BharatNotes What is La Nina?
La Nina is the cold phase of the El Nino-Southern Oscillation (ENSO) — a periodic, coupled ocean-atmosphere fluctuation centred on the tropical Pacific. During La Nina, the easterly trade winds strengthen, dragging warm surface water westward toward the maritime continent (Indonesia/Australia) and allowing cold, nutrient-rich water to upwell in the central and eastern equatorial Pacific. The result is below-average sea-surface temperatures (SSTs) across the east-central Pacific, the mirror image of El Nino's warming (NOAA).
The atmosphere responds in step: the east-west Walker circulation intensifies, with stronger rising air (convection and rainfall) over the western Pacific and stronger sinking air over the cooler eastern Pacific. Because ocean and atmosphere reinforce each other, ENSO is described as a "coupled" system.
How is it measured?
NOAA's Climate Prediction Center declares La Nina conditions when SSTs in the Nino-3.4 region of the equatorial Pacific are at least 0.5 degrees C below average, with the anomaly persisting (or expected to persist) for five consecutive overlapping three-month periods (NOAA definition). The atmospheric side is tracked via the Southern Oscillation Index (SOI) — the sea-level pressure difference between Tahiti and Darwin — which turns strongly positive during La Nina.
| Feature | El Nino (warm phase) | La Nina (cold phase) |
|---|---|---|
| Central/east Pacific SST | Above average (warming) | Below average (cooling) |
| Trade winds | Weaken | Strengthen |
| Walker circulation | Weakens | Strengthens |
| Indian SW monsoon | Generally suppressed | Generally enhanced |
| Nino-3.4 anomaly | At least +0.5 deg C | At least -0.5 deg C |
Significance for India
For India, La Nina is broadly favourable for the southwest monsoon. Stronger trade winds and an intensified Walker circulation push more moisture-laden flow toward the subcontinent, and La Nina years are statistically associated with above-normal monsoon rainfall — though this is a tendency, not a guarantee, and the outcome is modulated by other drivers such as the Indian Ocean Dipole (IOD). La Nina winters in India also tend to be colder than usual. Good monsoon rainfall has direct knock-on effects for kharif agriculture, reservoir storage, hydropower and rural demand.
Current status (as of mid-2026)
The 2024-25 La Nina has ended, and the equatorial Pacific has returned to ENSO-neutral conditions, with forecasts pointing toward a developing El Nino during the 2026 season (NOAA; IMD outlook). Consistent with an emerging El Nino, the India Meteorological Department (IMD) has projected below-normal monsoon rainfall for 2026. This transition illustrates why ENSO phases must always be date-stamped — the dominant phase shifts every one to three years.
UPSC angle
This is a foundational climatology concept — there is no single verified PYQ for the exact term, but it underpins questions on monsoon mechanism, ocean currents, atmospheric circulation and climate variability. Aspirants should master the El Nino versus La Nina contrast, the role of the Walker circulation and trade winds, and the distinction between ENSO and the IOD, which are independent but interacting drivers of Indian monsoon rainfall.
