Influence of environmental factors on the tropical cyclone activity in the western North Pacific during the past 30 years (1980-2009)

Effects of environmental factors (sea-surface temperature, vertical wind shear, outflow temperature, upper-level divergence, low-level convergence and vorticity) on the tropical cyclone (TC) activity in the western North Pacific (WNP) during 1980-2009 are investigated using NCEP/NCAR reanalysis and Joint Typhoon Warning Center (JTWC) best track data sets. TC activity is represented by the annual frequency of TC occurrence and power dissipation index (PDI). Time series of the TC frequency and PDI in WNP revealed large inter-annual and inter-decadal variations. In the spatial distribution of the correlation coefficient between the TC frequency and the sea-surface temperature (SST), significant positive correlation is presented in the eastern part of WNP. The TC frequency is also positively correlated with the 850-hPa vorticity and 200-hPa divergence in the central part of WNP, while vertical wind shear and 850 hPa convergence are negatively correlated in the large area of WNP. Significant negative correlation between the outflow temperature and the TC frequency is shown in the northeastern part of WNP. The spatial distributions of the correlation coefficient between PDI and environmental factors are generally similar to those between the TC frequency and environmental factors. However, in the central part of WNP, the areas of the negative correlation between PDI and vertical wind shear decrease and those of the positive correlation between PDI and 850-hPa vorticity increase.