Observational Analysis And Numerical Simulation Of The Upper Ocean Response To Typhoon Linfa In The Northeastern South China Sea

The passage of typhoon can usually cause remarkable responses of marine environment.Based on the observational analysis and numerical simulation of the upper ocean response in the northeast South China Sea(NESCS)caused by the Typhoon Linfa(1510),in this paper,the response process of typhoon was studied from the aspects of thermodynamics and dynamics.By the observational analysis,it was found that there were two obvious deflections of typhoon moving direction after the underlying surface changed from land to ocean(when the typhoon left from Luzon Island)and from ocean to land(before the typhoon landed from eastern Guangdong)during the typhoon period.There were two stages of the sea surface cooling caused by typhoon:the first stage was mainly caused by the heat absorption in the center of typhoon(the sea surface heat loss could reach 60 W/m~2 per second)and more significant on the left of the typhoon track.The second stage was mainly caused by Ekman pumping after the typhoon with a stronger cooling than the first and more significant on the right of the typhoon track.When typhoon approached the coastal upwelling area of eastern Guangdong,the vertical coastal current current could be significantly increased.Typhoon could cause vertical temperature decrease for about 2.5?,which lagged the flow for about1?2 days.The vertical salinity decreased for about 1.3 psu which lagged the flow for about 2?3days.The EOF analyses on the wind,SST and precipitation were conducted to study the temporal and spatial distribution characteristics of these variables.It was found that the wind field in the NESCS showed a pattern with opposite spatial pattern in eastern and western areas and the wind enhancement lasted for about 5 days,which showed more obvious tendency of increase to the right of typhoon track and showed obvious local characteristics near the shore.The precipitation increased consistently in the entire NESCS and the increase lasted for about4 days,which showed a stronger increase to the left of the typhoon track and a obvious local characteristics near the northwestern Luzon Island.The SST decreased in most part of the NESCS and the decrease lasted for more than 8 days which lagged behind the wind field for about 2?3 days.The whole cooling process from July 5th to 15th was more affected by Ekman pumping than by sea surface heat loss,which showed that the cooling effect was more significant on the right of the typhoon track.By the numerical simulation,it was found that the typhoon caused the increase of the Isothermal Layer Depth(ILD)in the NESCS and it was most significant in the northwest of Luzon Island.Affected by the typhoon structure,the ILD was obviously different inside and outside the typhoon center.There were three stages of thermodynamics in the turning area of typhoon track.In the first stage,before the typhoon center reached the area,the flow field caused by the Coriolis force to form the onshore current and the parallel coastal current along the southwest.In the second stage,when the typhoon center reached the area,the advection term and diffusion term of momentum caused a strong parallel coastal current along the northeast.In the third stage,after the typhoon center passed through the sea area,the southwest parallel coastal current caused by diffusion was significantly enhanced.The intensity and moving speed of typhoon were important to the ocean response.The idealized typhoon simulation was ideal in the open sea area,and the simulation error is large in the nearshore and land.Gradient wind played an important role in the variation of typhoon wind speed.With the increase of gradient wind,the velocity of vertical movement of sea water increased,resulting in the vertical circulation of central cold water upwelling and peripheral warm water descending.