Comparison of physical and biological responses to tropical cyclones between the low and middle latitude zones of the western North Pacific

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Upper ocean conditions vary considerably between latitudes, which results in different responses of the ocean to tropical cyclones (TCs). During the TC season, many TCs in the western North Pacific (WNP) extend northward from the low latitude zone (LLZ) to the middle latitude zone (MLZ). We studied the differences in the influence of TCs on the LLZ and MLZ by comparing the changes in the mixed-layer depth (MLD), sea surface temperature (SST), and chlorophyll-a (Chl-a) concentrations before and after 32 typical TCs that occurred from 2006 to 2019 in these two regions. Although the mean intensity and transit time of these TCs in the MLZ were lower than in the LLZ, the mean MLD deepening was greater, the SST cooling was more significant, and the surface Chl-a concentration increases were more pronounced in the MLZ after TCs. Differences in the responses of these two WNP regions to TCs were directly related to the differences in their upper ocean conditions. Compared to the LLZ, the MLZ had a shallower MLD, nutricline, and subsurface chlorophyll maxima depth, lower temperature, and higher nutrient concentrations in the subsurface layers. The upper ocean conditions in this region were relatively more susceptible to alteration after TCs, and the subsurface water, nutrients, and phytoplankton were more readily brought to the surface, largely contributing to the SST cooling and surface phytoplankton bloom. In particular, as the latitude increases, the degree of response of MLD, SST and surface Chl-a concentration after TCs may be more pronounced. These findings indicate that differences in responses to TCs in ocean areas at different latitudes were mainly due to differences in upper ocean physical and biochemical structure rather than to intensity and transit time of TCs.