Diversity of 10–20-day propagation of genesis potential index and its roles in tropical cyclogenesis over the South China Sea

The genesis potential index (GPI) is used to quantify the major large-scale environmental parameters associated with tropical cyclogenesis (TCG). It is demonstrated that the 10–20-day GPI dominates the intraseasonal variability of the GPI over the South China Sea (SCS) from June to September during 1979–2021. The spatiotemporal evolutions of the 10–20-day GPI in the TCG processes show distinct diverse characteristics. Through extended empirical orthogonal function (EEOF) analysis and spatial correlation coefficients, the 10–20-day GPI propagation characteristics of the TCG processes are mainly classified into 3 categories: WN-Pattern, N-Pattern, and W-Pattern. The WN-Pattern originates from the tropical western North Pacific (WNP), then gradually intensifies and propagates northwestward to the northern SCS. The N-Pattern moves northward from Indonesia to the central-northern SCS and distinctly strengthens from 2 days before the cyclogenesis. The W-Pattern features westward propagation from the subtropical WNP to the northern SCS. These three propagation patterns correspond to the respective locations of SCS TCG. In the GPI terms of these three patterns, the low-level absolute vorticity and mid-level relative humidity are considered as the dominant environmental factors. Furthermore, the significant cyclonic circulation anomalies and water vapor flux anomalies are well coordinated with the 10–20-day GPI in all the three patterns, constituting favorable dynamical and thermodynamic conditions for SCS TCG. This study may be beneficial for deeper comprehension of the subseasonal large-scale environmental factors about SCS TCG.