Characteristics And Causes Of Precipitation Changes During Autumn Flood Season In Hainan Islan

The main peak of precipitation in Hainan Island occurs in September–October,which is significantly different from other areas in South China.Based on the observed daily precipitation data from 18 meteorological stations in Hainan Island,TRMM daily precipitation data,UK Hadley Centre sea surface temperature(SST)and ERA5 reanalysis datasets from 1998 to 2017,this paper systematically studies the variation characteristics and possible causes of formation of the autumn heavy rainy season over Hainan Island(AHRSHI)using statistical analysis and numerical model simulations.The main conclusions are as follows:(1)In autumn,the East Asian summer monsoon begins to shift to the winter monsoon.Influenced by the northeast monsoon,the air over Hainan Island is in the easterly flow belt,and the precipitation in the AHRSHI is obviously more in the east than in the west.Moreover,the precipitation center is located on the windward side of the mountain front,and the easterly flow under the blocking and lifting effect of the bell-shaped topography let to the increase of precipitation in the east.At this time,the SST of the South China Sea(SCS)in autumn is still in a relatively high state,which is conducive to maintaining and enhancing the AHRSHI.It is verified that the topography of Hainan Island,the sea-land thermal contrast,and the high SST in the SCS play a decisive role in the formation of the AHRSHI by a series of numerical model simulations.(2)The interannual variation of precipitation in the AHRSHI is obvious,which is mainly affected by the large-scale circulation anomalies.There is an abnormal anticyclone over Taiwan and an abnormal cyclone over the northern SCS,which makes the equatorial Indian Ocean and Pacific Ocean transport water vapor to the vicinity of Hainan Island,resulting in the convergence of water vapor over the region and providing abundant water vapor conditions when precipitation is more than normal.In addition,the Walker circulation has increased abnormally when precipitation is more than normal.However,the opposite is true when precipitation is less than normal.(3)There is a significant positive correlation between the South Atlantic SST in the previous spring and the interannual component of precipitation in the AHRSHI.In the previous spring,the South Atlantic SST continued abnormal warming,which caused abnormal easterly wind in the central and eastern Pacific through the two possible influence paths of the zonal vertical circulation anomaly in the equatorial region and the teleconnected Rossby wave train in the middle and high latitudes of the northern hemisphere,and promoted the negative SST anomaly in the equatorial eastern Pacific.Through the Bjerknes positive feedback mechanism of the tropical air-sea coupling system,the SST gradient of the equatorial Pacific cold in the east and warm in the west is increased,resulting in the enhancement of the Walker circulation anomaly,which is conducive to the generation of ascending motion to form precipitation.