The Influence Of Tropical Cyclones On The Upper Layer Ocean In The South China Sea

Using multiple satellite observed datasets and tropical cyclones dataset, combinedwith the Sverdrup theory,1.5-layer reduced gravity model and4.5-layer model, thepaper investigates the influence of tropical cyclones (TCs) on the upper layer ocean inSouth China Sea (SCS).There are a few tropical cyclones which influence the SCS every year, and theinfluence of these tropical cyclones on the SCS circulation is unclear. The researchshows that the TCs have an important influence on the basin scale and mesoscalecirculation in SCS. The results show that the TCs can induce a positive (negative)wind stress curl in the northwestern (southeastern) SCS in summer and a positivewind stress curl in whole SCS in winter. With these wind stress curl induced by theTCs, the cyclonic gyre in northern SCS and anti-cyclonic gyre in the southern SCS areintensified in summer. In winter monsoon period, the northern cyclonic gyre isintensified and the gyre in the southern SCS is weakened except in November andDecember when both the two gyres are enhanced. The model results also show thatthe dipole structure off central Vietnam in summer is intensified and in winter theeddy off northwest Luzon Island is weakended by TCs.Some of the TCs which influence the SCS are from the Northwestern Pacific andthe others are generated in the local SCS, and the different effect of the two kinds ofTCs on the SCS circulation is also unclear. We further investigate the different effectsof the SCS TCs and the NWP TCs on the SCS circulation. In summer of thesouthwesterly monsoon season, both SCS and NWP TCs enhance the meancirculation pattern of the cyclonic gyre in the northern SCS and the anti-cyclonic gyrein the southern SCS. However, the effect of SCS TCs is much larger than that of NWP TCs although the number of SCS TCs is smaller than NWP TCs. The reason is theSCS TCs-induced wind stress curl pattern is favorable for enhancing the summer SCSmean circulation.The TCs also have an important influence on the surface air-sea flux and theupper layer ocean structure. The results of our research show that:(1) From May toDecember, the wind energy input to the surface currents and waves induced by thetropical cyclones are1.26GW and11.65GW，and account for9.87%and5.44%of thetotal wind energy input to the surface currents and waves, respectively.(2) The heavyrainfall is always companying with the TCs. From May to December, the averagetotal precipitation in SCS is192.90mm/month, and25.41mm/month is induced bythe tropical cyclones, which accounts for13.71%of the total precipitation.(3) Insummer, the TCs will reduce the temperature of the mixed layer and deepen themixed layer in nearly whole South China Sea, and the TCs will deepen (shoal) thedepth of seasonal thermocline in the southern (nothern) SCS, the temperature of theseasonal thermocline is reduced in most areas of SCS.