ENSO modulating tropical cyclone geneses in fall and winter seasons over the South China SeaENSO Modulating tropical cyclone geneses in fall and winter seasons over the South China SeaY. Shi et al.

Understanding the Tropical Cyclone (TC) geneses is essential to prevent associated disasters in Asian countries. The TC geneses are strongly modulated by the El Niño-Southern Oscillation (ENSO) over the South China Sea (SCS). However, the physical processes of such modulation in the boreal fall and winter seasons remain unclear. This study shows that the TC geneses are much higher over the SCS in late fall and early winter (October to December) during the developing year of La Niña than that of El Niño. The relative environmental factors, including vertical velocity and mid-level relative humidity, are the critical factors contributing to the TC genesis. The dynamic and thermodynamic environmental factors are tele-connected with the ENSO. During an El Niño event, the tropical eastern-central Pacific warming weakens the Walker Cell, leading to an anomalous descending motion and suppressed convection over the western North Pacific. These induce negative anomalies of mid-level relative humidity that suppress TC geneses. Conversely, a La Niña event provides favorable large-scale conditions, including anomalous cyclonic circulation in the lower-troposphere and positive low-level relative vorticity, enhanced convection, and anomalous mid-troposphere relative humidity over the SCS. These factors, particularly the enhanced middle-tropospheric upward motion and higher relative humidity, support TC geneses in La Niña years during fall and winter. Our research enhances understanding of the ENSO influence on the TC geneses and benefits the interannual predictions.