Comprehensive Investigation on Spatiotemporal Variations of Tropical Cyclone Activities in the Western North Pacific, 1950-2019

Tropical Cyclones (TCs) are the most severe natural disasters in the Western North Pacific Ocean (WNP). While previous studies reported evident changes over certain regions or seasons between typical periods, there is a lack of a complete picture of the long-term variations in TC activities in the WNP. This study carried out a comprehensive investigation on the spatiotemporal variations in TC genesis locations and transit pathways in the WNP, based on the TC best-track datasets from the China Meteorological Administration Shanghai Typhoon Institute. The results showed that the TC genesis and occurrence frequencies showed drastic decreases and westward shifts in the WNP from 1950 to 2019. The greatest decrease in TC genesis occurred for Tropical Depressions (TDs: 10.8-17.1 m/s) and Typhoons (TYs: > 32.7 m/s). The number of Tropical Storms (TSs: 17.2-32.6 m/s) and the mean intensity (2 min maximum sustained wind speed) of TCs overall showed no evident change. The decadal average of TC genesis frequency increased by 63% in the near-coast seas (WNPO), but decreased by 46% near the central Pacific Ocean (WNCP), demonstrating a westward shift for TC genesis locations. The TC genesis and occurrence frequencies also showed significant declines in the southern Philippine Sea (SPS) and South China Sea (SCS), while they showed a lower reduction in the Eastern East China Sea (EECS), the northern ocean edge of TC genesis areas, resulting in an increase in the average latitude of TC genesis locations, a spurious northward shift. The La Nina and El Nino years showed contrasting effects on TC genesis frequency and landfall ratios. There were greater instances of TC genesis and greater landfall ratios during the La Nina mature phase, while there were fewer TC formations and lower landfall ratios during the short duration (SD) El Nino developing phase. The TC genesis locations showed a distinct northwestward shift during La Nina years compared to those during El Nino years.