Indian Summer Monsoon Rainfall (ISMR) is strongly influenced by large-scale ocean–atmosphere interactions, particularly the El Niño–Southern Oscillation (ENSO) over the tropical Pacific and the Indian Ocean Dipole (IOD) over the Indian Ocean. El Niño represents the warm phase of ENSO, while La Niña is the cold phase. Respectively, they cause drought and flood-like conditions in India. IOD is analogous to ENSO and refers to the differential change in Sea Surface Temperature (SST) between the western and eastern parts of the Indian Ocean. The positive IOD, with warmer SST in the western Indian Ocean, favours a stronger monsoon, while the negative IOD, with colder SST in the western Indian Ocean, is associated with reduced monsoon rainfall in India.

Figure 1 Upper panel shows the relationship between ENSO (NINO3.4) and Dipole Mode Index (DMI) during 1981–2026, with correlation coefficient 0.65. Bottom panel represents the evolution of ENSO and IOD in recent time. [Data Source: NOAA Sea Surface Temperature Data]

ENSO and IOD interaction play a key role in modulating Indian Summer Monsoon Rainfall (ISMR). ENSO in the tropical Pacific forces changes in the Walker Circulation, and Madden–Julian Oscillation (MJO), altering equatorial wind stress across the Indian Ocean and either triggering or suppressing IOD events. Strong El Niño events without a compensating positive IOD produced major droughts, whereas a positive IOD can mitigate El Niño impacts, and La Niña with neutral/positive IOD favours above-normal monsoon rainfall.

Figure 2 Comparison of actual and long-term normal precipitation during the June–September Southwest Monsoon (SWM) season over India. [Data Source: IMD gridded rainfall data]

The current El Niño (2026) is one of the strongest events in recent history, while the IOD was passing through the neutral phase, significantly (negatively) affected the early phase of ISMR, and now, leading towards the positive phase, may slightly suppress much of the negative impact of El Niño on modulating the ISMR.