Comparison of the Madden Julian Oscillation Related Tropical Cyclone Genesis over the South China Sea and Western North Pacific under Different El Nino Southern Oscillation Conditions
This paper explores the Madden Julian Oscillation (MJO) modulation of tropical cyclone (TC; hereafter, MJO TC) genesis over the South China Sea (SCS) and Western North Pacific (WNP) under different El Nino 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. The results showed that the MJO has significant modulation on both the SCS and WNP TC genesis in neutral years, with more (fewer) TCs forming during the active (inactive) MJO phases. However, during the El Nino and La Nina years, the MJO TC genesis modulation over the two regions differs from each other. Over the SCS, the MJO modulation of TC genesis is stronger in the La Nina years, while it becomes weaker in the El Nino years. Over the WNP, the MJO has a stronger influence on TC genesis in the El Nino years compared to that in the La Nina years. Related Genesis Potential Index (GPI) analysis suggests that midlevel moisture is the primary factor and vorticity is the secondary factor, for the MJO TC genesis modulation over the SCS in the La Nina years. Over the WNP, midlevel moisture is the dominant factor for the MJO TC genesis modulation during the El Nino years. These results can be explained by increased water vapor transport from the Bay of Bengal, associated with enhanced westerly wind anomalies, during the active phases relative to the inactive MJO phases; these conditions prevail over the SCS during the La Nina years, and over the WNP during the El Nino years.