El Nino and La Nina effects of tropical cyclones: the mechanisms

Abstract

The effects that El Nino and La Nina events exert on western North Pacific tropical cyclones, and the physical mechanisms involved were examined using best track data from the Joint Typhoon Warning Center and NCEP reanalysis data. During El Nino and La Nina events, equatorial heating anomalies induce anomalous tropical and extratropical atmospheric wave trains which alter circulation, vertical shear, and steering flow. The shear changes cause tropical cyclones to form farther south and east (north and west) than normal during El Nino (La Nina) events. These formation differences lead to longer (shorter) tracks and stronger (weaker) tropical cyclones during El Nino (La Nina) events. Late in the tropical cyclone season, the anomalous extratropical waves alter the subtropical ridge and steering flow to favor recurving (straight running) tropical cyclones during El Nino (La Nina). These track differences lead to a much higher number of land falling tropical cyclones in southeast Asia during La Nina events. A preliminary study of the North Atlantic shows that there are more, and stronger, tropical cyclones during La Nina than El Nino. This is the result of extratropical Rossby wave trains that originate in the east Asia and extend into the North Atlantic. There they alter the vertical shear, so that La Nina favor more formations in the tropical Atlantic, where other conditions are favorable for the development of strong tropical cyclones