Influence of the Eastern Pacific and Central Pacific Types of ENSO on the South Asian Summer Monsoon

Based on observational and reanalysis data, the relationships between the eastern Pacific (EP) and central Pacific (CP) types of El Niño−Southern Oscillation (ENSO) during the developing summer and the South Asian summer monsoon (SASM) are examined. The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s. While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically, the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s. The drought-producing El Niño warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line. Accordingly, compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent, leading to suppressed convection and decreased precipitation over these regions. Moreover, anomalously less moisture flux into South Asia associated with developing EP El Niño and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Niño, may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods, respectively. El Niño events with the same “flavor” may not necessarily produce consistent Indian monsoon rainfall anomalies, while similar Indian monsoon droughts may be induced by different types of El Niño, implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.