中国南海与西北太平洋热带气旋初始尺度分布差异:BSISO的调制作用OA

This study investigates the differences in the initial size distribution of tropical cyclones(TCs)generated over Western North Pacific(WNP)and South China Sea(SCS)from July to October during 1981-2017.Results show that although the median initial sizes of TC in the two basins are similar,the WNP exhibits greater variance with a right-skewed distribution,while SCS TC sizes follow a distribution closer to normal.Further analysis indicates that during the non-convective phase of Boreal Summer Intraseasonal Oscillation(BSISO),the median initial TC sizes in both regions do not differ significantly.However,during the convective phase,WNP experiences a pronounced increase in TC size,whereas SCS shows only limited growth.In terms of environment conditions,although SCS has stronger low-level relative vorticity and humidity,its higher vertical wind shear leads to a marked wavenumber-1 asymmetry in TC convective structure.During the BSISO convective phase,environmental factors such as low-level vorticity and humidity are significantly improved over WNP,resulting in systematically enhanced TC convection and more efficient inward transport of angular momentum,favoring the formation of larger TCs.In contrast,SCS sees no substantial weakening in vertical wind shear,and its asymmetric structure becomes more pronounced.The overall TC circulation and central convection do not exhibit significant strengthening,leading to a weaker response of TC size.These findings highlight that the modulation of environmental conditions and TC structure by the BSISO convective phase contributes importantly to the differences in initial TC size distribution between the WNP and SCS.