The role played by tropical cyclones-induced freshwater flux forcing in the upper-ocean responses: A case for Typhoon Yutu (2018)

Tropical cyclones (TCs) produce heavy precipitation and provide excessive freshwater flux (FWF) to the ocean, which alters surface salinity and upper-ocean stratification, leading to changes in ocean physical processes. At present, roles played by TC-induced FWF (FWFTC) forcing in the upper-ocean responses have received less attention than those by TC-induced wind stress (τTC) forcing. An ocean model is used to investigate the role played by FWFTC forcing in upper-ocean responses in the Pacific; several numerical experiments and observation-based analyses are performed, focusing on Typhoon Yutu (2018) . The FWFTC and τTC components are explicitly extracted from atmospheric reanalysis fields and are then taken into account or not in model sensitivity experiments. As expected, the FWFTC forcing acts to freshen the ocean surface, enhance the ocean stratification and reduce the mixing and entrainment of subsurface cold water into the mixed layer (ML), leading to potentially favoring surface warming; this FWF effect is opposite to that induced by τTC forcing (i.e., a cooling effect on sea surface temperature (SST)). The combined effects of FWFTC and τTC forcings lead to local ocean responses to Typhoon Yutu that are characterized by surface cooling and salting, which are dominated by the τTC forcing and are also modulated by the FWFTC forcing, depending on their relative magnitudes. When FWFTC forcing reaches a certain intensity and the τTC forcing is weak, however, the warming effect on SST due to FWFTC forcing can even exceed the cooling effect due to τTC forcing. When FWFTC forcing is relatively stronger than τTC forcing (e.g., TC-induced precipitation >10.37mmh−1 and τTC∼0.14Nm−2), the warming effect on SST due to FWFTC forcing can even exceed the cooling effect due to τTC forcing. Meanwhile, with the combination of τTC forcing, the FWFTC forcing can also promote SST cooling induced by the τTC forcing, which is dominated by mixing and entrainment. In this case, the FWFTC forcing increases surface heat loss from the ocean due to enhanced stratification, which contributes to τTC-induced vertical mixing and surface cooling. Further case studies are needed to thoroughly evaluate FWFTC-related effects on the upper ocean.