Abstract:This study utilises best-track datasets of tropical cyclones (TCs) from the Joint Typhoon Warning Center (JTWC), Japan Meteorological Agency Tokyo Regional Specialised Meteorological Center (RSMC), and Shanghai Typhoon Institute of China Meteorological Administration (CMA), along with NCEP reanalysis data and correlation analysis methods, to investigate the fundamental characteristics of TC intensity and frequency in different seasons (July-September for summer and October-November for autumn) and subregions over the Western North Pacific (WNP) during 1989-2020, as well as their relationships with local environmental factors. The results indicate that: (1) The total TC frequency observed in the three summer WNP subregions (Area I: 10°-25°N, 110°-145°E; Area II: 10°-25°N, 145°E-180°; Area III: 25°-37.5°N, 125°E-180°) is 10164, while the total TC frequency in the three autumn subregions (Area I: 5°-17.5°N, 110°E-180°; Area II: 17.5°-35°N, 142.5°E-180°; Area III: 17.5°-35°N, 120°-142.5°E) is 4984. This demonstrates significantly higher TC activity in summer than in autumn. (2) TC activity during both summer and autumn exhibits statistically significant correlations with 850 hPa relative vorticity (RVOR), except for Area II in autumn. In addition to low-level RVOR, summer TC frequency in Area I is associated with 500 hPa vertical velocity (OMEGA) and 700-500 hPa relative humidity (RHUM). Summer TC intensity in Area II is linked to 700-500 hPa RHUM, 500 hPa OMEGA, and vertical wind shear (VWS), while summer TC frequency in Area II correlates with 500 hPa OMEGA and VWS. Autumn TC frequency in Area I shows a relationship with 500 hPa OMEGA. Regardless of season, TC activity generally displays either nonsignificant correlations or significantly negative correlations with oceanic factors. The local environmental factors influencing TC activity vary across regions and seasons, depending on seasonal and regional divisions, which indicates complex interactions. (3) A strong monsoon trough enhances TC activity in summer and autumn Area I through low-level high RVOR, mid-level ascending motion, and high humidity. The warming of sea surface temperature in the Nino3.4 region can induce anomalous westerly winds at 850 hPa in Area II during summer, leading to increased low-level RVOR. Concurrently, an anomalous anticyclonic circulation emerges at 200 hPa, resulting in reduced VWS. The combination of the enhanced low-level RVOR and upper-level anticyclonic circulation further strengthens mid-level upward motion and moisture convergence, thereby influencing TC activity in this region. When the Western Pacific subtropical high extends westward (retreats eastward), it generates localised negative (positive) vorticity anomalies, leading to reduced (increased) TC frequency entering summer Area III. Strengthened mid-level steering flows further enhance the intrusion of intense TCs into this region. The convergence in the relative low region between two highs enhances low-level RVOR, which consequently modulates TC activity in Area III during autumn.
