How Does Shifting Pacific Ocean Warming Modulate on Tropical Cyclone Frequency over the South China Sea?

The different modulation of El Nino Modoki and canonical El Nino events on tropical cyclone (TC) frequency over the South China Sea (SCS) during boreal summer and fall for 1960-2009 is investigated. The bootstrap resampling method and two-sample permutation procedure are applied to simulate sampling distributions and conduct statistical tests, respectively. Results from the hypothesis testing indicate that the above-normal TC frequency over the SCS occurs during June-August (JJA) for the El Nino Modoki years, whereas the below-normal TC frequency is significant during September-November (SON) for the canonical El Nino years. The remarkably opposite modulations can be attributed to the different large-scale circulation anomalies, which are consistent with Matsuno-Gill-type responses to the tropical heating source/sink over the western North Pacific (WNP) and Maritime Continent for two kinds of Pacific Ocean warming events. In response to a broad-scale convection anomaly over the WNP during JJA for El Nino Modoki, a zonally elongated cyclonic anomaly dominates the WNP and SCS, leading to enhanced TC activity. In contrast, during SON for the canonical El Nino, a markedly strengthened cooling source centered in the Maritime Continent induces an anticyclonic anomaly over the SCS, resulting in suppressed TC activity.