The archipelago-induced wakes and their impact on typhoon air-sea interactions in the northwestern South China Sea

The topography of an archipelago can induce strong wakes in the surrounding ocean. However, the effects of these wakes on air-sea interactions during typhoons are often overlooked in numerical simulations. In this study, a high-resolution (~1 km) coupled atmosphere-ocean model is used to investigate the wakes and eddies induced around the Xisha Islands in the South China Sea and their impact on air-sea interactions during two typhoons: Ketsana (2009) and Doksuri (2017). The results of comparative experiments with and without the topography show that the sub-mesoscale eddies in the wakes around the Xisha Islands enhance the exchange of water between layers up to 120m deep by 50% before the arrival of the typhoons. This preconditioning cools the surface layer and shoals the mixed layer, creating a thermodynamic environment that is unfavorable for typhoons. During the typhoon passage, the wind-induced mixing and upwelling become even stronger. The reef basin-induced wakes further increased the vertical water exchange of the upper ocean by 30-50%. These wakes enhanced vertical advection, cooled the upper ocean, and reduced the sea surface heat flux, resulting in weaker typhoon intensities by up to 8.7 hPa for Ketsana (2009) and 8.8 hPa for Doksuri (2017). These findings highlight the significant impact of wakes around the archipelago on regional ocean conditions and typhoon intensity. Accurately resolving these fine-scale topographic effects in coupled models is therefore essential for improving the simulation and prediction of typhoon-ocean feedbacks in archipelago regions.