Impact of south Indian Ocean Dipole on tropical cyclone genesis over the South China Sea

This study investigates the impact of the south Indian Ocean Dipole (SIOD) on the frequency of tropical cyclones (TCs) over the South China Sea (SCS) in summer (May-August) and winter (September-December) for 1975-2012. The frequency of TCs over the SCS shows significant inter-annual variations in the two seasons and relatively obvious inter-decadal variability in summer. A prominent relationship occurs between the TC generation over the SCS and the SIOD, and the latter can be regarded as a good predictor of the former on inter-annual scale. In winter, vertical circulation is obvious due to a strong SIOD induces intensified ascending motions and high sea surface temperature over the northern SCS with sufficient moisture that supplies a favourable environment for TC formation. The impact of SIOD is weaker in summer but a convergent zone with upwards motion also can be found over northeastern SCS. In addition, the SIOD and El Nino-Southern Oscillation are two primary impacting factors in winter and TC numbers increase when La Nina and positive SIOD events occur simultaneously. However, the SIOD is a dominant and independent impacting factor in summer and El Nino events just play a secondary role.