How forecasters predict El Niño months ahead: models, buoys and satellites

Agencies talked about this El Niño months before it arrived. How do they see it coming?

Eyes on the ocean. IMD lists the tools used to monitor ENSO: satellites, ocean buoys, research vessels, Argo floats, surface weather stations and atmospheric observations. They track sea surface temperature, heat stored in the ocean, surface winds, air pressure, rainfall and circulation.

The buoy network. NOAA's Pacific Marine Environmental Laboratory says its Tropical Atmosphere Ocean (TAO) array was built after the 1982–83 El Niño, which was "neither predicted nor detected until nearly at its peak". The array was completed in December 1994 with nearly 70 moorings. Since 2000 it has been TAO/TRITON, run with Japan's JAMSTEC, and the buoys send data every day.

Models and people. The observations feed coupled ocean–atmosphere computer models. IMD uses its Monsoon Mission Coupled Forecasting System, run many times as an ensemble. At NOAA, a team of about ten forecasters combines many models with the latest observations and publishes odds for nine upcoming seasons on the second Thursday of each month.

How far ahead? IMD says models can often predict ENSO one to six months in advance. Skill is highest after the northern spring "predictability barrier", so forecasts made in March to May are the least reliable, and confidence grows as an event develops.

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