Improvement of genesis potential index for Western North Pacific tropical cyclones

The genesis potential index (GPI) suggested by Emanuel and Nolan (2004) and Murakami and Wang (2010) is widely used, which simulate well the climate distribution of tropical cyclone (TC) genesis, but with a poor simulation of interannual variation of TC genesis. Our study attempts to improve the GPI over western North Pacific (WNP) by considering the important impact of relative vorticity on interannual variation of TC genesis. Based on the Best-track from Joint Typhoon Warning Center and NCEP/NCAR reanalysis data from 1979 to 2011, we substitute the absolute vorticity in previous GPI with a modified relative vorticity. The Coriolis factor is still remained. The difference of tropical cyclone formation between the South China Sea (SCS) and the WNP is also taken into account and develop the GPI for the SCS (5-25°N, 100-120°E) and the WNP (5-40°N, 120-180°E) respectively. The modified GPI improve the simulation of the climate distribution of TC genesis. Moreover, the modified GPI much improve the simulation of the interannual variation of TC genesis, compared with the previous GPI.