Diagnosis of the ENSO modulation of tropical cyclogenesis over the southern South China Sea using a genesis potential index

Abstract

The modulation of tropical cyclogenesis over the southern South China Sea (SSCS) by the El Nio-Southern Oscillation (ENSO) is examined in October-December (OND), when tropical cyclone (TC) activities are most active in this region. The results reveal that there were more TCs formed over the SSCS during La Nia years and less TCs during El Nio years. How different environmental factors (including low-level vorticity, mid-level relative humidity, vertical wind shear, and potential intensity) contribute to this influence is investigated, using a genesis potential (GP) index developed by Emanuel and Nolan. Composite anomalies of the GP index are produced for El Nio and La Nia years separately, which could account for the changes of TC frequency over the SSCS in different ENSO phases. The degree of contribution by each factor is determined quantitatively by producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology. The results show that the mid-level relative humidity makes the largest contribution to the ENSO modulation of tropical cyclogenesis over the SSCS. Although warmer sea surface temperatures (SSTs) and larger amount of evaporation from the ocean surface were observed over the SSCS during El Nio years, anomalous descending motions due to the anomalous Walker circulations inhibited the upward transports of water vapor and led to less moisture contents in the middle troposphere, which suppressed TC formations.