Estimation of biophysical properties of areas in the Western North Pacific with a high frequency of tropical cyclones

Tropical cyclones (TCs) are among the most important factors that influence the upper oceanic environment of the Western North Pacific (WNP). To better understand the effects of TCs on the changes in the physical and biological processes of the WNP, two regions with similar properties but different TC frequencies were studied. Unlike previous studies, the long-term effects of TCs on the biophysical properties of the WNP throughout the TC season were discussed. The biophysical properties of both regions during the TC season were compared using satellite-derived measurements and Argo float data during 2003–2017. During the TC season, short-term rapid enhancement of the wind during TC passages had a direct effect on the cooling of the sea surface temperature and the deepening of the mixed-layer depth. Moreover, surface nitrate concentrations in the TC-prone region were significantly higher than those in the region not heavily influenced by TCs. As a result, the mean surface chlorophyll-a (Chl-a) concentration in the region with a high TC frequency was significantly higher than that with a low TC frequency. The Chl-a concentrations also tended to be higher during the El Niño years because of more strong TC events in both regions, while they were generally lower during the La Niña years when there were fewer strong TCs. Based on a conservative estimate, new primary production in TC season caused by TCs in the region with a high TC frequency accounted for approximately 1–7% of the annual amount in this region during the study years. These findings suggest that the TCs play an important role in the enhancement of biological carbon fixation in parts of the WNP.