Interannual Changes of Tropical Cyclone Intensity in the Western North Pacific

The individual contributions of changes in sea surface temperature (SST), vertical wind shear and tropical cyclone (TC) tracks to the interannual TC intensity change in the western North Pacific (WNP) basin are examined based on the selected 7 warm years and 7 cold years during the period 1970-2007. The selected warm and cold years are defined by the Nino-3.4 SST anomalies index, and correspond to El Nino and La Nina events, respectively. The intensity model used in this study can simulate the spatial distribution and differences of TC intensity when the model is integrated along the observed TC tracks in the warm and cold years. It is found that the change of TC tracks plays a dominant role in the observed TC intensity difference between warm and cold years. During the warm years, TC formation is enhanced in the southeast quadrant, and more TCs take a northwestward track during the warm years than during the cold years because of the interannual change in the large-scale steering flows. As a result, TCs have a longer time for intensification and develop into intense TCs during the warm years when compared to the cold years.