The Response Of The South China Sea To The Tropical Cyclones Over The Northwestern Pacific Induced Wind Anomalies In The Outer Area

Based on the tropical cyclone dataset named ‘best track data',atmospheric and oceanic reanalysis data,and model data,combined with synthetic analysis and dynamic diagnosis method,the present paper invetigated the impact of the wind anomalies induced by the tropical cyclones(TCs)over the northwestern Pacific(NWP)on the tropical cyclones geneses and the sea surface temperature(SST)in the South China Sea(SCS)in summer(May-September).The results show that:(1)62% of the total SCS TCs geneses in summer are accompanying with the NWP TCs.66% of SCS TCs form in the area with 900-2700 km distance to the NWP TCs,the minimum distance is about 906 km.The genesis location of SCS TC shifts eastward(northward)when the location of the NWP TC is more eastward(northward).The result of the composite analysis shows that the NWP TCs induce an anomalous cyclonic atmospheric circulation in the SCS,which enhances the southwest monsoon in the central-southern SCS and the wind jet off central Vietnam.The anomalous wind field induced by the NWP TCs increases the relative vorticity in the lower layer in the northern SCS and the vertical velocity and relative humidity of the middle layer in the central-southern SCS,and reduces the vertical wind shear between the upper and lower layers in the northern SCS These changes of the environmental factors are favorable for the TC genesis,leading to a significant increase in the genesis potential index(GPI)in the northern SCS.Further analysis shows that the daily genesis rate of SCS TCs during the NWP TCs period is 53% higher than the climatological value.In addition,compared to the climatological developed ratio of tropical cloud cluster(TCC)(12.3%),the developed ratio of the TCCs that generated before the NWP TCs entering the core region of NWP(the Pre-TCCs)in SCS during the NWP TCs period significantly increases to 16.4%.What's more,the daily genesis rate of TCC is significantly higher than the climatological value,which indicates the NWP TCs can trigger the TCCs geneses and promote the TCCs developing to TCs.(2)The anomalous atmospheric cyclonic circulation induced by the NWP TCs strengthenes the soutwest monsoon in the central-southern SCS,which increases theevaporation of sea surface water vapor,and then promotes ocean lose heat,resulting in SST decreases in the central-southern SCS.In the northern shelf area,the northeast wind anomaly weakens the ocean upwelling,leading to an increase in SST.Further analysis of the net heat flux term and the horizontal advection term of the temperature equation reveals that the contribution of the net heat flux term is larger.The NWP TCs induce ocean lose heat in the central-southern SCS,which causes SST decrease.The contribution of the horizontal advection term to the temperature during the NWP TCs period is mainly to increase the eastward transfer of cold water in the area east of Vietnam,making SST decease in the central SCS.In addition,the intensified southwest monsoon induced by the NWP TCs can enhance the wind jet and ocean eastward jet off central Vietnam.The stronger wind jet off central Vietnam can make the ocean lose heat and the intensified ocean eastward jet can bring more cold water to the east,resulting in the cold tongue east of Vietnam is enhanced and extends to the east and north.