Explain the ENSO phenomenon and its historically observed correlation with the Indian summer monsoon.

The El Niño–Southern Oscillation (ENSO) is a coupled ocean–atmosphere oscillation in the tropical Pacific, whose warm and cool phases shift irregularly every two to seven years and disturb wind and rainfall patterns across the tropics [1]. For India, where the June–September southwest monsoon delivers the bulk of annual rainfall, ENSO is the single most-tracked predictor of seasonal performance.

Mechanism of the phenomenon

• Oceanic limb (El Niño/La Niña): abnormal warming (El Niño) or cooling (La Niña) of sea-surface temperatures in the central and eastern equatorial Pacific [1].
• Atmospheric limb (Southern Oscillation): a see-saw of surface pressure between Tahiti and Darwin; the two limbs are coupled, hence "ENSO" [1].
• Walker circulation shift: during El Niño the rising limb of convection migrates eastward toward the mid-Pacific, weakening convergence over the Indian Ocean–maritime continent belt.

Normal year:   warm W-Pacific -> rising air over Indonesia -> strong monsoon flow
El Nino year:  warm C/E-Pacific -> rising limb shifts east -> subsidence over India -> weak monsoon

Observed correlation with the Indian summer monsoon

• El Niño years are statistically associated with below-normal, delayed or erratic monsoons, and La Niña years with normal-to-excess rainfall — the reason IMD's Monsoon Mission Climate Forecast System explicitly tracks ENSO evolution in its Long Range Forecast [2].
• The correlation is strong but not deterministic: several El Niño years have still yielded near-normal rainfall, since the Indian Ocean Dipole (IOD) and other drivers can offset it — a positive IOD, for instance, was projected alongside developing El Niño conditions for 2026 [2].
• The socio-economic transmission is direct: with agriculture still employing about 43% of the workforce (PLFS 2025), deficient rains compress farm incomes, rural demand and food prices [3].

ENSO thus modulates, rather than dictates, the monsoon. Strengthening dynamical forecasting, expanding micro-irrigation and crop insurance, and ENSO-linked contingency cropping plans can convert forecast skill into farmer resilience — aligning monsoon management with SDG-2 (zero hunger) and SDG-13 (climate action).