Variability of tropical cyclone occurrence date in the South China Sea and its relationship with SST warming

Abstract

Analyses of tropical cyclone (TC) occurrence dates in the South China Sea (SCS) for the past 60 yrs indicate a trend toward an earlier occurrence of the first annual TC in the SCS. On the other hand, a significant increasing trend in sea surface temperature (SST) in early summer (May–June) has been observed in the SCS. The negative correlation between the first annual TC occurrence date and SST in early summer during the period 1945–2009 suggests that the earlier occurrence of the first annual TC is related not only to the increasing of SST in the SCS, but also to the variability of SST in the Niño3.4 region. Quantitative analysis of the SCS TC occurrence date and SST by quantile regression also reveals such a relationship and confirms that the SCS early-season TCs tend to occur earlier when SSTs in the SCS and Niño3.4 region are increasing. Since the SCS SST anomalies are influenced by the El Niño-Southern Oscillation (ENSO), the relationship between the first annual TC occurrence date and ENSO-related large-scale atmospheric circulation including 850-hPa relative vorticity (RV), vertical wind shear (VWS), and moist static energy (MSE) in early summer are also investigated. It is found that variations of VWS and MSE have influences on first annual SCS TC occurrence dates, although there is not a statistically significant relationship between 850-hPa RV and first annual SCS TC occurrence date. These results suggest that the earlier occurrence of the first annual TC in the SCS is influenced not only by local SST, but also by ENSO through the alternation of early summer VWS and MSE in the SCS.