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

In this study, we have investigated contributions of tropical India Ocean (IO) sea surface temperature (SST) warming and El Niño-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 emperical orthogonal function (S-EOF) of tropical IO and tropical Pacific SST with trend and multi-decadal variability removed are defined as the IOBM index and ENSO index, respectivly. The results show that the IO warming year is usually the El Niño 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 Niño 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 has not related to ENSO, with the 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 the IO warming is investigated by using the global climate model of European Centre Hamburg Model (ECHAM) with imposed SST anomalies (SSTA) over the tropical IO. The results show that tropical IO warming can lead to the formation of an anomalous anticyclonic circulation over the tropical Northwest Pacific off the equator during boreal summer when central Pacific warming has dissipated and the center of the IO warming is moving northward. The IO warming induces upward motion and divergence in the upper troposphere over the northern IO while downward motion and upper-tropospheric convergence over the WNP in boreal summer, maintaining the anomalous anticyclonic circulation until early boreal fall over the WNP. Prior to boreal summer, the anomalous anticyclonic circulation mainly results from the Rossby-wave response to the suppressed convective heating induced by both the in situ SST cooling and the subsidence forced remotely by the central Pacific SSTA, as suggested in earlier studies.