Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China

摘要

The present study identifies a significant influence of the sea surface temperature gradient (SSTG) between the tropical Indian Ocean (TIO;15°S-15°N,40°-90°E) and the western Pacific warm pool (WWP;0°-15°N,125°-155°E) in boreal spring on tropical cyclone (TC) landfall frequency in mainland China in boreal summer.During the period 1979-2015,a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence,mid-tropospheric ascendance and upper-level divergence over the west-central TIO,and the opposite situation over the WWP,which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP.This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling,which facilitates the persistence of the anomaly until the summer.Consequently,lower-level negative vorticity,strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific (WNP),which decreases the TC genesis frequency.Meanwhile,there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region,meaning a westerly anomaly dominates over coastal regions of mainland China,which is unfavorable for steering TCs to make landfall in mainland China during summer.This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.