Modoki, Indian Ocean Dipole, and western North Pacific typhoons: possible implications for extreme events

Abstract

An interesting feature during El Niño Modoki summers (June–August) is the occurrence of surplus rainfall anomalies over the South China Sea (SCS) and off-equatorial tropical Northwest Pacific. Based on analyses of observed and reanalyzed data, it is noted that anomalous warming in central tropical Pacific during an El Niño Modoki induces a quasi-stationary Rossby wave response in the form of a low-level cyclonic flow over the northern SCS. This anomalous response favors rainfall enhancement over the SCS and also promotes the seasonal typhoon activity. We find that this scenario over SCS and sub-tropical NW Pacific is further amplified when an El Niño Modoki co-occurs with a positive Indian Ocean Dipole (IOD) as in 1994. The combination is apparently more conducive for extreme convection events over the region as compared to a pure El Niño Modoki case. The respective impacts of positive IOD and El Niño, on the other hand, are opposite to one another. The modulation of the typhoon activity by IOD events, however, is prominent only since late 1970s.