基于交叉小波分析方法的西太平洋副热带高压年际变率与热带海温及大气环流异常的相关性研究

西太平洋副热带高压的年际变率受热带多个关键海区的海-气相互作用过程调控, 但彼此间的因果关联和影响机制尚不清楚。为揭示西太平洋副热带高压的年际变率与热带海温及大气环流异常之间的内在关联特性, 定义了三个关键海区以及赤道纬向西风区的特征指数, 并分别与西太平洋副热带高压强度、脊线指数进行了交叉小波和相干小波分析。研究发现:西太平洋副热带高压指数存在显著的2~3年和准5年的周期振荡, 20世纪八九十年代后, 由于暖池区海温及赤道纬向西风区的Hadley环流强迫加强, 致使副热带高压特征指数的2~3年周期振荡加强; 从位相关系看, 先是西太平洋副热带高压减弱南撤导致纬向西风加强, 其后影响赤道东太平洋海温升高, 同时暖水向东传, 使赤道中太平洋以及暖池区海温逐渐升高, 在Hadley环流作用下使副高加强北抬。基于上述西太平洋副热带高压的年际变率与热带海温及大气环流异常变化相关性诊断研究, 进一步探讨了造成这种相关性的影响机理和因果关联, 为揭示西太平洋副热带高压年际变率与热带海温及大气环流异常的相关性做探索研究。      

2013年夏季西太平洋副高异常特征及其对湖南高温干旱的影响

利用湖南96个测站的逐日降水、日最高气温和NCEP/NCAR再分析资料、海温资料,分析了2013年夏季西太平洋副热带高压异常活动特征、成因及其对湖南高温干旱的影响。结果表明,2013年夏季西太平洋副高异常偏西、偏强,使得湖南一直处在高压下沉气流控制下,形成持续高温干旱天气。造成副高变异的原因主要有:(1)2012年冬季至2013年春季,赤道东太平洋海表温度持续偏低,印度洋—赤道西太平洋海表温度持续偏高,使得Walker环流和Hadley环流的上升和下沉运动得到加强,西太平洋副高西伸、加强;(2)南亚高压一次次东伸,通过强烈高空负涡度平流的动力强迫,造成西太平洋副高区内的下沉运动,导致副高稳定维持,天气晴热高温;(3)西风急流较常年偏北,纬向环流偏强,导致副热带高压在偏北位置稳定维持,200 h Pa高空辐合增强,辐合中心位于30°N以北,造成500 h Pa副高下沉运动区位置偏北、偏强。     

太平洋中低纬度海表温差与副热带高压异常的数值模拟

中国夏季天气变化与太平洋副热带高压关系密切,而中低纬度热力差异可能是副热带高压的强度和位置发生变化的莺要原因,文中利用NCAR/NCEP再分析位势高度、垂商速度和海表温度场资料,在对太平洋海表温度合理分区的基础上.根据海表温度EOF分解的第一模态时间系数与副热带高压的相关关系,定义了太平洋中、低纬度海表温差指数,并通过统计分析和数值模拟方法分析了温差的年代际变化特征及其对副热带高压的影响.结果表明:副热带高压的变化分别与中纬度太平洋的(30°-40°N,180°-140°W)和低纬度太平洋的(10°S-10°N,140°～100°W)两块区域海温关系密切,对由此两区域定义的温差指数分析发现,1976年前后温差指数出现一次显著的由弱变强的年代际突变,且温差的年代际变化特征与副热带高压异常有很好的对应关系,温差大值年,副热带高压偏强,面积增大,西伸尤其明显;温差小值年,副热带高压偏弱,面积减小,东撤明显.进一步的统计分析和NCAR/CAM3.0模式数值模拟都发现,夏季中低纬海表温差增大将引起哈得来环流加强,副热带的下沉速度加大,使副热带高压增强;夏季中低纬海表温差减小将引起哈得来环流减弱,副热带的下沉速度减小,使副热带高压减弱.冈此夏季中低纬海表温差的变化是导致副热带高压强度和位置异常的重要原因之一.     

北半球中纬地区平均纬向垂直环流的特征

Reginald.E.Newell等曾计算了冬夏两季5℃和5°N的沃克环流(Walker Cell),给出了赤道地区纬向气流的特征。Krishnamurti曾根据200mb位势流的计算结果,推论出北半球夏季0°-30°N和北半球冬季15°S—15°N的理想纬向垂直环流圈,给出了热带和副热带地区纬向垂直环流的特征。叶笃正等指出青藏高原地形及其夏季的热源作用,使其与西半球、南半球和中、东太平洋的天气系统产生遥相关,给出了青藏高原地区和遥     

热带太平洋-印度洋海温异常综合模对南亚高压的影响

从综合考虑热带太平洋和印度洋海温异常特征出发,研究了热带太平洋-印度洋海温异常综合模对南亚高压的影响.当热带太平洋-印度洋海温异常综合模为正位相(西印度洋和东太平洋海温距平为正,东印度洋-西太平洋海温距平为负),南亚高压偏弱,位置偏东偏南;当热带太平洋-印度洋海温异常综合模为负位相(西印度洋和东太平洋海温距平为负,东印度洋-西太平洋海温距平为正),南亚高压偏强,位置偏西偏北.热带太平洋-印度洋海温异常综合模影响南亚高压主要通过三种机制:一是通过影响亚洲季风从而影响了降水潜热形成的大气加热场分布,在正(负)位相年,青藏高原大气热源为负(正)异常,因此青藏高原上空空气上升减弱(加强),南亚高压偏弱(偏强);南海季风和热带辐合带加强(减弱),菲律宾附近的大气热源加强(减弱),有利于上空青藏高原东南侧反气旋(气旋)式的距平环流,因此南亚高压偏东偏南(偏西偏北).二是热带太平洋-印度洋海温的纬向热力对比引起赤道纬向垂直(Walker)环流异常,必将引起高空纬向风异常,在正(负)位相年,南亚高压南部的印度洋高空会出现西(东)风异常,导致南亚高压偏弱(偏强).三是综合模的正(负)异常加强(减小)西印度洋经度范围的区域Hadley环流,其北侧伊朗高原上的异常下沉(上升)支,造成南亚高压偏弱(偏强),位置偏东偏南(偏西偏北).     

夏季北太平洋副热带高压系统的活动

文中根据 NCEP/NCAR再分析资料 ,分析了北太平洋副热带高压系统的变化。 5～ 9月由于亚洲夏季风的建立及活动 ,北半球副热带高压系统在 6 0～ 1 2 0°E出现断裂 ,夏季西太平洋副热带高压脊点平均伸展到 1 2 0°E,其年际变化反映了亚洲夏季风的强弱。强夏季风年 5 0 0h Pa西太平洋副热带高压脊线位于 3 0°N以北 ,并分裂成两个中心 ,印度低压强 ;弱夏季风年西太平洋副热带高压脊线位于 3 0°N以南 ,表现为北太平洋高压中心向西伸展的高压脊 ,印度低压弱。夏季西太平洋副热带高压的季内活动有两种模态 :第 1种表现为副热带高压系统以 2 0～ 3 0 d的周期从北太平洋中部的副热带高压中心一次次地向西扩张到 1 2 0°E以西 ,这类过程大多出现在亚洲夏季风强度偏弱年 ;第 2种模态表现为副热带高压系统以 2 0～ 3 0 d的周期一次次地由东向西扩充时 ,在 1 2 5～ 1 5 5°E停滞 ,这类过程大多出现在亚洲夏季风强度偏强年。江淮流域梅雨的中断和结束与北太平洋副热带高压系统 2 0～ 3 0 d季内振荡有关。西太平洋副热带高压 5～ 1 0 d的短期活动受 3 5～ 45°N西风带活动的影响 ,当西风槽在中国沿海和西太平洋地区向南伸展到 3 0°N以南后 ,西太平洋副热带高压有一次加强活动     

一次El Niño背景下副高异常事件的基本特征

分析了 2019年一次El Ni?o事件发展的基本特征,并探讨了该次事件背景下副高偏强偏南的原因.研究发现:赤道中东太平洋纬向流反馈偏弱和西太平洋海域区域性的东风异常,都对该次事件表现出向中部型转变的趋势有一定程度的贡献.西太平洋海域上层大气的强烈下沉运动以及对流层低层存在的异常反气旋环流在一定程度上促进了副高的发展,在该次El Ni?o事件发展期间,导致西太平洋副高向南增强,位置偏南;Hadley环流下沉支的异常进一步加强了西太平洋副高的强度.2018年期间,从东太平洋海域西传的冷性Rossby波到达西太平洋海域且维持,产生异常Rossby波响应,同时2019年5月,MJO来到印度洋海域第8位相,西太平洋海域为反气旋性环流异常,二者共同影响,有利于热带西太平洋及我国南海地区出现异常的反气旋环流,进一步促进西太平洋副高加强西伸、强度偏强、位置偏南.     

西北太平洋区域纬向风垂直切变的气候特征研究

西北太平洋区域纬向风垂直切变的变化是影响西北太平洋热带气旋生成和发展的一个重要的动力因子,弱的纬向风切变有利于热带气旋的发生、发展。文中将西北太平洋区域纬向风垂直切变幅度(MWS)定义为850与200 hPa的纬向风之差的绝对值,以研究MWS的气候特征。结果表明,西北太平洋区域的MWS有两个主要空间模态,第1空间模态表现为在15°N以南的热带西太平洋存在MWS东西向变化相反的两个区域,20°N附近的热带西太平洋MWS的变化与其以北海区的MWS的变化相反。第2空间模态表现为在热带太平洋140°E东、西的变化相反。研究了两个模态相关的大气环流特征,发现去掉强ENSO信号后,第1模态不但与低纬度大气环流有关,而且还与南、北半球中高纬度的大气环流有关,第2模态主要与热带西太平洋和北太平洋局地大气环流有关。另外,第1模态的时间系数与赤道东太平洋海温、西北太平洋台风生成频次有着密切联系;第2模态时间系数与西北太平洋台风活动频次联系密切。     

两类中太平洋El Nio特征的对比分析

利用1979—2012年逐月Hadley中心海表温度、欧洲中期天气预报中心次表层海温、NCEP/NCAR风场再分析资料,对两类中太平洋(CP)El Nio及耦合的大气环流特征进行分析。结果表明,第一类CP El Nio(CP-ⅠEl Nio)增暖中心位于Nio4区且关于赤道对称;第二类CP El Nio(CP-ⅡEl Nio)的Nio4区与热带东北太平洋区域(NEP,130~110°W、15~25°N)同位相变化,冬季成熟后形成关于赤道非对称的带状增暖结构。进一步的研究表明,两类CP El Nio次表层结构存在差异:CP-ⅠEl Nio冬季次表层海温异常(SOTA)在中东太平洋与西太平洋呈显著偶极分布;CP-ⅡEl Nio在中、西太平洋位相相反但东太平洋异常较弱,且经向异常主要在赤道及其以北。两类CP El Nio耦合的大气环流特征不同:CP-ⅠEl Nio冬季异常Walker环流上升中心位于赤道上空,经向风向赤道辐合,低纬地区Hadley环流加强;CP-ⅡEl Nio冬季低层向北越赤道气流加强,Walker环流上升中心移到赤道以北,低纬地区Hadley环流减弱。     

北半球副热带—中纬度太平洋大气季节内扰动的纬向传播与东亚夏季旱涝

用时空滤波和Morlet小波方法,分析了1958—2000年夏季东亚(20°—45°N,110°—135°E)不同纬带(由南到北分为4个区域)的降水分别与太平洋同一纬带上大气30—60 d振荡(ISO)沿纬圈传播的关系及其成因机制。发现太平洋上经向风ISO向西传播的强或弱,是东亚夏季风区降水偏多或偏少的必要条件。对逐年夏季的分析表明,无论当年东亚夏季风强与否,在所划分的几个东亚季风区所有涝的年份里,太平洋同一纬带上大气ISO向西传播都明显较强,而在这些区域绝大多数旱的年份里,相应的ISO向西传播明显较弱。进一步分析发现,经向风ISO的纬向传播对应着大气经向型环流系统的移动,向西传影响东亚夏季风区降水的ISO有来自低纬中东太平洋东风流中的低频气旋(如副热带东风带中ISO的演变);也有来自中高纬度阿拉斯加湾及鄂霍次克海一带低频低压(如洋中槽)和高压(如阻塞高压和东北太平洋高压)的向南向西频散。因此东亚夏季旱涝不但与热带季风有关,而且与中东太平洋副热带东风系统中ISO的向西传播、中高纬度长波调整时低频扰动向西南经北太平洋副热带的传播密切相关。     

夏季西北太平洋副热带高压指数

利用在特定区域上平均的夏季（6、7、8月）平均850hPa位势高度异常,我们定义了两种指数,分别用来描述夏季北太平洋副热带高压在东西方向和南北方向上的偏移。对于东西向指数,平均的区域为副高的西侧（110°-150°E,10°-30°N）;对于南北向指数,平均的区域为副高的西北侧（120°-150°E,30°-40°N）。发现这两种指数是相互独立的。基于南北向指数的合成分析结果与以往的研究结果吻合得相当好。在年际时间尺度上,将这两种指数与国家气候中心公布的副高指数进行了比较,发现尽管有一些微弱的差别,本文定义的指数与国家气候中心的副高指数大致具有相似的年际变化,因而本文的指数与国家气候中心的指数也对应着相似的环流和降水型。进而,对本文的指数与国家气候中心的指数对应的环流（降水）型之间的不同进行了分析,表明本文的指数比国家气候中心的指数能够更好地描述对应的环流和降水型。一个重要的结果是,不论根据本文指数,还是根据国家气候中心指数,东西向指数（或西伸指数）都比南北向指数（或北界指数）对应着更显著的降水异常,特别是在东亚地区和菲律宾海。 这两种指数还可以用来描述副高在夏季里的季节推进,即,北移和东退。副高在7月中旬迅速北移和东退。发现在副高平均处于偏北或偏东的夏季里,北移     

Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon (EAWM) were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell (ZWC) in the tropic, the Hadley cell in the western Pacific (WPHC), the midlatitude zonal cell (MZC) over the central North Pacific, and the Hadley cell in the eastern Pacific (EPHC)] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly (SSTA).     

The impact of tropical western Pacific convection on the North Pacific atmospheric circulation during the boreal winter

In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.     

Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: a comparison of observations to 600-years control run of Bergen Climate Model

Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.     

Interdecadal Change in the Interannual Variation of the Western Edge of theWestern North Pacific Subtropical High During Early Summer and the Influenceof Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Ni?o decaying pattern before 1990 while rapid transformation from El Ni?o to La Ni?a after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Ni?o SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.     

ENSO teleconnections in projections of future climate in ECHAM5/MPI-OM

The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO₂ simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.     

Impact of the western North Pacific subtropical high on the East Asian monsoon precipitation and the Indian Ocean precipitation in the boreal summertime

The western North Pacific subtropical high (WNPSH) is a crucial component of the East Asian summer monsoon (EASM) system and significantly influences the precipitation in East Asia. In this study, distinguished role of WNPSH on the EASM and Indian Ocean monsoon (IOM) are investigated. Based on the boreal summer mean field of 850-hPa geopotential height and its interannual variability, the WNPSH index (WNPSHI) is defined by the areaaveraged geopotential height over the region [110°–150°E, 15°–30°N]. The WNPSHI is significantly related to the precipitation over the East Asian monsoon (EAM) region [105°–150°E, 30°–40°N] and IOM region [70°–105°E, 5°–15°N]. Rainfalls over these two regions have good correlation with WNPSH developments and the geopotential height fields at 850 hPa related to the EAM precipitation and IOM precipitation have remarkably different teleconnection patterns in boreal summer. These features exhibit that EAM and IOM precipitations have different type of development processes associated with different type of WNPSH each other. Focusing on the relationships among the EAM precipitation, IOM precipitation, and the WNPSH variabilities, we assume that WNPSH and EAM precipitation are usually fluctuated simultaneously through the sea surface temperature (SST)-subtropical ridge-monsoon rainfall feedback, whereas the IOM precipitation varies through the different process. To clarify the relationships among WNPSH, EAM, and IOM, two cases are selected. The first one is the case that all of WNPSH, EAM, and IOM are in phase (WE(+)I(+)), and the second one is the case that WNPSH and EAM are in phase and WNPSH/EAM and IOM is out of phase (WE(+)I(?)). These two cases are connected to the thermal forcing associated with SST anomalies over the eastern Pacific and Indian Ocean. This different thermal forcing induces the change in circulation fields, and then anomalous circulation fields influence the moisture convergence over Asian monsoon regions interactively. Therefore, the monsoon rainfall may be changed according to the thermal conditions over the tropics.     

热带大气垂直环流对海表温度异常的响应——一个初步的理论分析

本文提出了一个β平面定常的线性二维模式,并考虑了海面边界层和赤道侧向边界层,讨论了热带海表温度异常对大气所产生的垂直环流——经向环流和纬向环流。结果表明:热带大洋东部(例如太平洋)海表温度比平均状态暖而西部较冷时,其上空经向环流(Hadley环流)比平均状态强,而纬向环流(如在太平洋上,称Walker环流)弱。相反,当热带大洋西部暖而东部冷时,经向环流减弱,纬向环流加强。这些是与观测事实比较一致的。     

Surface currents and equatorial thermocline in a coupled upper ocean-atmosphere GCM

The Oregon State University coupled upper ocean-atmosphere GCM is evaluated in terms of the simulated winds, ocean currents and thermocline depth variations. Although the zonal wind velocities in the model are underestimated by a factor of about three and the zonal current velocities are underestimated by a factor of about five, the model is seen to qualitatively simulate the major features of the gyral scale currents, and the phases of the seasonal variation of the principal equatorial currents are in reasonable agreement with observations. The simulated tropical currents are dominated by Ekman transport and the eastern boundary currents do not penetrate far enough equatorward, while the western boundary currents do not penetrate far enough poleward. The subtropical trade wind belt and the mid-latitude westerlies are displaced equatorward of observations; hence, the mid-latitude eastward currents, principally the Kuroshio-North Pacific Drift and the Gulf Stream-North Atlantic Current are displaced equatorward. In spite of these shortcomings the surface current simulation of this two-layer upper ocean model is comparable with that of other ocean GCMs of coarse resolution. The coupled model successfully simulates the deepening of the thermocline westward across Pacific as a consequence of the prevailing Walker circulation. The region of most intense simulated surface forcing is located in the western Pacific due to a southwestward displacement of the northeast trade winds relative to observations; hence the equatorial Pacific is dominated by eastward propagation of thermocline depth variations. The excessively strong Ekman divergence and upwelling in the western Pacific cools the local warm pool, while incorrectly simulated westerlies in the eastern Pacific suppress upwelling and inhibit cooling from below. These features reduce the simulated trans-Pacific sea-surface temperature gradient, weakening the Walker circulation and the anomalies associated with the simulated Southern Oscillation. Offprint requests to: KR Sperber     

印度洋海温异常对西北太平洋台风的影响及机制研究

基于近40 a NCEP/NCAR再分析月平均高度场、风场、涡度场、垂直速度场以及NOAA重构的海面温度（sea surface temperature,SST）资料和美国联合台风预警中心（Joint Typhoon Warning Center, JTWC）热带气旋最佳路径资料,利用合成分析方法,研究了前期春季及同期夏季印度洋海面温度同夏季西北太平洋台风活动的关系。结果表明：1）前期春季印度洋海温异常（sea surface temperature anoma1y,SSTA）尤其是关键区位于赤道偏北印度洋和西南印度洋地区对西北太平洋台风活动具有显著的影响,春季印度洋海温异常偏暖年,后期夏季,110°～180°E的经向垂直环流表现为异常下沉气流,对应风场的低层低频风辐散、高层辐合的形势,这种环流形势使得低层水汽无法向上输送,对流层中层水汽异常偏少,纬向风垂直切变偏大,从而夏季西北太平洋台风频数偏少、强度偏弱,而异常偏冷年份则正好相反。2）春季印度洋异常暖年,西北太平洋副热带高压加强、西伸;而春季印度洋异常冷年,后期夏季西北太平洋副热带高压减弱、东退,这可能是引起夏季西北太平洋台风变化的另一原因。