Long-lasting upper ocean temperature responses induced by intense typhoons in mid-latitude.

The sea surface temperature (SST) drops rapidly when a typhoon passes over the western North Pacific, and the cold SST is known as cold wake. In general, more intense typhoons on the day of arrival cause stronger SST cooling via turbulent oceanic vertical mixing. Moreover, after intense typhoons have passed, there are cases in which the SST decreases further, and the cold conditions persist for approximately 2 weeks. In this study, we suggest possible mechanisms by which long-lasting cold SST responses to typhoon forcing are related to the generation of cold-core-like ocean circulation. The atmospheric surface cyclonic circulation causes divergent anticlockwise upper ocean currents owing to the Ekman transport, which in turn induces further upwelling and strengthens the cold SST. In the European Center for Medium-Range Weather Forecasts Ocean Reanalysis System 5, cold-core-like ocean current responses were strong in 5 typhoons among the 12 intense typhoons that passed through 30°N in the western North Pacific region from 2001 to 2019. The favorable conditions for a cold-core circulation to occur can be summarized as a slow typhoon migration speed with strong intensity, well stratification of vertical ocean layers, and the absence of large-scale strong background currents.