REGIONAL DISCREPANCIES OF THE IMPACT OF TROPICAL INDIAN OCEAN WARMING ON NORTHWEST PACIFIC TROPICAL CYCLONE FREQUENCY IN THE YEARS OF DECAYING EL NIÑO

Abstract:

This study investigates the influences of tropical Indian Ocean (TIO) warming on tropical cyclone (TC) genesis in different regions of the western North Pacific (WNP) from July to October (JASO) during the decaying El Niño. The results show significant negative TC frequency anomalies localized in the southeastern WNP. Correlation analysis indicates that a warm sea surface temperature anomaly (SSTA) in the TIO strongly suppresses TC genesis south of 21°N and east of 140°E in JASO. Reduced TC genesis over the southeastern WNP results from a weak monsoon trough and divergence and subsidence anomalies associated with an equatorial baroclinic Kelvin wave. Moreover, suppressed convection in response to a cold local SSTA, induced by the increased northeasterly connected by the wind-evaporation-SST positive feedback mechanism, is found unfavorable for TC genesis. Positive TC genesis anomalies are observed over higher latitudinal regions (at around 21°N, 140°E) and the western WNP because of enhanced convection along the northern flank of the WNP anomalous anticyclone and low-level convergence, respectively. Although local modulation (e.g., local SST) could have greater dominance over TC activity at higher latitudes in certain anomalous years (e.g., 1988), a warm TIO SSTA can still suppress TC genesis in lower latitudinal regions of the WNP. A better understanding of the contributions of TIO warming could help improve seasonal TC predictions over different regions of the WNP in years of decaying El Niño.