Strengthening El Niño: how it is measured and what it means for Kenya's short rains

El Niño, an unusual warming of the central and eastern tropical Pacific, is now established and strengthening, and it is linked to wetter October–December rains in East Africa. The World Meteorological Organization (WMO) said on 8 October that its forecast has the event peaking around December. Its Secretary-General, Celeste Saulo, said "El Niño is firmly established and strengthening rapidly."

The Kenya Meteorological Service Authority's outlook for the October–December "short rains", issued on 31 August, expects above-average rainfall in Nairobi, Mombasa, Kisumu and dozens of other counties.

El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO), a Pacific Ocean cycle that also has a cool phase, La Niña, according to the US National Oceanic and Atmospheric Administration (NOAA). WMO describes it as unusually warm surface water in the central and eastern Pacific, arising when the easterly trade winds weaken. NOAA says it occurs on average every two to seven years.

To declare an El Niño, NOAA says, sea-surface temperatures in the central and eastern tropical Pacific must run at least 0.5°C above normal for at least five consecutive overlapping three-month periods, and the atmosphere must respond by weakening the trade winds.

The yardstick is the temperature "anomaly" (the departure from a long-term average) in a Pacific box called Niño-3.4. NOAA's Climate Prediction Center now uses the Relative Oceanic Niño Index (RONI) for official classifications. RONI takes the Niño-3.4 anomaly and subtracts the average anomaly across the tropical oceans between 20°S and 20°N, so that general ocean warming does not inflate the reading. The 0.5°C thresholds are unchanged. The Center's information circular gives the 2023–24 El Niño as an example: it reads +1.5°C under RONI against +2.1°C under the older index, and the Center says its impacts remain the same.

NOAA forecasters classify events as weak, moderate, strong or very strong. In a news release on 11 June, NOAA said it considers an event "very strong" if sea-surface temperatures in the monitored region run more than 2.0°C above normal. For a "historic" event, one that would exceed every El Niño since 1950, the Center uses a three-month RONI of +2.5°C or more.

On 8 October the Center kept its El Niño Advisory in force. It said Niño-3.4 had reached +2.1°C and that a strong-to-very strong El Niño was likely through January–March 2027. It put the chance of a historic event at 54% for September–November, 83% for October–December and 70% for November–January.

WMO uses a different baseline, the 1993–2009 average, and a different forecast. It said the Niño-3.4 anomaly rose from +1.6°C in June to +2.5°C in August and is forecast at about +3.7°C for October–December, which would be the highest on record. The previous record was +2.6°C in November 2015–January 2016. WMO gave a near-100% likelihood that El Niño lasts through February 2027, based on a consensus of 13 national and international centres.

WMO says El Niño is associated with increased rainfall over East Africa, often giving a wetter-than-normal short-rains season. Its 8 October release said wetter-than-normal conditions are more likely across the Greater Horn of Africa in October–December. WMO also says each event's effects vary with its intensity, duration and the time of year it develops.

The Kenyan outlook lists above-average rainfall for most counties, including Nairobi, Mombasa, Kisumu, Kilifi, Kwale, Lamu and Mandera, and near-average to above-average rainfall for Turkana and parts of West Pokot, Samburu and Marsabit. It also expects warmer-than-average temperatures, with higher probabilities over Kwale, Mombasa, Kilifi, Lamu and parts of Tana River, Taita/Taveta and Garissa. The outlook says the season brings more than 30% of annual rainfall in several counties, including Makueni, Garissa, Wajir, Mandera, Marsabit and Mombasa. It says the Indian Ocean Dipole, an Indian Ocean climate pattern, influences the season's duration and performance, and that El Niño, which it calls currently strong, also enhances rainfall when it occurs together with a positive dipole.

Capital FM reported on 5 October that the Kenya Meteorological Service Authority had announced the start of the October–December season, with rain already under way in 24 counties although onset would not be uniform. According to the report, rain had been recorded in parts of western Kenya, the Rift Valley and the Coast, and the areas likely to receive the heaviest rainfall include Kwale, Mombasa, Kilifi, Lamu and Mandera. Edward Muriuki, the authority's acting head, said rainfall is influenced by several climate systems, including El Niño and the Indian Ocean Dipole, as quoted by Capital FM.

NOAA's next ENSO Diagnostic Discussion is scheduled for 12 November 2026.