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 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 empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multidecadal variability removed are defined as the IOBM index and ENSO index, respectively. 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 1968During -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.

Eastward Shift of Northwest Pacific Tropical Cyclone Genesis Frequency Anomaly in Decaying El Niño

“…1. This agrees well with recent studies (Du et al 2011;Zhan et al 2011a;Tao et al 2012) stating that the WNP TC activity is suppressed in summer following strong El Niño events. However, TCs in October is slightly higher than the climatological mean by 0.9 (Fig.…”

“…Recent studies have revealed that the TCGF significantly decreases over the WNP in the EDC years (Du et al 2011;Zhan et al 2011a, b;Tao et al 2012). In this study, we focus on the longitudinal evolution of TCGF anomaly over the WNP from July to October under the influences of large-scale circulation and environmental conditions in the EDC mode.…”

“…The atmospheric circulation anomalies in the two modes show distinct features from summer to the subsequent fall, and the different environmental conditions exert distinct impacts on the synoptic-scale disturbances over the WNP, including TC activities. Many studies focus on the WNP TC activity in the EDV mode (Chan 2000;Chia and Ropelewski 2002;Wang and Chan 2002;Camargo and Sobel 2005;Zhan et al 2011a), while recent studies reveal an obvious negative correlation between the WNP TC frequency and SST anomaly (SSTA) in the tropical Indian Ocean (TIO; Du et al 2011;Zhan et al 2011a, b;Tao et al 2012). In particular, warm TIO SSTA forces an equatorial baroclinic Kelvin wave in the lower troposphere, propagating to the tropical Pacific, which strongly impacts the WNP anomalous anticyclonic circulation and further suppresses the WNP TC activity in the EDC summer and fall (Xie et al 2009;Du et al 2011).…”

Anomalous Tropical Cyclone Activity in the Western North Pacific in August 2014

“…24.1h). The sea level pressure anomaly was slightly positive over the WNP in August 2014, which is different from that in 1998, when the TIO warming after the strong El Niño (1997) induced strong Kelvin wave propagation into WNP, leading to anomalous high pressure over the WNP (Du et al 2011;Tao et al 2012; Supplemental Fig. S24.6).…”

Features of tropical cyclone landfalls over East Asia corresponding to three types of Pacific warming decaying phase

“…Moreover, the influence of ENSO on the WNP TC activity takes place not only in its peak year but also lingers after the mature phase. Although the suppressed cyclogenesis is observed over the central WNP in the decaying El Niño years [9,10], the slightly enhanced TC genesis in the western WNP could lead to increased landfalling TCs in Vietnam [11]. Furthermore, a robust asymmetry of the duration between the eastern Pacific El Niño and La Niña events [12] affects the evolution of the ENSO cycle when the warm SSTA over the equatorial eastern Pacific has dissipated.…”