ENSO对中国西北地区秋季异常降水的影响

以奇异值分解（SVD）方法为基础,用1960～1994年35年的资料,分析了ENSO对中国西北地区秋季降水异常的影响,然后用奇异向量的海温场时间系数与500hPa高度场和海平面气压场进行交叉相关,讨论了在ENSO年引起西北地区降水异常的环流特征。结果表明,赤道中东太平洋海表温度异常与西北秋季大范围的区域性降水异常有较好的对应关系,在ElNino年,西北地区秋季大部分地区降水异常偏少,其中青藏高原东北侧的青海东部、甘肃中东部、宁夏南部和陕西北部等西北主要的雨养农业区降水明显偏少。引起这种异常的环流特征表现为500hPa高度场出现PNA异常流型,强异常中心多分布在低纬度,印缅槽偏弱,西太平洋副高加强西伸,脊线南移,南海副高和北美副高加强;东亚中纬度冬季盛行的1波流型在秋季异常发展,新疆脊偏强,东亚大槽加深西移。海平面气压场上,乌拉尔山北部高压、欧洲南部低压及阿留申低压等较高纬度系统异常加强（加深）。LaNino年情况相反。     

Seasonality of the Interaction between Convection over the Western Pacific and General Circulation in the Northern Hemisphere

The seasonality of the interaction between convection over the western Pacific and general circulation in the Northern Hemisphere (NH) is analyzed in the present paper with singular value decomposition (SVD) and empirical orthogonal function (EOF) analysis approaches, based on 500 hPa monthly mean geopotential height data and high-cloud amount data. The analyses demonstrate that coupled dominant patterns in the interaction between the convection over the western Pacific and the general circulation in NH are different in various seasons. In spring, the convection over the western Pacific is closely related with the western Atlantic (WA) and North Pacific (NP) like patterns of the general circulation in NH, and some associations between the WA and NP like pat?terns and the El Ni?o / Southern Oscillation (ENSO) cycle are also existed. The Pacific Japan (PJ) pattern is the dom?inant pattern in the interaction between the interannual variabilities of the convection over the western Pacific and the general circulation in NH summer. The WA like pattern and 3-4 year period oscillation are also relatively obvious for the summer case. In autumn, the convection over the western Pacific is closely linked with the Eurasian (EU) like pattern and the Atlantic oscillation in the general circulation in NH, it is suggested that in autumn the vitiation of convective activity over the western Pacific is largely affected by the general circulation anomaly (cold air from high latitudes) through EU like teleconnection pattern. Abrupt change happened by the end of 1980’s in the autumn interaction. The strong interaction between the western Pacific (WP) and EU like patterns in the general circulation in NH and the convection over the western Pacific and a linear trend of increasing of this interaction are also suggested in winter. It is also demonstrated that the interaction in summer and winter is stronger than in the transition seasons (spring and autumn).     

INTENSITY INDEX OF SOUTH CHINA SEA MONSOON AND ITS VARIATION CHARACTERISTICS

The region of south China is sometimes subject to major climatic catastrophes in winter. To have a clear understanding, the time in which extremely severe cold months occur in the south China wintertime over the past 45 years are determined and characteristics of the 500-hPa geopotential fields and SST fields are studied for the simultaneous and preceding 6-month periods. Similarity exists in the 500-hPa geopotential fields between each current severely cold month, with the geopotential pattern of being high in the north, but low in the south, of Asian-Pacific region and meridional circulation developing. The work presents anomalies of the months with significant differences in the 500-hPa geopotential field of the previous periods. The SSTA is continuous in the distribution from each extremely severe cold winter month back to the 6 months leading up to it for the region of south China while the SST pattern is of El Ni?o in January and the preceding 1 ~ 6 months for equatorial eastern Pacific but of La Ni?a in February and December. It is concluded that the prediction of severely cold winter months are possible with the use of the geopotential field at 500 hPa and the SST fields for the months ahead of the target time.     

THE CHARACTERISTICS OF 500 hPa GEOPOTENTIAL AND SST FIELD OF EXTREMELY SEVERE COLD MONTHS IN SOUTH CHINA WINTER

The region of south China is sometimes subject to major climatic catastrophes in winter. To have a clear understanding, the time in which extremely severe cold months occur in the south China wintertime over the past 45 years are determined and characteristics of the 500-hPa geopotential fields and SST fields are studied for the simultaneous and preceding 6-month periods. Similarity exists in the 500-hPa geopotential fields between each current severely cold month, with the geopotential pattern of being high in the north, but low in the south, of Asian-Pacific region and meridional circulation developing. The work presents anomalies of the months with significant differences in the 500-hPa geopotential field of the previous periods. The SSTA is continuous in the distribution from each extremely severe cold winter month back to the 6 months leading up to it for the region of south China while the SST pattern is of El Ni?o in January and the preceding 1 ~ 6 months for equatorial eastern Pacific but of La Ni?a in February and December. It is concluded that the prediction of severely cold winter months are possible with the use of the geopotential field at 500 hPa and the SST fields for the months ahead of the target time.     

Interannual variability of the upper tropospheric circulation

Summary Seventeen years of sea level pressure (SLP), 200-hPa zonal wind and 500-hPa geopotential height data were used to investigate the boreal winter and summer interannual (IA) circulation patterns. The IA patterns for these variables and for their zonally asymmetric (ZA) part were determined by performing empirical orthogonal function (EOF) analyses on the SLP and on ZA SLP. The corresponding patterns for the other variables were obtained by correlating their time series with the amplitude time series of these EOF analyses. For both seasons, the SLP and ZA SLP show a zonal wavenumber one pattern extending from the tropics into the winter hemisphere extratropics, which is consistent with the circulation anomalies related to the El Niño/Southern Oscillation (ENSO) cycles. The zonal wavenumber one pattern observed for the boreal winter describes the SLP and ZA SLP variations related to the mature state of the El Niño and La Niña episodes, and that for the summer, the SLP and ZA SLP variations associated with the initial or decay stages of these phenomena. The 200-hPa zonal wind and 500-hPa geopotential height patterns exhibit strong seasonal dependence, and the ZA parts of these two variables show even more pronounced seasonal differences. These results indicate that the seasonal cycle of the atmospheric circulation, in particular at the upper tropospheric levels, might play an important role in extending the IA wavetrain-like structure into the subtropics as noted for the 200-hPa zonal wind and its ZA part in the Pacific/Americas sector. This wavetrain-like structure shows its Southern Hemisphere (SH) and Northern Hemisphere (NH) branches for the boreal winter, and only its SH branch, for the boreal summer. Thus, the effects of the seasonal cycle of the atmospheric circulation on the IA patterns seem to be stronger for the NH.With 9 Figures     

EL NINO EPISODE-RELATED NH 100 hpa CIRCULATION ANOMALY

Investigation is carried out of winter Northern Hemisphere (NH) 100 hpa teleconnection, indicating the existence of a PNA pattern whose wavetrain well resembles that at 500 hpa level. During the climax of the EI Nino episode (winter), the 100 PNA becomes strong, a fact that manifests itself more clearly than at the 500 hpa level. Moreover, study is made of the anomaly in the tropical monthly windfield and mean circulation on a seasonal basis in a range of phases of the EI Nino event. Results show that the 100 hpa geopotential height is lowered on a large scale in the spring and previous winter with the event happening as compared to a La Nina year, wherewith diagnosis is performed of the 1991/1992 EI Nino episode.     

厄尔尼诺和反厄尔尼诺事件与西北太平洋台风活动

用统计相关和典型年合成方法分析了厄尔尼诺和反厄尔尼诺事件与西北太平洋台风活动的关系,指出厄尔尼诺年台风活动减少,反厄尔尼诺年台风活动增加,而且台风活动与厄尔尼诺、反厄尔尼诺事件起始和终止时间、强度、台风生成区域有关。利用厄尔尼诺和反厄尔尼诺年台风活动频数的统计特征,及台风频数与海温等要素的时滞相关关系,为台风频数的预测提供了有益的信息。还应用奇异值分解方法,分析了高度场和海温场的相关关系。结果表明,厄尔尼诺年海气耦合作用将造成不利于台风发展的环流条件,因此台风偏少,反厄尔尼诺年则出现相反的情况。     

冬季太平洋SSTA对北太平洋风暴轴年际变化的影响

研究了冬季北太平洋风暴轴的年际异常及其与500hPa高度以及热带和北太平洋海温的联系。结果发现，冬季北太平洋风暴轴中心有线性增强、偏北、偏东的趋势。对15个冬季北太平洋风暴轴区域 500 hPa天气尺度滤波位势高度方差与同期热带和北太平洋海温的 SVD分析表明，第一对空间典型分布反映了 ENSO区海温异常对风暴轴年际变化的影响，而第二对空间典型分布反映了黑潮区域海温异常对风暴轴年际变化的影响。进一步的合成分析显示，ENSO区海温异常可以通过激发500hPa高度场上的PNA遥相关型影响冬季北太平洋风暴轴的东西摆动和中、东端的强度变化，而黑潮区域海温异常则通过激发 500 hPa高度场上的 WP遥相关型，主要影响冬季北太平洋风暴轴中、西端的强度变化和南北位移。     

1998年秋到1999年春华南特大干旱气候成因

利用奇异值分解 (SVD) ,配合统计相关技术的应用 ,研究了广西秋季到第 2年 3月降水与 5 0 0 h Pa高度场和热带、北太平洋海温场的时间和空间结构及其相互关系。结果表明 ,第一耦合场明显反映了 ENSO信息 ,具有很高的相关 ;而海气配对的第二耦合场反映了作为关键区的日本海域以东的海温与 1 0 0°E以东 5 0 0 h Pa高度有较好的负相关。分析 1 998年以来海气场的变化 ,高度场上北太平洋中高纬 5 0 0 h Pa高度明显偏低 ,亚洲大陆和北太平洋中低纬高度偏高 ,因此其间冷空气明显偏弱 ,路径偏东 ;而海温场上 ENSO的变化 ,L ANina的发展过程与第一耦合场的海温变化模态一致 ;后期海、气关键区变化与第二耦合场相符。因此 ENSO的变化 ,日本海域以东升温造成了环流形势的异常 ,这是华南特大干旱的原因。利用时间系数互为因子法对广西 1 998年 8月到 1 999年 3月逐月降水作 SVD定量预报 ,预报降水持续偏少 ,1 999年春季干旱将非常严重。     

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES*

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short) of the tropical Pacific from January 1970 to December 1989,main spatial patterns of tropical Pacific SSTA and SSWA coupling features in the transform course from the warm phase to the cold phase of El Nino-southern Oscillation(ENSO) cycles are discussed.The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of El Nino(La Nina) are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at the mature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcing of distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with the California cold current and SSTA is mainly responsible for weakening of El Nino;(3)the second sea temperature increase along the South American coast in the decaying course of El Nino results from the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacific forced by anomalous west wind stress:(4)La Nina results from the joint effect of Walker circulation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.     

北太平洋次表层海温异常对中国夏季降水影响的可能途径

利用Godas月平均次表层海温资料, 分析了冬、春季和夏季北太平洋次表层海温层际相似性特征, 据此对次表层海温进行分层。在此基础上研究了500 hPa位势高度场、北太平洋次表层海温、中国夏季降水三者之间的时滞相关关系, 发现春季北太平洋次表层海温场是联系前、后期大气环流的关键因素。前期冬季大气环流对春季北太平洋次表层海温场影响最显著, 春季北太平洋次表层海温场又持续影响同期及后期夏季大气环流异常。异常的夏季大气环流与同期表层、次表层海温相互作用, 共同造成夏季长江流域与华北、华南降水出现相反异常的分布型式。     

Phase‐locked and asymmetric correlations of the wintertime atmospheric patterns with the ENSO

Abstract

Teleconnections between sea surface temperature (SST) anomalies over the Pacific and the dominant patterns of wintertime Northern Hemisphere 500‐hPa height are examined by applying statistical techniques such as rotated principal component analysis and composite analysis. It is shown that the Pacific/North American (PNA) patterns in December through March are correlated most significantly with the ENSO‐related SST anomalies in the previous October, while the western Pacific (WP) patterns in December through February are most closely linked to the ENSO‐related SST anomalies in the same season. In addition, the PNA response to the ENSO signal during La Niña events is more significant than that during El Niño events, while the WP response is stronger during El Niño events than during La Niña events. A composite analysis shows that in the El Niño winters the North Pacific centre of the PNA pattern is located about 10 degrees east of its normal position, leading to a less significant correlation between the ENSO signal and the PNA pattern in these winters.

The ENSO‐related SST anomalies include a large centre of action over the tropical Pacific and an oppositely signed anomaly centre over the North Pacific. The North Pacific centre appears to the west of the dateline in September and October. This ENSO‐related seed of SST anomalies slowly moves eastward in the following months, gradually cutting off its connection with SST anomalies over the tropical Pacific and being coupled with the PNA pattern. It is pointed out that, although the wintertime SST anomaly over the North Pacific may appear as a mode linearly independent of the ENSO signal in the same season, it is partially related to the ENSO signal in the preceding autumn.

Possible dynamical explanations of the above results are discussed. It is suggested that the WP pattern can be linked to the tropical Pacific heat source via advection of vorticity by the upper‐tropospheric divergent/convergent flow, and the intensification of vorticity gradients associated with a stronger east Asian jet is likely to be responsible for a more significant WP pattern response to the ENSO signal in the El Niño winters. On the other hand, the ENSO‐related PNA pattern could be considered a manifestation of the eastward extension (El Niño) or westward withdrawal (La Niña) of the east Asian jet stream due to the local Hadley cell over the Pacific. In addition, the ENSO‐related seed of extratropical SST anomaly over the western Pacific in autumn may also play an important role in the development of the PNA pattern in the following winter.     

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short)of thetropical Pacific from January 1970 to December 1989,main spatial patterns of tropical PacificSSTA and SSWA coupling features in the transform course from the warm phase to the cold phaseof El Nino-southern Oscillation(ENSO)cycles are discussed.The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of ElNino(La Nina)are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at themature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcingof distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with theCalifornia cold current and SSTA is mainly responsible for weakening of El Nino;(3)the secondsea temperature increase along the South American coast in the decaying course of El Nino resultsfrom the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacificforced by anomalous west wind stress:(4)La Nina results from the joint effect of Walkercirculation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.     

El Nino事件当年和次年夏季低纬环流特征的初步分析

分析1950年以来中等强度以上的ElNiño和LaNina当年和次年夏季(7月)低纬度风场和500hPa高度距平场,发现有不同的特征。其中中-西太平洋近赤道地区850、200hPa上风场异常变化十分明显,这种异常变化与南北半球大气相互作用有密切关系。在500hPa高度距平场上,北半球中、高和低纬地区表现出不同的正、负距平分布型式。上述特点对气候短期预测有一定参考意义。     

ElNino事件当年和次年夏季低纬环流特征的初步分析

分析１９５０年以来中等强度以上的ＥｌＮｉｎｏ和ＬａＮｉｎａ当年和次年夏季低纬度风场和５００ｈＰａ高度距平场发现有不同特征。其中中－西太平洋近赤道地区８５０、２００ｈＰａ上风场异常变化十分明显,这种异常变化与南北半球大气相互作用有密切关系。     

“短期气候预测专刊”

A premonitory sign of an anomalous SST over the eastern equatorial Pacific shows up in the North Pacific Subtropical Mode Water (STMW) 18 months earlier,and the air-sea relationship between the STMW and the anomalous SST over the eastern equatorial Pacific is shown.This premonitory connection involves an air-sea coupling between the longtime persistent mid-latitude sea surface temperature anomaly (SSTA) induced by the remote re-emergence of the STMW and the following spring subtropical atmospheric circulation anomalies.An examination of the air-sea interaction reveals that the following spring subtropical atmospheric circulation,which responds to the longtime persistent SSTA,is dominated by the anomalous negative (positive) geopotential height downstream of the negative (positive) SSTA in the strong (weak) STMW case.Thus,the tropics adjust to these anomalies through coupled dynamics,producing positive (negative) SST anomalies over the eastern equatorial Pacific.A cold water event that occurred over the eastern equatorial Pacific during winter 2008-09 was successfully forecasted by the weak summer STMW index in 2007.The evolution of this process for the air-sea interactions from the autumn of 2007 to December 2008 is presented.     

Year-to-Year and interdecadal variability in the activity of intraseasonal fluctuations in the Northern Hemisphere wintertime circulation

Summary Interannual variability in the activity of fluctuations with subseasonal time scales is investigated based upon observed data of the extratropical Northern Hemisphere circulation over the recent 38 winters. Their activity is represented in the root mean square (RMS) field of filtered geopotential height in which the fluctuations with time scales between 10 days and a season are retained. The singular value decomposition (SVD) was applied to the covariance matrix between the seasonal mean and RMS fields for the 500-hPa height.The leading SVD mode for the north Pacific represents the strong relationship between the polarity of the Pacific/North American (PNA) pattern in the seasonal-mean anomalies and the amplitude of a meridionally-oriented dipole-like oscillation within the season. It tends to be more active when the seasonal-mean jet stream is strongly diffluent over the central Pacific than when the jet is extended zonally across the Pacific. The leading SVD mode for the north Atlantic is indicative of stronger intraseasonal fluctuations near Greenland in the presence of anticyclonic seasonal-mean anomalies associated with the North Atlantic Oscillation (NAO).The intraseasonal variability in the extratropics is strongly correlated with the underlying sea surface temperature (SST) anomalies, and that in the north Pacific also exhibits significant but rather weak correlation with SST anomalies in the equatorial Pacific. The activity of the atmospheric intraseasonal fluctuations is found to be modulated in accordance with interdecadal variability in the seasonal-mean circulation and SST.On leave from Department of Earth & Planetary Physics, University of Tokyo.With 12 Figures     

Circulation Indices of the Aleutian Low Pressure System: Definitions and Relationships to Climate Anomalies in the Northern Hemisphere

In this study, a group of indices were defined regarding intensity (P), area (S) and central position (λ^c, Φ^c) of the Aleutian low (AL) in the Northern Hemisphere in winter, using seasonal and monthly mean height field at 1000-hPa. These indices were calculated over 60 winter seasons from 1948/1949 to 2007/2008 using reanalysis data. Climatic and anomalous characteristics of the AL were analyzed based on these indices and relationships between the AL, and general circulations were explored using correlations between indices P, λ^c, and Pacific SST, as well as Northern Hemisphere temperature and precipitation. The main results are these: (1) AL is the strongest in January, when the center shifts to the south and west of its climatological position, and it is the weakest in December when the center shifts to the north and east. (2) AL intensity (P) is negatively correlated with its longitude (λ^c): a deeper low occurs toward the east and a shallower low occurs toward the west. On a decadal scale, the AL has been persistently strong and has shifted eastward since the 1970s, but reversal signs have been observed in recent years. (3) The AL is stronger and is located toward the east during strong El Nino winters and vice versa during strong La Nina years; this tendency is particularly evident after 1975. The AL is also strongly correlated with SST in the North Pacific. It intensifies and moves eastward with negative SST anomalies, and it weakens and moves westward with positive SST anomalies. (4) Maps of significance correlation between AL intensity and Northern Hemisphere temperature and rainfall resemble the PNA teleconnection pattern in mid-latitudes in the North Pacific and across North America. The AL and the Mongolian High are two permanent atmospheric pressure systems adjacent to each other during boreal winter over the middle and high latitudes in the Northern Hemisphere, but their relationships with the El Nino/La Nina events and with temperature and precipitation in the Northern Hemisphere are significantly different.     

华东冬季异常冷暖与大气环流和海温的关系

利用1951-2007年华东地区14个代表站冬季(12-2月)温度资料和北半球500 hPa高度及北太平洋海温资料,通过合成分析、相关分析等方法,研究了华东地区冬季气温的气候变化及其与北半球500 hPa高度场、北太平洋海温场的关系.结果表明:华东地区冬季气温具有明显的年代际气候变化特征;前期夏季北半球500 hPa高度距平场和前期春季北太平洋海温距平场分布可作为华东冬季异常冷暖年的前兆信号;夏季北太平洋中部地区500 hPa高度场变化及前期10月西太平洋副高强弱变化,对华东地区冬季气温变化具有很好的指示性;春季南赤道海流区和西风漂流区海温异常变化,对华东地区冬季气温变化也具有很好的指示意义.     

INFLUENCE ON NORTHERN PACIFIC STORM TRACK OF EQUATO-RIAL CENTRAL AND EASTERN PACIFIC SSTA DURING WINTER

Singular value decomposition (SVD) is conducted of 15 winter tropical Pacific SST with 500 hPa filtered potential height variance over the northern Pacific storm track. It is shown that the first coupled mode obtained depicts the effect on the track of SSTA over equatorial central and eastern Pacific. Further composite analysis indicates that the SSTA over there during winter can give rise to or invigorate PNA teleconnection response pattern in 500 hPa height field which, in turn, exerts crucial influence on the interannual variability in vigor and east-west displacement of the Pacific storm track, especially over its central and eastern part.