Decrease of tropical cyclone genesis frequency in the western North Pacific since 1960s

Abstract

Tropical cyclone (TC) genesis frequency in the western North Pacific (WNP) during 1960–2014 shows a step-by-step decrease on interdecadal timescale, in accordance to the phase of the Interdecadal Pacific Oscillation (IPO). The environmental parameters responsible for the interdecadal change of TC genesis frequency were investigated. It was found that vertical wind shear especially the zonal wind shear plays a critical role, while other parameters such as sea surface temperature (SST), vertical velocity, divergence, humidity and maximum potential intensity cannot explain the step-by-step decrease of TC genesis frequency.A further diagnosis shows that the interdecadal change of vertical wind shear is caused by SST and associated rainfall pattern changes across the Indo-Pacific Ocean. A stronger warming in the Indian Ocean/western Pacific from 1960–1976 to 1977–1998 led to enhanced convection over the Maritime Continent and thus strengthened vertical shear over the key TC genesis region in the WNP. A La Nina-like SST pattern change from 1977–1998 to 1999–2014 led to a strengthened Walker circulation in the tropical Pacific, which further enhanced the vertical shear and decreased TC genesis frequency in the WNP.