Contribution of Tropical Indo-Pacific Precipitation Anomaly to Seasonal Differences of Long-term Changes in Tropical Cyclone Genesis Frequency over the Western North Pacific

Based on model experiments, we examined potential dynamical mechanism of seasonality of long-term changes in tropical cyclone genesis frequency over the western North Pacific (WNP) during 1982-2020, which was reported in a previous study by Chang et al. (2021). They reported a remarkable reduction in the genesis frequency of tropical cyclones over the WNP only during from October to December (OND). They also stated that an anomalous anticyclone throughout the main development region of tropical cyclones only during OND would be the main cause, which could be related to the rich-get-richer mechanism. Because their investigations are only based on observation, we conducted model experiments to confirm mechanisms proposed by them. MPAS-A experiments forced by sea surface temperature warming suggest that the precipitation increased in the climatological convective regions of the tropical Indo-Pacific Ocean, which is matched to the rich-get-richer mechanism. Furthermore, the anomalous anticyclone is stronger and expand more westward during OND than other seasons. We also performed the Linear Baroclinic Model (LBM) experiments forced by diabatic heating related to seasonal precipitation anomalies. Results show that the large-scale circulation over the WNP is likely driven by these diabatic heating in the climatological convective areas. The climatological convective area locates further westward (5°S-10°N, 80°E-110°E) during OND than the other seasons, the anomalous anticyclone, which induced by increased convection, could more extend to the western edge of the WNP.