Upper Oceanic Response Of East China Sea To Typhoon

Typhoons are extremely disastrous weather systems.The passage of typhoons leads to severe strong wind and storm surge along the coast,and generates huge waves on the sea surface.Typhoon causes serious losses to the lives and properties of people in coastal areas and to maritime transport,marine engineering and fisheries.Raninim is a typhoon in August 2004.It had a very high wind speed of over 58 m/s and high rainfall intensity with maximum rainfall exceeding 900 mm.The affected area reached to 50 counties and cities and was a very intense typhoon to land on the mainland of China since 1979.It caused serious losses to the lives and properties along the southeast coast of China.A followed typhoon,Megi happened to cross the East China Sea continental shelf from south to north.The study of these two typhoons is not only of great scientific significance,but also of great social value.In this paper,the upper oceanic response to two consecutive typhoons in the East China Sea is studied by using the MITgcm model.The results shows that when the two typhoons pass through the East China Sea,they causes an extensive sea surface temperature(SST)cooling in the East China Sea with a maximum cooling of 4℃.The SST cooling lags the typhoon by one day and shows significant rightward bias to the typhoon track.Due to the strong stirring action of typhoon,the mixed layer deepens by 10 to 20 m and the seawater temperature in the mixed layer decreases.The analysis of temperature control equation and continuity equation shows that the strong vertical diffusion caused by flow shear,i.e.entrainment and the upwelling caused by vertical convection are the main driving mechanisms of the seawater cooling.During two consecutive typhoons,the seawater cooling has cumulative effects.During the second Typhoon Megi,the SST continues to cool and the depth of the mixed layer continues to deepen.But both the extent and rate of SST cooling and mixed layer deepening is much smaller than the first typhoon Rananim.When Typhoons pass through the East China Sea,it causes a cyclonic current field on the surface and the current direction is about 45 degrees angle to the right of the wind stress.Once the surface current forms,it rotates clockwise quickly.After the typhoon disappears,the current is featured by a near-inertial oscillation with period of about 1 day.The current direction in the mixed layer is opposite to that in the deep layer.Vertically,the current propagates with an upward phase.The upward phase means that energy is propagated downward and the near-inertial waves propagates the mechanical energy in the upper ocean input by typhoon into the lower ocean.