El Nino-Southern Oscillation (ENSO)
ENSO refers to a coupled ocean-atmosphere phenomenon involving periodic fluctuations in sea surface temperatures (SSTs) and atmospheric pressure across the tropical Pacific Ocean. El Nino (the warm phase) occurs when SSTs in the central and eastern equatorial Pacific are anomalously warm, while La Nina (the cool phase) involves anomalous cooling of the same region. The Southern Oscillation Index (SOI) — measuring the atmospheric component — was first described by Sir Gilbert Walker in the 1920s. ENSO is the dominant source of inter-annual climate variability globally and affects rainfall, temperature, and extreme weather events on every inhabited continent.
- During El Nino, the Walker Circulation (the east-west tropical Pacific circulation cell) weakens and shifts eastward, suppressing convection over the Indo-Pacific and weakening the Indian monsoon.
- El Nino years historically correspond with below-normal Indian Summer Monsoon Rainfall (ISMR); La Nina years tend to produce above-normal rainfall (e.g., 2010–11, 2020–22).
- The Nino 3.4 region (5°N–5°S, 170°W–120°W) SST anomaly is the key index used by WMO and IMD to classify ENSO phases.
- The El Nino-monsoon teleconnection: warming of the Pacific shifts the ascending limb of the Hadley Cell eastward, reducing moisture supply and weakening the low-level southwesterly jet stream that drives India's southwest monsoon.
● Tracked since February 19, 2026 · last seen August 14, 2026 · updates as the daily brief publishes