Kenya's short rains: what the Indian Ocean Dipole is and how it moves the rain

The Indian Ocean Dipole (IOD) is the difference in sea-surface temperature between the western and eastern Indian Ocean, and when the west is the warmer side, Kenya's October–December short rains tend to be heavier. The Kenya Meteorological Department (KMD) says in its outlook of 31 August 2026 that the dipole influences how long the season lasts and how it performs, and “enhances rainfall when in the positive phase”.

In a positive phase, water in the western Indian Ocean is warmer than normal and water in the east is cooler. The Cabot Institute at the University of Bristol wrote on 11 March 2024 that such events tend to occur in the second half of the year and can last for months.

The US National Oceanic and Atmospheric Administration's Climate Prediction Center tracks the dipole with a Dipole Mode Index: the temperature anomaly (the departure from the 1991–2020 average) in a western box of ocean (50°E–70°E, 10°S–10°N) minus the anomaly in a south-eastern box (90°E–110°E, 10°S to the equator). It uses a threshold of ±0.4°C to mark positive and negative periods. Its table gives 0.68°C for August 2026, the only 2026 month so far above +0.4°C.

Hannah Kimani, an assistant director at KMD, said, as quoted by the farming news site FarmBizAfrica on 18 September, that “for now it is still neutral”, and that “We are expecting that it will become positive as we move closer to the season.” The India Meteorological Department said in releases of 30 September and 8 October that neutral conditions were prevailing. The World Meteorological Organization's seasonal update of 8 October predicted a positive dipole that peaks in October and then declines, with a mean anomaly of +0.5°C for October–December.

Evidence that the dipole moves East Africa's rain comes from past seasons and from models. In a July 2023 watch for that year's short rains, the IGAD Climate Prediction and Applications Centre (ICPAC) said a positive event “contributes to enhanced rainfall across the Eastern Africa region”, and pointed to October–December 2019 as an example. The Cabot Institute described a study that used a large set of climate simulations: positive dipoles raise the likelihood of very wet days, and the effect on the most extreme days grows as the dipole gets stronger.

The same blog said the short rains at the end of 2023 brought severe flooding to Kenya, Somalia and Tanzania. Kimani said 2023 had a strong El Niño and a positive dipole together.

El Niño, the warming of the equatorial Pacific, adds to the effect. KMD's outlook says El Niño, which it describes as “currently strong”, also enhances rainfall when it occurs together with a positive dipole. ICPAC's outlook of 18 August said El Niño has developed and the dipole is also expected to be positive, with a 90% chance of enhanced rainfall over southern Ethiopia, central to southern Somalia and north-eastern Kenya.

For Kenya, KMD expects above-average rainfall in Nairobi, Mombasa, Kisumu, Nakuru and most other counties, and says heavy rainfall events are likely in several parts of them. The season supplies more than 30% of annual rainfall in counties including Nairobi and Mombasa, and more than 700mm falls in some places. Turkana and a few parts of West Pokot, Samburu and Marsabit are forecast near-average to above-average.

KMD gives the second to third week of October as the onset for Nairobi, Kiambu and other central counties, and for Mombasa, Kwale, Lamu and Kilifi. For Machakos, Kitui, Makueni, Kajiado and Taita-Taveta it gives the third to fourth week of October. Rains in Nairobi and the central counties are expected to continue into January; on the coast they are expected to end in the first to second week of December.

The outlook lists urban flooding, flash floods, waterlogged roads, railways and runways, power outages and a higher risk of malaria, Rift Valley fever, yellow fever, chikungunya and cholera among the season's risks. For farmers it advises early land preparation, dry planting and long-maturing, high-yielding maize varieties. It also lists recharged groundwater and fuller dams as benefits.

KMD says the probability of drought is low over most of the country, with a 15% to 20% chance of a mild drought event in Turkana.