Modulations of El Nin≈o-Southern oscillation on tropical cyclone activity affecting Taiwan

Tropical cyclone (TC) activity affecting Taiwan and El Nin??o-Southern Oscillation (ENSO) exhibit systematic relationships in the July???September season. El Nin??o events can induce more [EN(more TC)] or fewer [EN(fewer TC)] TCs affecting Taiwan, but fewer TCs are dominantly caused by La Nin??a events [LN(fewer TC)]. In the EN (more TC) type, a meridionally stratified circulation pattern exists consisting of a westward-extending anomalous cyclone in the south over the western North Pacific (WNP) and anomalous anticyclonic shears in the north. Consequently, TC genesis tends to increase in the tropical WNP south of 20oN and decrease to the north. In-creases of northward and northwestward TC movement from the tropical WNP toward the subtropical north-western WNP result in more TCs affecting Taiwan. Variability features of TC activity and large-scale oceanic -atmospheric patterns in the LN(fewer TC) type are generally opposite to those of the EN(more TC) type. In the EN (fewer TC) type, an anomalous cyclone elongates northwestward from the tropical WNP toward Taiwan and Japan. TC genesis increases in the tropical WNP east of 150???E. These TCs exhibit enhanced northward move-ment. In the 120o-150???E region, suppressed TC formation is followed by reduced westward TC movement from the WNP toward Taiwan, leading to fewer TCs affecting Taiwan. For atmospheric circulation variability in the WNP, both the monsoon trough (MT) and western Pacific subtropical high (WPSH) weaken in the LN(fewer TC) type, but intensify in the EN(more TC) type. A significant southeastward intensification of the MT and an eastward retreat of the WPSH occur in the EN(fewer TC) type.