Typhoon Impact On Coastal Upwelling Off The Eastern Hainan Island

The upwelling along the eastern coast of Hainan Island is an important phenomenon in the northern part of the South China Sea.Based on observation analysis and numerical simulation of upwelling area in the east of Hainan Island during the passage of Typhoon Rammasun(No.9)in 2014,the response process during typhoon was studied from the perspective of thermodynamics and dynamics.Through the analysis of SST observation data during the passage of typhoon,it was found that the response of SST lagged behind the change of wind field by 1-2 days,and the chlorophyll A concentration increased significantly in the upwelling area 4-5 days after the passage of typhoon.The upwelling intensity was weakened during the passage of typhoon and the center of the upwelling area moved southwest.Through numerical simulation,it is found that the mixing layer depth in the upwelling area caused by typhoon deepens and Ekman pump rate has lag of energy transfer in the process of typhoon transit.The coastal upwelling rate calculated by using the coastal wind stress verifies the conclusion that typhoon weakens the upwelling intensity during typhoon transit.It is found that coriolis force and pressure gradient force play a dominant role in analyzing the change of specific momentum term during typhoon passage.It is found that advection term and diffusion term have certain contribution in controlling the local temperate change rate during typhoon transit.The advection term shows obvious internal wave signal near the southeast shelf of Hainan Island.At the same time,a near-inertial oscillation with a frequency of about 0.705 CPD was triggered in the coastal upwelling of Qiongdong.The near-inertial wave oscillations lasted for about two weeks and caused strong vertical mixing of the upwelling along the Qiongdong coast after the typhoon made landfall.In general,the wind field and near-inertial wave during the typhoon jointly dominate the temperature change of upwelling along qiongdong coastal area during the passage of Typhoon Rammasun.