南海土台风生成及发展过程海气热通量交换特征

THE CHARACTERISTICS OF AIR-SEA HEAT FLUX EXCHANGE DURING THE GENERATION AND DEVELOPMENT OF THE LOCAL TYPHOON OVER THE SOUTH CHINA SEA

利用1985—2007年西北太平洋热带气旋(TC)资料,定义生成于南海范围内并且发展强度达到热带风暴(TS)等级及以上的热带气旋为南海土台风,统计了南海土台风的季节演变特征,发现南海生成的TC约有68%发展成为土台风,其强度普遍较弱且与TC生成纬度和路径均有关。其频数的季节变化呈双峰结构,5月和7—9月是南海土台风的高发期。结合同期美国伍兹霍尔海洋研究所的1°×1°客观分析海气通量(WHOI_OAFlux)日平均资料,分析了南海土台风生成及发展各阶段的海气热通量分布特征。结果表明:南海土台风形成过程中,海洋向大气释放的热通量逐日递增,台风眼南侧的海洋为台风形成提供主要能量来源,随着台风发展热通量高值区都沿顺时针方向向台风北侧传播,体现了台风外围涡旋罗斯贝波的能量频散特征,土台风形成后,热通量的加强不再明显。在土台风整个形成及发展过程中,净热通量、潜热通量和感热通量三者的变化较为一致,以潜热对净热的贡献为主,最大热量交换位于台风移动方向的南半圆,可能与南海西南季风作用有关。

A South China Sea （SCS） local typhoon （SLT） is defined as a tropical cyclone （TC） that forms within the SCS region and can reach the grade of tropical storm （TS） or above. The statistical features of the SLTs from 1985 to 2007 were analyzed first. It was found that the intensities of the TCs were relatively weak and about 68% of them could develop into TSs. The SLT intensity was associated with its genesis latitude as well as its track. The SLT frequency number presented a seasonal variation with two peaks in May and June through September. Based on the daily heat flux data from the Woods Hole Oceanographic Institution Objectively Analyzed air-sea Fluxes （WHOI_OAFlux） in the same period, the air-sea exchange during the process of generation and development of the SLT was studied. Results showed that the heat fluxes released to the atmosphere were increasing significantly day by day before cyclogenesis. The ocean at the south to the TC center provided the main energy. Along with the development of SLT, the regions with larger heat fluxes spread clockwise to the north of TC, which reflected the energy dispersion property of vortex Rossby-waves in peripheral bands of TC. However, once the SLT was formed there were no more increments in the fluxes. During the whole process, the net heat, latent heat and sensible heat flux displayed a similar evolution when the latent heat made a main contribution to the net heat flux. The maximum air-sea heat exchange always locates at the left side of the TC moving direction, which may reflect influence of the SCS summer monsoon on TC structure.