Multi-decadal modulation of the El Niño–Indian monsoon relationship by Indian Ocean variability

The role of leading modes of Indo-Pacific climate variability is investigated for modulation of the strength of the Indian summer monsoon during the period 1877–2006. In particular, the effect of Indian Ocean conditions on the relationship between the El Niño–Southern Oscillation (ENSO) and the Indian monsoon is explored. Using an extended classification for ENSO and Indian Ocean dipole (IOD) events for the past 130 years and reanalyses, we have expanded previous interannual work to show that variations in Indian Ocean conditions modulate the ENSO–Indian monsoon relationship also on decadal timescales. El Niño events are frequently accompanied by a significantly reduced Indian monsoon and widespread drought conditions due to anomalous subsidence associated with a shift in the descending branch of the zonal Walker circulation. However, for El Niño events that co-occur with positive IOD (pIOD) events, Indian Ocean conditions act to counter El Niño's drought-inducing subsidence by enhancing moisture convergence over the Indian subcontinent, with an average monsoon season resulting. Decadal modulations of the frequency of independent and combined El Niño and pIOD events are consistent with a strengthened El Niño–Indian monsoon relationship observed at the start of the 20th century and the apparent recent weakening of the El Niño–Indian monsoon relationship.