2010年西北太平洋与南海热带气旋活动异常的成因分析

利用中国气象局热带气旋（TC）资料、NCEP/NCAR 再分析资料和美国 NOAA 向外长波辐射（OLR）等资料，分析了2010年西北太平洋（WNP）及南海（SCS）热带气旋活动异常的可能成因，讨论了同期大气环流配置和海温外强迫对TC生成和登陆的动力和热力条件的影响。结果表明，2010年生成TC频数明显偏少，生成源地显著偏西，而登陆TC频数与常年持平。导致7～10月TC频数明显偏少的大尺度环境场特征为：副热带高压较常年异常偏强、西伸脊点偏西，季风槽位置异常偏西，弱垂直风切变带位置也较常年偏西且范围偏小，南亚高压异常偏强，贝加尔湖附近对流层低高层均为反气旋距平环流，这些关键环流因子的特征和配置都不利于 TC 在WNP的东部生成。影响TC活动的外强迫场特征为：2010年热带太平洋经历了El Ni?o事件于春末夏初消亡、La Ni?a事件于7月形成的转换；7～10月，WNP海表温度维持正距平，140°E以东为负距平且对流活动受到抑制；暖池次表层海温异常偏暖，对应上空850 hPa为东风距平，有利于季风槽偏西和TC在WNP的西北侧海域生成。WNP海表温度和暖池次表层海温的特征是2010年TC生成频数偏少、生成源地异常偏西的重要外强迫信号。有利于7～10月热带气旋西行和登陆的500 hPa风场特征为：北太平洋为反气旋环流距平，其南侧为东风异常，该东风异常南缘可到25°N，并向西扩展至中国大陆地区；南海和西北太平洋地区15°N以南的低纬也为东风异常；在这样的风场分布型下，TC容易受偏东气流引导西行并登陆我国沿海地区。这是2010年生成TC偏少但登陆TC并不少的重要环流条件。

Analysis of Anomalous Tropical Cyclone Activities over the Western North Pacific and the South China Sea in 2010

The possible reasons for anomalous tropical cyclone (TC) activity over the western North Pacific (WNP) and South China Sea (SCS) were studied, and the possible effects of dynamic conditions and the thermal state on the frequency of TC genesis and landfall, derived from circulation patterns and boundary forcing, were analyzed using TC data from the China Meteorological Administration, NCEP/NACR reanalysis data, and the outgoing long-wave radiation data from the US NOAA. The results showed that TC genesis was much less frequent than normal but that TC landfall was near normal in 2010. The TC genesis locations were west of the usual genesis longitudes. The circulation patterns leading to fewer TCs were characterized as strong western North Pacific subtropical highs and South Asia highs, westward monsoon troughs, and vertical wind shear, anticyclone anomalies at low and high levels of the troposphere over Lake Baikal. Each of these features does not favor the genesis of TC in the eastern part of the WNP. Boundary forcing showed that the tropical Pacific experienced an El Niño event that ended in the late spring and early summer 2010 while a new La Niña event formed that July. From July to October, there was a positive sea surface temperature (SST) anomaly over the WNP west of 140°E while convection was suppressed east of 140°E. The subsurface temperature over the warm pool was much higher than normal, corresponding to an 850 hPa easterly wind anomaly. This thermal state led to the monsoon trough moving west of its average position and TC genesis in the eastern WNP. The SST and subsurface temperature features in the warm pool were the key thermal factors in the lower frequency of TCs in the western WNP in 2010. Further analysis showed that there was an anticyclone 500 hPa wind anomaly over the middle latitudes of the North Pacific from July to October 2010. The easterly anomaly to the south of the anticyclone was accompanied by another in the west of the WNP and SCS. The easterly anomaly extended southward to 25°N and westward to mainland China and the western anomaly extended northward to 15°N. Such a wind anomaly pattern favors the westward movement of TCs and then making landfall in mainland China. These are, therefore, the circulation conditions required for much less frequent TC genesis and almost normal TC landfall.