WMO: Without Ocean Data, ENSO Forecast Lead Time Falls From 4.5 to 3.2 Months

Ocean observing networks are the reason the 2026 El Niño was identified and forecast months ahead of its arrival, according to an article published in the WMO Bulletin by authors from the World Meteorological Organization, the Global Ocean Observing System (GOOS) and partner institutions. The piece, issued on 6 November 2026, sets out how satellites, moored buoys, ships and autonomous floats feed the prediction systems behind seasonal ENSO outlooks — and quantifies what would be lost without them.

The authors cite research showing that ENSO prediction lead time decreases from about 4.5 months to 3.2 months when ocean observations are withheld from ocean data assimilation. That loss of more than a month of warning, the WMO article argues, illustrates why sustained observation matters for anticipating events of this kind.

A strong event on an unusually warm ocean

The WMO authors place the current event against what they describe as an extraordinary backdrop. Ocean heat content reached its highest level in the 66-year observational record in 2025, exceeding the record set in 2024, and each of the last nine years has set a new record, the article states. The rate of ocean warming observed since 2005 is more than twice that recorded between 1960 and 2005.

In the Niño 3.4 region (170 °W–120 °W, 5 °S–5 °N), conditions at the start of 2026 still reflected weak La Niña, with sea-surface temperature anomalies slightly below average, according to WMO. Temperatures then returned to near-average before rising sharply, with anomalies increasing by more than 1.5 °C between March and August 2026. From mid-May onward, anomalies exceeded those seen at the equivalent stage of any other major El Niño included in WMO's comparison since 1950. El Niño conditions emerged in June and were firmly established by mid-August.

The atmosphere followed. WMO reports that the Southern Oscillation Index, which tracks the air-pressure difference between the western and eastern tropical Pacific, moved from positive values early in the year to strongly negative values from April, reaching –24.9 in June and –29.1 in July. Those were the lowest June and July readings among the major El Niño events in the comparison, which the article says indicates a marked weakening of the Walker circulation.

Multi-model ensemble forecasts issued in August pointed to seasonal average anomalies of approximately 3.6 °C (±0.4) for September–October–November, while September forecasts indicated 3.7 °C (±0.4) for October–November–December 2026. WMO notes that a very strong El Niño is generally defined by a three-month average anomaly of at least 2.0 °C in Niño 3.4, and that the forecasts suggested the event would exceed that threshold during the northern hemisphere autumn and persist into early 2027. If accurate, the event would rank among the strongest since 1950, potentially exceeding 1997/98 and 2015/16 — though the authors caution that the forecast anomalies use a fixed 1993–2009 reference period while historical anomalies use centred 30-year periods.

From float to forecast

The article describes GOOS as the framework bringing together 17 global ocean observing networks, ten of which directly contribute to ENSO monitoring, with the Tropical Pacific Observing System coordinating observations across the basin. The international Argo programme operates autonomous floats that typically measure from the surface to 2,000 metres every 10 days and transmit data in near real time, which WMO says are essential for tracking upper-ocean heat content and the eastward propagation of subsurface downwelling Kelvin waves. Expendable bathythermographs deployed from ships, uncrewed surface vehicles, gliders, moored buoys, voluntary observing ships and sea-level stations round out the picture.

Those data are quality-controlled and exchanged internationally before being assimilated into coupled ocean–atmosphere models run by WMO Global Producing Centres for Seasonal Prediction, with results consolidated by the WMO Lead Centre for Seasonal Prediction Multi-Model Ensemble. The WMO El Niño/La Niña Update adds expert assessment, the article explains, because ENSO cannot be identified or predicted reliably from a single variable, index or threshold.

What the forecasts can and cannot say

WMO is explicit about the limits. An ENSO forecast indicates likely timing, strength and evolution, shifting the odds of wetter, drier, warmer or cooler conditions, but it does not predict with certainty that a particular drought, flood or heatwave will occur. A very strong event raises the likelihood of typical teleconnection patterns without guaranteeing them, and outcomes depend on the pattern of Pacific warming, the atmospheric response, timing, duration and other climate drivers.

The authors contrast the present situation with 1982/83, an event they say caught the scientific community by surprise when tropical Pacific observations were far more limited. WMO advises users to consult forecasts and updates from their Regional Climate Centres and National Meteorological and Hydrological Services, and notes that seasonal forecasts should be treated as evolving rather than static. The full article is available on the WMO Bulletin site.