Upper Ocean Response Of The Northwestern South China Sea To Tropical Cyclone

The South China Sea has a frequent occurrence of tropical cyclones. Tropical cyclone could transport the momentum into the ocean and take away the heat during the process of moving, and in a short time it can cause significant changes of heat and kinetics in the ocean. As an important physical process in the South China Sea, tropical cyclone has an important impact on ocean physics, chemistry and ecological environment. So it is very meaningful to understand the response of the upper ocean to tropical cyclones.In this paper we use moored observation data and the remote sensing data of tropical storm Washi to study the response characteristics of upper ocean on dynamic and thermodynamic in the Northwestern South China Sea. We have drawn some important conclusions, and the mechanism of the process is analyzed in the paper.Tropical storm Washi passed the subsurface moorings in the Northwestern South China Sea on 29 to 30 July 2005 and the upper ocean thermal and dynamical response was examined in detail. Intense sea surface temperature cooling was induced by Washi, and the maximum cooling was 3.6 °C on the right track of Washi. The whole observed water column(4-75 m) was cooled by Washi, with maximum cooling of 2 °C at the mooring station. Because of the strong mixing process, the mixed layer depth deepens as much as 10-20 m. There is a strong upwelling in the low temperature zone during the period of the tropical storm, and the Ekman pumping speed is greater than 1.6×10-4 m/s. By using the heat balance of the mixed layer, the heat loss caused by each dynamic factor is calculated. The results show that the vertical mixing plays a leading role, accounting for 70%, and the vertical convection is relatively weak. After the passage of Washi, strong inertial currents are generated by the storm with the max currents up to 0.2 m/s in the upper mix layer and with a period of 35 h, which is slightly less than the local inertial oscillation. Based on the field observation data, the turbulent eddy diffusivity is found to have increased by more than 3 times.