初夏孟加拉湾地区风暴活动与季风爆发早晚的关系及其前兆信号

RELATIONSHIP BETWEEN STORM ACTIVITY AND SUMMER MONSOON ONSET IN BAY OF BENGAL BASIN AND THEIR PRECURSOR SIGNALS

孟加拉湾风暴(简称孟湾风暴)和孟加拉湾夏季风(简称孟湾夏季风)是初夏活动于孟加拉湾(简称孟湾)地区的两个重要环流系统,对中国西南地区的天气气候和亚洲夏季风的季节进程有十分重要的影响。为了进一步认识两个环流系统活动异常的特征及其影响因子,从气候角度出发初步分析了两者活动的相互联系及其影响的前兆信号因子和环流变化特征。结果表明两者有一定的联系,孟湾夏季风爆发偏早时孟湾风暴活动时间偏早且活动频数偏多,反之,风暴活动时间偏晚且活动频数偏少。初夏两个系统的异常活动与前期环流变化密切联系,由于孟湾夏季风爆发早晚与大尺度环流的季节转换有关,爆发早晚其前期的环流变化有十分显著的差异,对流层低层赤道印度洋异常西风(东风)和孟湾异常气旋(反气旋)活动,以及对流层高层东亚持续稳定的异常反气旋(气旋)是孟湾夏季风爆发偏早(偏晚)的重要前兆信号特征,而这些异常环流变化为孟湾风暴的生成发展提供了极为有利(不利)的环流背景条件。进一步分析发现,前期冬季1月赤道太平洋地区西暖东冷的海温异常分布对后期2-4月孟湾区域异常气旋环流的生成发展有十分重要的影响,其中赤道西北太平洋异常暖海温的变化有利于在其西北侧南海南部激发异常气旋环流,而赤道太平洋西暖东冷的海温差异对赤道印度洋异常西风的加强及其南北两侧气旋对的发展西移有重要作用,是影响孟湾区域气旋性环流发生发展的重要外强迫因子。

Storm and summer monsoon are two important atmospheric circulation systems in the Bay of Bengal(BOB)basin during early summer.They exert strong influences on the weather and climate over southwestern China,as well as the seasonal process of the Asian monsoon activity.To further understand the characteristics of the abnormal activities of the two circulation systems and their influencing factors,the relationship between the two systems and the precursory features of circulation variation are analyzed from the perspective of climate.The results show that the earlier the onset of the summer monsoon,the earlier the formation of the storm and the higher the frequency of the storm,and vice versa.The anomalous activities of the two systems are related to the changes of the early circulation.Because the onset time of summer monsoon is related to the seasonal transition of large circulation,there are significant differences in the previous circulation anomalies.Variation of the anomalous westerly(easterly)wind in the lower troposphere of the equatorial Indian Ocean with symmetric cyclone(anticyclone)pair activity on the north and south sides,the stable and persistent anomalous anticyclone(cyclone)in the upper troposphere are important precursor signals of the onset of summer monsoon.These variations may provide a favorable(unfavorable)circulation background condition for the occurrence and development of storms.Further analysis shows that the anomalous sea surface temperature anomalies(SSTA),i.e.,being warm in the west and cold in the east of the equatorial Pacific region in January,have a very important influence on the formation and development of the anomalous cyclonic circulation in the BOB basin from February to April.The SSTA in the northwestern equatorial Pacific Ocean is favorable for anomalous cyclonic circulation generation in its northwestern side,i.e.,the southern part of the South China Sea.The SSTA difference between the warm west part and cold east part of the equatorial Pacific Ocean has an important influence on the strengthening of the anomalous westerly wind in the equatorial Indian Ocean and westward movement of the cyclones on both sides of the equatorial Indian Ocean,and this SSTA difference is an important external forcing factor affecting the occurrence and development of the cyclonic circulation in the BOB.