INTERRELATION BETWEEN EAST-ASIAN WINTER MONSOON AND INDIAN/PACIFIC SST WITH THE INTERDECADAL VARIATION*

Investigated statistically is the interrelation between East Asian winter monsoon (EAWM) and SST over sensitive areas of the Indian and Pacific Oceans.with focus on the relation of EAWM to strong ENSO signal area.i.e.,the equatorial eastern Pacific (EEP) SST.Evidence suggests that the EAWM variation is intimately associated not only with the EEP SST but with the equatorial western Pacific "warm pool" and equatorial Indian/northwestern Pacific Kuroshio SST as well:the EAWM and ENSO interact strongly with each other on the interannual time scales,exhibiting pronounced interdecadal variation mainly under the joint effect of the monsoon QBO and the monsoon/SST background field features on an interdecadal basis-when both fields are in the same phase(anti-phase).strong EAWM contributes to EEP SST rise(drop)in the following winter,corresponding to a warm(cold)ENSO cycle;the EAWM QBO causes ENSO cycle to be strong phase-locked with seasonal variation,making the EEP SST rise lasting from April-May to May-June of the next year,which plays an important role in maintaining a warm ENSO phase.     

The interannual variability of East Asian Winter Monsoon and its relation to the summer monsoon

1.IntroductionOvertheEastAsiaregion,themostprominentsurfacefeatureofthewintermonsoonisstrongnortheasterliesalongtheeastflankoftheSiberianhighandthecoastofEastAsia.At500hPathereisabroadtroughcenteredaboutatthelongitudesofJapan.Thedominantfea-tureat2O0hPaistheEastAsianjetwithitsmaximumlocatedatjustsoutheastofJapan.Thisktisassociatedwithintensebaroclinicity,largeverticalwindshearandstrongadvectionofcoldair(StaffmembersofAcademiaSinica,l957,LauandChang,1987;BoyleandChen,1987;Chenetal.,1991…     

冬夏东亚季风环流对太平洋热状况的响应

冬夏隔季韵律关系一直是我国长期天气预报和短期气候预测的一个重要依据,然而迄今为止对它们之间的物理过程及成因机理并不十分清楚。利用NCEP/NCAR全球2.5°×2.5°网格月平均再分析资料,研究1951~2000年冬夏东亚季风环流异常变化与太平洋海面温度(SST)的关系及对关键海温区响应机理。研究指出:冬夏东亚季风环流隔季韵律关系及其年际变化与赤道东太平洋海面温度异常(SSTA)变化密切相关,冬季赤道东太平洋出现La Ni~na(El Ni~no)型的SST分布,有利冬、夏东亚季风环流加强(减弱),其影响过程通过赤道Walker环流强(弱)以及东亚地区Hadley环流强(弱)过程完成。冬季赤道东太平洋海温变化是冬、夏东亚环流季节以及年际变化的一个重要外强迫因子。     

东亚冬季风指数的定义及其年际年代际异常

利用NCAR/NCEP再分析资料,通过相关分析方法分析了多种可以表征东亚冬季风活动的气象要素指数的相关性特征,选取地面温度、海平面海陆气压差和500hPa高度3个要素,综合定义了一个东亚冬季风指数。结果表明,该指数表征的东亚冬季风有明显的QTO和QFO周期振荡特征,这一特征与冬季风年代际背景有关,强冬季风年代背景下的振荡比弱冬季风年代背景下的振荡显著,同时该指数能很好地表征东亚冬季风活动的各个方面。对强弱冬季风年的大气环流的结构及其相应的低纬度对流活动、海洋和高低层大气热力状况的分析表明,强弱冬季风活动不仅表现在中高纬度高低层大气环流变化的差异,在局地Hadley环流和赤道附近Walker环流、低纬度对流活动、海洋和大气的热力状况的变化方面也表现出了显著的差异。     

ENSO 循环各阶段东亚夏季风特征的诊断研究

利用NCEP／NCAR再分析资料和NCAR海温资料及中国测站地温资料，对ENSO循环不同阶段东亚夏季风强弱变化进行了分析．并从此期间的海陆热力差异和季风低压变化来探讨海温异常对东亚夏季风的影响，结果表明：东亚夏季风指数有明显的年际变化和年代际变化，且与赤道东太平洋SST有较好的负相关关系，其中又以与三个月前的海温变化关系最好．在Ninol 2区为冷、暖水之后的三个月中，冷水期对应的东亚夏季风指数大于暖水期对应的东亚夏季风指数，东亚夏季风比暖水期强。赤道东太平洋SST变化期间亚洲大陆的地面温度和地面气压也有明显变化，这是引起ENSO不同阶段东亚夏季风变化的主要原因。     

关于东亚冬季风指数的一个讨论——东亚中、低纬冬季风的差异

利用NCEP再分析资料和NOAA海表温度等资料,分析了东亚冬季风在不同纬度上的表现。根据我们定义的东亚中纬度冬季风(The mid latitudinal East Asian winter monsoon, 简称EAWM-M)和低纬度冬季风(The low latitudinal East Asian winter monsoon, 简称EAWM-L)指数,探讨了它们对应的影响系统,并重点分析了它们与海温异常之间联系的异同。研究主要发现:(1)EAWM-L和EAWM-M指数所反映的东亚冬季大气环流形势不尽相同。在对流层低层,EAWM-L与中国南海、菲律宾附近环流异常的关系密切,EAWM-M与贝加尔湖阻塞高压的关系更为密切;在对流层中层,EAWM-M同样与贝加尔湖阻塞高压异常的联系相对更为紧密,而EAWM-L指数则与东亚大槽的关系更为紧密。在对流层高层,副热带西风急流强度变化通过调制次级环流进而与EAWM-L联系起来,而EAWM-M强弱变化主要与副热带西风急流北界的位置有关。(2)EAWM-L与冬季赤道中东太平洋和热带印度洋海温异常的联系都很紧密,而EAWM-M变化与冬季热带印度洋海温异常的联系更为密切,与赤道中东太平洋海温异常的关系相对偏弱。EAWM-L与冬季赤道中东太平洋和热带印度洋海温异常的紧密联系在年际和年代际尺度上都是存在的,而EAWM-M与冬季热带印度洋海温异常的紧密联系主要体现在年代际尺度上。     

An Index of East Asian Winter Monsoon Applied to the Description of China's Mainland Winter Temperature Changes

Using the NCEP/NCAR reanalysis data （Version 1.0） and the observation data of China from January 1951 to February 2007, a new index of East Asian winter monsoon circulation （I EAWM） was defined based on the comparison of previous different winter monsoon indices and circulation factors influencing the winter climate over China. Its relationships with winter temperature over China and large-scale circulation were analyzed. Results show that IEAWM can successfully describe the variation of China＇s mainland winter temperature and the East Asian winter monsoon （EAWM） system. This index reflects the integrated effect of the circulations over high and low latitudes and the thermal difference between the continent and the ocean. While in the previous studies, most monsoon indices only describe the single monsoon member. The IEAWM is a good indicator of the intensity of the EAWM. Positive values of/EAWM correspond to the strong EAWM, the stronger Siberian high and East Asian trough than normal , and the strengthening of the meridional shear of 500-hPa zonal wind between high and low latitudes over East Asia, and therefore, the southward cold advection becomes stronger and leads to the decrease in surface temperature over China; and vice versa. The IEAWM inter decadal change is obviously positive before the mid-1980s, but negative since the mid-1980s, in good agreement with the fact of the winter warming in China after 1985.     

THE CONTRASTS BETWEEN STRONG AND WEAK MJO ACTIVITY OVER THE EQUATORIAL WESTERN PACIFIC IN WINTER

This paper investigates the contrasts between strong and weak Madden-Julian Oscillation (MJO) activity over the equatorial western Pacific during winter using the NCEP reanalysis data. It is shown that the MJO over the equatorial western Pacific in winter shows significant interannual and interdecadal variabilities. During the winters with strong MJO activity, an anomalous cyclonic circulation lies east of the Philippines, strong anomalous easterlies control the equatorial eastern Pacific, and anomalous westerlies extend from the Indian Ocean to the western Pacific in the lower troposphere, which strengthens the convergence and convection over the equatorial western Pacific. The moisture convergence in the lower troposphere is also enhanced over the western Pacific, which is favorable to the activity of MJO. Eastward propagation is a significant feature of the MJO, though there is some westward propagation. The space-time spectral power and center period of the MJO are higher during strong MJO activity winters. The anomalous activity of MJO is closely related to the sea surface temperature (SST) and East Asian winter monsoon (EAWM). During strong MJO activity winters, there are positive/negative anomalies at high/low latitudes in both sea level pressure and 500 hPa geopotential height, and the temperature is lower over the central part of the Chinese mainland, which indicates a strong EAWM. China experiences more rainfall between the Yellow and Yangtze Rivers, but less rainfall south of the Yangtze River. The SSTA is negative near the Taiwan Island due to the impact of strong EAWM and shows a La Ni?a pattern anomaly over the eastern Pacific. During the weak MJO activity winters, the situation is reversed.     

Relationship Between East Asian Winter Monsoon and Summer Monsoon

Using National Centers for Environmental Prediction/National Centre for Atmospheric Research(NCEP/NCAR) reanalysis data and monthly Hadley Center sea surface temperature(SST) data,and selecting a representative East Asian winter monsoon(EAWM) index,this study investigated the relationship between EAWM and East Asian summer monsoon(EASM) using statistical analyses and numerical simulations.Some possible mechanisms regarding this relationship were also explored.Results indicate a close relationship between EAWM and EASM:a strong EAWM led to a strong EASM in the following summer,and a weak EAWM led to a weak EASM in the following summer.Anomalous EAWM has persistent impacts on the variation of SST in the tropical Indian Ocean and the South China Sea,and on the equatorial atmospheric thermal anomalies at both lower and upper levels.Through these impacts,the EAWM influences the land-sea thermal contrast in summer and the low-level atmospheric divergence and convergence over the Indo-Pacific region.It further affects the meridional monsoon circulation and other features of the EASM.Numerical simulations support the results of diagnostic analysis.The study provides useful information for predicting the EASM by analyzing the variations of preceding EAWM and tropical SST.     

一个反映中国大陆冬季气温变化的东亚冬季风指数的统计预测方法

利用中国160个站逐月温度、NCEP再分析和NOAA-CIRES20世纪再分析等资料, 采用统计分析方法, 就反映中国 东部大陆冬季一致性气温变化模态的能力方面, 对多种东亚冬季风指数进行了评估, 探讨了影响东亚冬季风强弱的主要前期因子及其相应的影响过程, 并据此建立了一个预测冬季风指数的预测模型。研究结果表明:1981 年前、后两个阶段, 朱艳峰 2008年定义的东亚冬季风指数都可以很好地反映中国东部大部分地区的冬季气温异常;北美大陆西侧北太平洋中纬度地区 (35°-50°N,145°-130°W)的前期秋季(9-10月)海温、北极喀拉海地区(75°-82°N,65°-85°E)的前秋海冰密集度和东亚中 纬度地区(30°-50°N,80°-140°E)的前秋高空(300-200hPa)温度异常都具有较强的持续性, 异常信号可从前秋一直持续到 冬季, 进而影响东亚冬季风的强度;根据上述3个前期因子建立了东亚冬季风统计预测模型, 评估发现该模型具有较强的预测 能力, 可用于冬季风强度以及相应的中国东部大陆冬季气温的定性预测。     

东亚冬季风对秋、冬季SSTA响应的数值试验

利用NCAR CCM2模式，通过设计多组数值试验方案，研究了东亚冬季风对赤道中东太平洋、中纬度太平洋秋、冬季SSTA的响应。模拟结果表明，东亚冬季风对秋、冬季太平洋SSTA均存在一定的响应，但冬季风对SSTA的响应存在显著的季节和海域差异，并表现出不同的异常响应形态。东亚冬季风对秋季SSTA的显著响应区域位于中纬度太平洋地区，该区域的SST的持续异常，可以引起欧亚、北美地区冬季大气环流的异常，并伴随有明显的EU型和PNA型波列特征，秋季中纬度太平洋SST持续异常偏冷（暖），能够引起有利于强（弱）东亚冬季风的环流异常。而冬季风对冬季SSTA的响应则为赤道中东太平洋和中纬度太平洋SSTA共同作用的结果，在两个区域SSTA的同时强迫下，大气环流表现出类似PNA及WP型波列分布的异常响应特征，赤道中东太平洋的正（负）的SSTA及中纬度太平洋负（正）的SSTA，将导致弱（强）冬季风的发生。     

冬季西太平洋和海洋性大陆热源变化特征及其影响

利用ERA-interim再分析资料、中国地面降水资料、全球海温(SST)和降水资料研究了冬季西太平洋和海洋性大陆大气热源变化特征及其对区域气候的影响。结果表明:冬季西太平洋和海洋性大陆热源具有显著的年际变化,海洋性大陆热源同时存在明显的年代际变化。冬季西太平洋热源强年相较弱年,东亚副热带急流轴线偏北3~4 °,东亚冬季风显著加强;热源强度与中国东部大部分地区冬春季降水呈显著负相关。由于西太平洋热源与ENSO事件密切相关,去除ENSO影响后,西太平洋热源对急流和冬季风的影响明显减弱,且与华东南部和华南地区降水无显著相关,但仍与华东中北部和华北地区呈显著负相关,相关关系仍可从同期冬季持续到后期春季。海洋性大陆热源在1993年左右发生显著突变,突变后海洋性大陆附近SST显著升高,海平面气压显著降低。进一步分析表明,海洋性大陆地区大气热源与Walker环流在年代际尺度上存在很好的对应关系,海洋性大陆岛屿整体增暖趋势快于周围海洋,导致海陆热力差异增大,这可能是触发局地热源和Walker环流长期变化的主要原因,进而影响太平洋SST分布和全球增暖的进程。      

近百年东亚冬季风与ENSO循环的相互关系及其年代际异常

运用相关及滑动相关的计算技术，讨论了近百年东亚冬季风与ENSO循环的相互关系及其年代际异常。研究指出，东亚冬季风与赤道东太平洋海温的年际关系具有年代际的变化特征；季风与ENSO循环的关系受到季风的QBO以及季风-海洋的年代际背景场配置关系的共同作用；当季风与海洋的背景场处于同样状态时，强冬季风有利于第二年冬季赤道东太平洋的升温，产生El Ni?o事件；当两者的背景场处于反位相状态时，强冬季风对应于第二年冬季的La Ni?a位相。     

How Well do Existing Indices Measure the Strength of the East Asian Winter Monsoon？

Defining the intensity of the East Asian winter monsoon (EAWM) with a simple index has been a difficult task. This paper elaborates on the meanings of 18 existing EAWM strength indices and classifies them into four categories: low level wind indices, upper zonal wind shear indices, east-west pressure contrast indices, and East Asian trough indices. The temporal/spatial performance and prediction potential of these indices are then analyzed for the 1957--2001 period. It reveals that on the decadal timescale, most indices except the east--west pressure contrast indices can well capture the continuous weakening of the EAWM around 1986. On the interannual timescale, the low level wind indices and East Asian trough indices have the best predictability based on knowledge of the El Nino-Southern Oscillation and Arctic Oscillation, respectively. All the 18 existing indices can well describe the EAWM-related circulation, precipitation, and lower tropospheric air temperature anomalies. However, the variations of surface air temperature over large areas of central China cannot be well captured by most indices, which is possibly related to topographic effects. The results of this study may provide a possible reference for future studies of the EAWM.     

THE RELATIONSHIP BETWEEN SCS SUMMER MONSOON INTENSITY AND OCEANIC THERMODYNAMIC VARIABLES AT DIFFERENT TIME SCALE

The oscillation characteristics of 1948 - 2003 South China Sea (SCS) summer monsoon intensity (SCSSMI) is analyzed by wavelet transform and the relationship between SCSSMI filtered by Lanczos filter at different time scale and oceanic thermal conditions is studied. The results show that SCSSMI exhibits dominant interannual (about 4 a), decadal (about 9 a) and interdecadal (about 38 a) oscillation periods. The interannual variation is the strongest and the interdecadal variation the weakest. The region of significant correlation between SCS summer monsoon intensity and oceanic thermodynamic variables at different time scale is greatly different. Significant correlation area of interannual variation of SCSSMI is concentrated in near equatorial region. Corresponding correlation displays quasi-biannual variability. If positive anomalies of SST and the depth of thermocline happen in eastern equatorial Indian Ocean and western equatorial Pacific, and negative anomalies of SST and the depth of thermocline happen in western equatorial Indian Ocean and eastern equatorial Pacific in previous autumn and winter, the interannual variation of SCSSMI will enhance. If the condition is contrary, interannual variation of SCSSMI will weaken. The interannual variation of SCSSMI will influence SST. The region surrounding SCS and east of Australia shows significantly negative correlation in autumn, and significantly positive correlation exhibits in west equatorial Indian Ocean, eastern equatorial Pacific and equatorial Atlantic in winter. The decadal variation of SCSSMI is modulated by PDO. Interdecadal variation of SCSSMI is relevant to the global warming and PDO.     

东亚冬季风强度的统计预测方法研究

利用1961～2008年NCEP再分析和NOAA延长重构的月平均海温资料, 基于海气系统关键区的前期信号分析, 建立了一个东亚冬季风强度的统计预测方法。东亚冬季风强度与前期 (9～10月) 黑潮及其延伸区和热带西印度洋海温异常 (SSTA) 密切相关。强东亚冬季风活动与黑潮及其延伸区正SSTA和热带西印度洋负SSTA相对应。东亚冬季风强度还和一个前期 (10月) 北半球环流型存在显著相关, 其中环流型的活动中心分别位于北太平洋中部、 太平洋东北部、 北美和北大西洋。文中探讨了这三个预测因子对东亚冬季风强度的预测意义, 并揭示了其影响东亚冬季风活动的可能物理过程。该预测方法的历史拟合率和试报准确率较高, 可用于东亚冬季风强度的定性预测。     

我国东北地区冬季气温变化的东亚冬季风背景

利用中国气象局国家气象信息中心1961—2011年我国东北地区72个气象站月平均气温资料及NCEP/NCAR月平均海平面气压、500 hPa高度场及200 hPa与850 hPa风场再分析资料,对东亚冬季风强度与我国东北地区冬季气温序列经去除线性趋势处理后的变化特征进行对比分析。结果表明:去除线性趋势后,东亚冬季风强度与我国东北地区冬季气温序列的相关系数为-0.69,较原始序列更为显著;两者变化的阶段性较为同步,我国东北地区冬季气温于2004年已转入低温阶段,这与东亚冬季风同时转为偏强阶段关系密切;两者均存在20年左右的长周期,同样存在相近的阶段性短周期;我国东北地区冬季气温的增温变化趋势在1986年前后的增暖性气候突变中起重要作用。东亚冬季风强度与我国东北地区冬季气温年代际信号的相关系数达-0.86,较原始序列年代际相关更为显著;两者的年代际变化存在21.5年左右的共同准周期。东亚冬季风强度与我国东北地区冬季气温的年际变化序列存在4年左右的共同准周期。我国东北地区冬季气温的年际和年代际异常存在与东亚冬季风相关联的200 hPa东亚急流、500 hPa东亚大槽、乌拉尔高压、850 hPa风场、地面西伯利亚高压等的异常背景。     

Interannual to decadal variability of the winter Aleutian Low intensity during 1900–2004

A new winter Aleutian Low (AL) intensity index was defined in this paper. A centurial-long time series of this index was constructed using the sea level pressure (SLP) data of nearly 100 years. The features of interannual and decadal variability of the winter AL intensity since 1900 were analyzed by applying the wavelet analysis. The relationship between the winter AL intensity and atmospheric circulation was examined. The cross-wavelet analysis technique was used to further reveal the relationship between the AL intensity and sea surface temperature (SST) in the equatorial eastern Pacific (EEP) and tropical Indian Ocean (TIO) in winter. The results indicate that: 1) On the interannual timescale, the winter AL intensity displays 3–7-yr oscillations, while on the decadal timescale, 8–10-yr and 16–22-yr oscillations are more obvious. 2) Of the linkage to atmospheric circulation, both AO (Arctic Oscillation) and PNA (Pacific North America pattern) are closely associated with winter AL intensity on the interannual timescale, but only PNA contributes to the variation of winter AL intensity on the decadal timescale. 3) As to the ocean impact, winter EEP SST is a major factor affecting the winter AL intensity on the interannual timescale, especially on the 3–7-yr periods. However, on the decadal timescale, though both the TIO and EEP SSTs are associated with the AL intensity in winter, the TIO SST impact is more significant.     

太平洋海气相互作用的时空变化

本文根据1957—1976年赤道太平洋海温和北太平洋海平面气压的月平均资料,计算了它们之间全年(1—12月)逐月的时滞相关,分析了北太平洋副热带反气旋影响赤道海温和赤道海温对副高反馈的季节变化。发现它们之间的联系不同季节、不同地区有明显差异:副热带反气旋对赤道海温的影响(负相关)以春季最大,秋季最小;赤道海温的反馈,对副高的不同部分作用不同,对副高主体的作用(正相关)以冬半年最大、夏半年较小(尤其是盛夏),对西部副高脊的作用(负相关)相反,以夏半年最大,冬半年较小。其过渡期为5月和11月。同时对其季节变化的可能原因也提出了一些初步看法。其中特别强调了大型环流背景的基本状态(包括平均垂直环流)对海气相互作用过程的重要性。     

Analysis of the decadal and interdecadal variations of the east asian winter monsoon as simulated by 20 coupled models in IPCC AR4

Using the output data of 20 coupled climate models used in IPCC AR4 and observational data from NCEP, the capability of the models to simulate the boreal winter climatology of the East Asian sea level pressure, 850-hPa wind, and surface air temperature; the decadal variations of the East Asian winter monsoon (EAWM) intensity and EAWM-related circulation, and the interdecadal variations of EAWM-related circulation are systematically evaluated. The results indicate that 16 models can weakly simulate the declining trend of the EAWM in the 1980s. More than half of the models produce relatively reasonable decadal variations of the EAWM-related circulation and the interdecadal differences of EAWM-related circulation between the boreal winters of 1960-1985 and 1986-1998, including the weakened Siberian high, Aleutian low, and East Asian trough, the enhanced Arctic oscillation and North Pacific oscillation, and a deepened polar vortex. It is found that the performance of the multi-selected-model ensemble in reproducing the spatial distribution of the variations is encouraging, although the variational amplitudes are generally smaller than the observations. In addition, it is found that BCCR-BCM2.0, CGCM3.1-T63, CNRM-CM3, CSIRO-MK3.0, GISS-ER, INM-CM3.0, and MRI-CGCM2.3.2 perform well in every aspect.