Modulation of boreal summer intraseasonal oscillation on tropical cyclone genesis over the South China Sea

The South China Sea (SCS) is affected by two components of boreal summer intraseasonal oscillation (BSISO): namely the 10-30d BSISO and 30-60d BSISO. In this study, the modulation of tropical cyclone genesis (TCG) over the SCS by individual and combined modes of BSISO is investigated from the view of dynamic and energy conversion. Results suggest that the number of TCG varies significantly with the life cycle of BSISO, with the influence of 30-60d mode being more significant than that of 10-30d mode. When 10-30d/30-60d BSISO is in convective phase, the cyclone triggered by active convection over the SCS leads to meridional shear of zonal winds and convergence of meridional winds, which favors synoptic-scale eddies gaining energy from BSISO flows through barotropic energy conversion. These growing eddies, together with beneficial environmental conditions such as low-level convergence and cyclonic vorticity, create a favorable environment for TCG over the SCS. Compared to 10-30d BSISO, synoptic-scale eddies gain more energy from 30-60d BSISO. When the SCS is under combined effect of dual modes, the modulation of 10-30d mode on TCG tends to be subjective to 30-60d mode. The impact of 10-30d BSISO is less obvious during nonconvective phase of 30-60d BSISO when compared to the convective counterparts over the SCS.