Upper Ocean Response To Tropical Cyclones In The Northern South China Sea

The South China Sea(SCS) is frequently affected by tropical cyclones(TCs) formed in the northwestern Pacific and knowledge of oceanic responses to TCs is helpful to understand the environment of the ocean, improve the forecast of TCs and prevent as well as reduce natural disasters. However, systematic researches on oceanic responses of the South China Sea to TCs are still few. The dissertation intends to explore the oceanic response of the abyssal, continental slope and continental shelf water to TCs systematically based on moorings, satellites and reanalysis data.In August 2011, Typhoon Nanmadol passed over the western Luzon Strait. The decrease of sea surface temperature(SST) occurs within 140 km from the track of Nanmadol, the right-hand bias of SST decrease is not evident and the maximum SST drop is-4.35℃. The resultant near-inertial waves(NIWs) concentrated in ocean interior with maximum velocity of 0.35 m/s and the vertical phase speed, group speed and wavenumber of the NIWs are 6.41 m/h、0.60 m/h and 225 m, respectively. The slowly varying, anticlockwise rotating wind is not favourable for the resonant interaction between the wind and the ocean. Thence the SST drop results mainly from upwelling which occurs near the typhoon center that consequently leads to a narrow SST drop band. Moreover, the weak resonant interaction results in weak NIWs because part of the wind energy enters to the Ekman current.In September 2008 and 2011, Typhoon Hagupit and Nesat passed over the continental slope of the SCS. The resultant SST drop happens along the typhoon track, shows a right-hand bias and recovers rather slowly. However, the resultant temperature changes, the maximum and the location of SST drop are different for Hagupit and Nesat. The oceanic dynamic response is NIWs that are stimulated by mixed layer(ML) currents and radiate into the ocean interior. The NIWs are blue-shifted and are dominated by the second baroclinic mode with vertical phase speed, group speed and wavenumber of 10 m/h, 2.10(2.80) m/h and 330 m, respectively.From July to September 2005, TC Washi, Vicente and Damrey passed over the continental shelf of the SCS. The oceanic thermal response to Washi are 15 m deepening and 2.00℃ cooling of the ML, 0.30℃ warming of the thermocline and 2.34% decline of the upper ocean heat content(OHC). The oceanic dynamic response to Washi is dominated by first mode NIWs that are blue-shifted and have a long e-folding time under the influenc of negative background vorticity. Thick ML cause inevident oceanic response to Vicente with 0.50℃ cooling of the ML and current less than 0.10 m/s. However, the oceanic response to Damrey is prominent with 15 m deepening and 4.50℃ cooling of the ML, 1.20℃ warming of the thermocline and 11.65% decline of OHC. The deepened ML and destroyed stratification cause weak dynamic response that is dominated by inertial oscillaitons.In summary, response of the continental shelf water is similar to that of the abyssal and continental slope water when intensity of TCs is relatively weak. The responses are deepening and cooling of the ML, warming of the thermocline, and downward propagating lower mode NIWs. However, when the intensigy of TCs is relatively strong the response of the continental shelf water shows different characteristics. The deepened ML and the destroyed stratification inhibit the baroclinic response resulting weaker dynamic response that is dominated by baratrophic inertial oscillations. Moreover, oceanic response to TCs in the northern SCS is affected by the parameters of the TCs and the background motinos of the ocean.