Influence of Tropical Indian Ocean Warming and ENSO on Tropical Cyclone Activity over the Western North Pacific

Abstract

In this study, we have investigated contributions of tropical Indian Ocean (IO) sea surface temperature (SST) warming and El Nino—Southern Oscillation (ENSO) to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) between 1948 and 2010 and the involved physical mechanisms. Both ENSO and tropical IO Basin Mode (IOBM) warming are found to play important roles in modulating the WNP TC genesis frequency, but their effects are significantly different. The time series of seasonal empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multi-decadal variability removed are defined as the IOBM index and ENSO index, respectively. The results show that the IO warming year is usually the El Nino decaying year. The number of total TCs, especially weak TCs, decreases during the tropical IO warming year when an anomalous anticyclonic circulation is observed over the tropical Northwest Pacific off the equator. On the other hand, the number of intense TCs increases in the El Nino developing year because of the eastward shifts of both the western Pacific monsoon trough and the cyclonic shear of the equatorial westerlies, and thus the eastward shift of the main TC genesis region. It is also found that the relationship between ENSO and the frequency of intense TCs has a decadal variation. During 1968–1987, the number of intense TCs was not related to ENSO with a correlation coefficient of only 0.19, while the correlation coefficient is 0.63 and 0.73 during 1948–1967 and 1988–2007, respectively. The extent to which the WNP anomalous anticyclone is forced by IO warming is investigated by using the global climate model (European Centre Hamburg Model, ECHAM) with imposed SST anomalies over the tropical IO. The results suggest that the WNP anomalous anticyclone that develops from June to September results mainly from the northward shift of the tropical IO warming from boreal spring to boreal summer.