Comparison of Madden–Julian Oscillation Modulation on tropical-cyclone genesis over the South China Sea and Western North Pacific under different El Niño Southern Oscillation conditions

This paper explores the modulation by Madden–Julian Oscillation (MJO) on tropical-cyclone (TC; hereafter, MJO TC) genesis over the Western North Pacific (WNP) and the South China Sea (SCS) under different El Niño Southern Oscillation (ENSO) conditions. Analyses used Joint Typhoon Warning Center (JTWC) Best Track data, the Real-Time Multivariate MJO (RMM) index, and European Center for Medium-Range Weather Forecasts (ECMWF) Interim (ERA-Interim) reanalysis data. Results showed that MJO has significant modulation on both SCS and WNP TC genesis in neutral years, with more (fewer) TCs forming during active (inactive) MJO phases. However, during El Niño and La Niña years, modulation over the two regions differs. Over the SCS, the modulation of TC genesis is strong in La Niña years, while it becomes weak in El Niño years. Over the WNP, MJO has stronger influence on TC genesis in El Niño years compared to that in La Niña years. Related Genesis Potential Index (GPI) analysis suggests that midlevel moisture is the primary factor for MJO modulation on SCS TC genesis in La Niña years, and vorticity is the secondary factor. Over the WNP, midlevel moisture is the dominant factor for MJO TC genesis modulation during El Niño years. The main reason is increased water-vapor transport from the Bay of Bengal associated with the active MJO phase related westerly wind anomalies; these features are a significant presence over the SCS during La Niña years, and over the WNP during El Niño years.