Ocean Response To Typhoon Morakot (2009) And The Influence On Its Activity

Typhoons are strong atmosphere systems happened above the ocean, which are the most intuitionistic expressions of air-sea interaction.The cyclonic stress with typhoons always force strong mixing processes which are important to the processes of the heat, and energy exchange between the upper ocean and the deep ocean.Therefore, it’s necessary to study the ocean response to the typhoons. Specially, the water masses around Taiwan Island formation and variety have many characteristics and the oceanic environment become complex. These may affect the ocean response. A couple coastal ocean model GRAPES-ECOM is been introduced in this paper. The model developed by Shanghai Typhoon Institute of China Meteorological Administration is used to study the ocean around Taiwan Island response to Hurricane Morakot.While previous studies indicate that typhoons can decrease sea surface temperature (SST) along their tracks, a few studies suggest that the cooling patterns in coastal areas are different from those in the open sea. However, little is known about to how the induced cooling coupled with the complex ocean circulation in the coastal areas can affect tropical cyclone track and intensity. The sea surface responses to the landfalling process of Typhoon Morakot (2009) are examined observationally and its influences on the activity of the typhoon are numerically simulated with the WRF model. The present study shows that the maximum SST cooling associated with Morakot occurred on the left-hand side of the typhoon track during its landfall. In addition, Morakot’s wind increased the mixed layer depth and induced strong upwelling along the southeastern coast of Taiwan island. Moreover, the cold eddy which locates the northeastern of Taiwan Island has been strengthened after the typhoon passed. Numerical simulations show that, together with the SST gradients associated with the coastal upwelling and mesoscale oceanic vortices, the resulting SST cooling can cause significant difference in the typhoon track, comparable to the current24-hour track forecasting error. It is strongly suggested that it is essential to include the non-uniform SST distribution in the coastal areas for further improvement in typhoon track forecast.