Characteristics Of Tropical Cyclones Over The South China Sea And The Relationship With Environmental Variables

Using the observational data IBTr ACS and the tracked data from the atmospheric reanalysis ERA-Interim over 1979-2019,the characteristics of the tropical cyclone spatiotemporal distribution,frequency,intensity,interannual and decadal variations in the South China Sea have been studied.The properties of several atmospheric environmental factors affecting the formation and development of tropical cyclones in the South China Sea have been investigated based on the ERA5 high-resolution monthly atmospheric reanalysis data,and their seasonal changes are obtained.The tropical cyclone genesis potential index GPI has been used to explore the ability of GPI in describing the characteristics of tropical cyclones in the South China Sea,and the contributions of various environmental factors to the generation of tropical cyclones been discussed.In addition,the environmental factors and the tropical cyclone characteristics at different development stages of El Ni(?)o have been discussed.The results show that:(1)In terms of seasonal changes,TCs(Tropical Cyclone)in the South China Sea mainly occur in the northern hemisphere summer and autumn,and its seasonal variation features a single-peak structure.Before October,TCs in the South China Sea mainly occur to the north of 12.5°N.After October,it mainly occurs to the south of 12.5°N.The seasonal distribution characteristics of environmental factors are highly consistent with the generation of TCs.(2)The GPI index can well describe the spatial distribution of TCs in the South China Sea and the seasonal variation of TC occurrence frequency.In different seasons,various environmental factors contribute to GPI to varying degrees.Vertical wind shear in May,October and November becomes the main contributor,and the potential intensity is the secondary contributor.The relative humidity contributes the most from June to September.The relative contribution of various environmental variables to GPI has a significant difference in the spatial distribution.(3)The frequency of TCs occurred in the South China Sea has an increasing trend before 2000 and a decreasing trend after entering the21 st century.The frequency of TCs that migrated into the South China Sea shows an increasing trend in the middle and late 1980s-1990 s,it then enters the low activity period from the late 1990 s to the mid-2000 s,and then slowly increases after that.(4)At different development stages of El Nino,the changes of environmental factors are different.There are fewer TCs in the El Nino year and the number of TCs in the El Nino decay year is more than that in the development year.This feature is especially obvious in November/December.In the El Nino development year,the absolute vorticity of 850 hPa plays a major role,the relative humidity of 600 hPa is the secondary factor,and the relative humidity is the major factor in the El Ni(?)o decay year.