Enhanced understanding of poleward migration of tropical cyclone genesis

The known trends of poleward migration for the tropical cyclone (TC) genesis in both hemispheres are discussed from different perspectives. It is shown that the poleward migration rate of the annually averaged latitude of TC genesis in the Northern Hemisphere is significantly affected by the regional variations of TC number in recent decades, especially an increase in the North Atlantic Ocean and a decrease in the western North Pacific Ocean. The poleward migration rates of TC genesis in the two hemispheres get closer when the effect of the regional TC number variation is excluded. The poleward migration of TC genesis without the effect of regional TC number variation is found to have a good correlation with the poleward shift of the edges of the tropics in both hemispheres. A decreasing trend of the cyclonic vorticity in the lower-troposphere over the tropical ocean regions is also identified in both hemispheres, which leads to a poleward shift of the equatorward boundary for TC genesis. The poleward migration of TC genesis after the effect of regional TC number variation is excluded and can thus be considered as a result of the tropical expansion. It is shown that the genesis of TCs with a different intensity has a different migration rate. When excluding the effect of the regional TC number variation, the poleward migration of TCs with a different intensity has a similar trend in both hemispheres. The tropical storms and intense typhoons have significant poleward migration trends, while the weak typhoons behave differently.