature, SST)的预测能力。结果表明:该模式对夏季中低纬海温具有一定的预测能力, 且在低纬地区的预测技巧尤为出色。对太平洋、热带印度洋和北大西洋这三个关键区进一步分析发现, 该模式对不同海区海温的预测能力有所不同。其中, 模式对夏季北太平洋海温及Ni?o 3.4指数表现出显著的预测技巧, 对热带印度洋、北大西洋海温及热带印度洋全区一致海温模态(Indian Ocean Basin-wide Warming, IOBW)也表现出一定的预测技巧, 而对北大西洋海温三极子模态(North Atlantic Tripole, NAT)的技巧相对较低。研究发现, 预测技巧与前冬的ENSO状态密切相关, 当前冬位于ENSO异常位相时, BCC_CSM模式对于三大海区夏季海温的预测技巧要高于前冬位于ENSO正常位相时, 且对NAT指数也具有更高的预测技巧。前冬ENSO所处的位相对于该模式对夏季Ni?o 3.4指数及IOBW指数的预测技巧影响不明显。此外, 该模式对夏季海温的预测技巧依赖于超前时间, 预测技巧在大部分情形下超前1个月的预测技巧相对更高。     

The asymmetric effects of El Niño and La Niña on the East Asian winter monsoon and their simulation by CMIP5 atmospheric models

El Niño–Southern Oscillation (ENSO) events significantly affect the year-by-year variations of the East Asian winter monsoon (EAWM). However, the effect of La Niña events on the EAWM is not a mirror image of that of El Niño events. Although the EAWM becomes generally weaker during El Niño events and stronger during La Niña winters, the enhanced precipitation over the southeastern China and warmer surface air temperature along the East Asian coastline during El Niño years are more significant. These asymmetric effects are caused by the asymmetric longitudinal positions of the western North Pacific (WNP) anticyclone during El Niño events and the WNP cyclone during La Niña events; specifically, the center of the WNP cyclone during La Niña events is westward-shifted relative to its El Niño counterpart. This central-position shift results from the longitudinal shift of remote El Niño and La Niña anomalous heating, and asymmetry in the amplitude of local sea surface temperature anomalies over the WNP. However, such asymmetric effects of ENSO on the EAWM are barely reproduced by the atmospheric models of Phase 5 of the Coupled Model Intercomparison Project (CMIP5), although the spatial patterns of anomalous circulations are reasonably reproduced. The major limitation of the CMIP5 models is an overestimation of the anomalous WNP anticyclone/cyclone, which leads to stronger EAWM rainfall responses. The overestimated latent heat flux anomalies near the South China Sea and the northern WNP might be a key factor behind the overestimated anomalous circulations.     

Combined effect of El Niño-Southern Oscillation and Pacific Decadal Oscillation on the East Asian winter monsoon

Using long-term observational data and numerical model experiments, the combined effect of the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the variability of the East Asian winter monsoon is examined. In the observations, it is found that when the ENSO and PDO are in-phase combinations (i.e., El Niño/positive PDO phase and La Niña/negative PDO phase), a negative relationship between ENSO and East Asian winter monsoon is significantly intensified. In other words, when El Niño (La Niña) occurs with positive (negative) PDO phase, anomalous warm (cold) temperatures are dominant over the East Asian winter continent. On the other hand, there are no significant temperature anomalies when the ENSO and PDO are out-of-phase combinations (i.e., El Niño/negative PDO phase and La Niña/positive PDO phase). Further analyses indicate that the anticyclone over the western North Pacific including the East Asian marginal seas plays an essential role in modulating the intensity of the East Asian winter monsoon under the changes of ENSO–PDO phase relationship. Long-lasting high pressure and warm sea surface temperature anomalies during the late fall/winter and following spring over the western North Pacific, which appear as the El Niño occurs with positive PDO phase, can lead to a weakened East Asian winter monsoon by transporting warm and wet conditions into the East Asian continent through the southerly wind anomalies along the western flank of the anomalous high pressure, and vice versa as the La Niña occurs with negative PDO phase. In contrast, the anomalous high pressure over the western North Pacific does not show a prominent change under the out-of-phase combinations of ENSO and PDO. Numerical model experiments confirm the observational results, accompanying dominant warm temperature anomalies over East Asia via strong anticyclonic circulation anomalies near the Philippine Sea as the El Niño occurs with positive PDO phase, whereas such warming is weakened as the El Niño occurs with negative PDO phase. This result supports the argument that the changes in the East Asian winter monsoon intensity with ENSO are largely affected by the strength of the anticyclone over the western North Pacific, which significantly changes according to the ENSO–PDO phase relationship.