Roles of different physical processes in upper ocean responses to Typhoon Rammasun (2008)-induced wind forcing

Abstract

This study investigates the roles of different physical processes in the oceanic response to tropical cyclones (TCs) in the Pacific, using an ocean general circulation model with several numerical experiments. A case study is focused on Typhoon Rammasun, which passed through the northwestern tropical Pacific in May 2008. TC-induced wind stress fields are extracted using a locally-weighted regression (Loess) method from a six-hourly Cross-Calibrated Multi-Platform satellite scatterometer wind product. By comparing model experiments with TC wind forcing being explicitly included or not, the effects of TC on the ocean are isolated in a clean way. The local oceanic response is characterized by a cooling in the surface layer that persists along the typhoon track as a cold wake, and a deepening of the mixed layer (ML). The TC-induced wind can affect the ocean through the momentum effects, the ML processes (the stirring effect on the ML depth), and heat flux (via wind speed), repectively. Analyses of numerical experiments with these different underlying processes explicitly represented or not indicate that vertical mixing and upwelling are dominant processes responsible for surface cooling, while the surface heat flux also plays a non-negligible role. Specifically, vertical mixing, upwelling and surface heat flux account for respectively ~53%, ~31% and ~16% of the sea surface temperature cooling. However, for the ML response, the vertical mixing and surface heat flux are dominant processes for the ML deepening, while the contribution from upwelling process is negligible. This study provides new insights into how TC-indcued wind forcing affects the ocean by isolating each different individual process in a clear way, which differs from previous direct heat budget analyses.