El Niño 2026–27 Explained: WMO Warning, RONI and Risks for India

Latest outlook: The World Meteorological Organization’s seasonal update issued on 21 October projects an average Niño 3.4 sea-surface temperature anomaly of about +3.8°C for November 2026–January 2027, with warming peaking around December. WMO reiterated the preparedness implications on 26 October.

Meaning of anomaly: An anomaly is a departure from a specified reference average. The projected figure concerns a defined Pacific region and season; it does not mean India’s air temperature will rise by that amount.

Forecast status: This is a model-based outlook, not a measurement of a completed season. The timing, strength and regional effects must be updated as observations arrive. NOAA’s October assessment also anticipates a strong-to-very strong event into early 2027.

Coupled system: ENSO is a recurring interaction between the tropical Pacific Ocean and the atmosphere. Its phases are El Niño, La Niña and neutral conditions; events occur irregularly rather than on a fixed annual timetable.

Warm phase: El Niño involves unusual warming in the central and eastern equatorial Pacific together with changes in winds, pressure and tropical rainfall.

Cool and neutral phases: La Niña involves unusual cooling and associated atmospheric changes. Neutral conditions mean neither phase dominates; they do not mean that weather everywhere will be average.

Atmospheric component: The Southern Oscillation refers to associated changes in tropical Pacific air pressure. ENSO is therefore more than an isolated patch of warm water.

Normal circulation: Easterly trade winds usually push warm surface water westward, helping concentrate rising air and heavy rain toward the western tropical Pacific. Cooler deep water rises near the eastern Pacific through upwelling.

El Niño circulation: Weaker trade winds and eastward redistribution of warm water alter convection, the upward movement of warm, moist air. Upwelling is often reduced in the eastern equatorial Pacific.

Walker circulation: This east–west tropical atmospheric circulation changes as the zones of rising and sinking air shift. Through teleconnections, meaning climate links between distant regions, the changes influence rainfall and temperature far from the Pacific.

Monitoring region: Niño 3.4 lies in the east-central equatorial Pacific, approximately 5°N–5°S and 170°W–120°W. It is one important region used to monitor ENSO.

Traditional measure: The Oceanic Niño Index, or ONI, uses a three-month running average of sea-surface temperature departures in that region from a climatological reference.

Relative measure: NOAA moved to operational use of the Relative Oceanic Niño Index, or RONI, in February 2026. It subtracts the tropical-wide ocean temperature departure from the Niño 3.4 departure and applies a scaling adjustment. It better captures how unusual the region is relative to the surrounding warm tropics.

Classification caution: NOAA’s RONI series uses ±0.5°C thresholds to identify warm and cool periods. Operational identification also considers persistence and the coupled atmospheric response. A single hot day or a single weekly value is insufficient.

Comparison rule: Raw Niño 3.4 anomalies, ONI and RONI are related measures with different constructions. A WMO sea-temperature forecast should not be labelled a measured RONI value or compared with an older index as though they were identical.

Informal terminology: These expressions communicate exceptional strength in media discussion. India’s Ministry of Earth Sciences has stated that “Super El Nino” is not an officially defined category of WMO or IMD.

Operational language: Weak, moderate, strong and very strong describe intensity in operational assessments. For examinations, the underlying index, reference period and forecasting agency matter more than a dramatic nickname.

Summer rainfall risk: El Niño often weakens the Indian southwest monsoon through changes in tropical circulation. The relationship is probabilistic: it increases the risk of deficient rain, but does not guarantee drought in every district.

Observed 2026 season: IMD reports June–September rainfall at 87% of the Long Period Average, or LPA. LPA is average rainfall for a defined region and period over a long reference interval. This implies a shortfall of about 13%, not an 87% fall.

Regional differences: National rainfall totals conceal the location and timing of rain. Crop stress depends on dry spells, sowing dates, soil moisture, irrigation and reservoir storage as well as the seasonal total.

Different winter-rain regime: The northeast monsoon brings important October–December rain to southeastern peninsular India. Its relationship with ENSO differs from the summer monsoon. The supplied report cites a normal seasonal outlook for the southern peninsula; this does not exclude local flood or dry-spell risks.

Indian Ocean Dipole: IOD measures the contrast in sea-surface temperature anomalies between the western and southeastern equatorial Indian Ocean. A positive IOD means the western region is relatively warmer than the eastern region.

Interaction with India: A positive IOD can sometimes support Indian summer rainfall and partly offset an unfavourable Pacific influence, but this is not a guaranteed cancellation.

Forecast horizon: WMO’s latest November–January outlook expects the positive IOD to weaken toward its average state. Conditions and forecasts for different three-month windows should not be treated as interchangeable.

Other controls: Monsoon depressions, intraseasonal active and break spells, surrounding ocean temperatures and land conditions also influence rainfall. Winter temperatures additionally depend on regional circulation and western disturbances, weather systems that bring rain and snow to northwestern India.

Delayed response: Ocean heat, soil moisture, reservoirs, crop cycles and food supplies respond over different timescales. A peak in a Pacific index need not coincide with the peak in economic or humanitarian damage.

India’s exposure: Lower stored water can affect irrigation and hydropower; warmer conditions can increase cooling demand and heat stress. A warm spell near a crop’s sensitive stage can reduce yields even when seasonal rainfall appears adequate.

Food-price link: Weather damage can reduce supplies and raise prices, but stocks, imports, logistics and policy responses influence the eventual inflation outcome. These are risks, not a fixed forecast for all commodities.

Winter caution: Above-normal seasonal temperature probabilities do not mean India will have no winter or no cold waves. Nor does persistence into early 2027 establish the outcome of the entire 2027 summer monsoon.

Contrasting hazards: WMO’s latest assessment highlights wetter conditions and flood risk in parts of East Africa, drier conditions in Southern Africa, and heat and rainfall deficits in Southeast Asia. Parts of Central America and the Amazon also face drought risk.

Local vulnerability: Similar rainfall departures can cause different harm depending on drainage, water storage, housing, health systems and dependence on rain-fed farming.

Climate-change interaction: ENSO is natural variability superimposed on a warming climate. Higher background temperatures can intensify heat exposure, but any particular flood, drought or cyclone requires event-specific analysis. El Niño is not a complete explanation for every weather extreme.

Probabilistic planning: A seasonal forecast describes the likelihood of broad conditions over months. It does not predict a particular storm on a particular day.

Institutional chain: WMO coordinates global climate information; national and regional agencies translate it into more local outlooks. In India, IMD and its regional climate services issue the relevant national, regional and district advisories.

Decision focus: Authorities should combine updated forecasts with district vulnerability, reservoir levels, crop calendars and public-health preparedness. Early action can reduce damage even when the forecast contains uncertainty.