El Niño / La Niña Phenomena
El Niño and La Niña are opposite phases of a naturally occurring climate pattern known as the El Niño-Southern Oscillation (ENSO), involving fluctuating ocean temperatures in the central and eastern equatorial Pacific, coupled with changes in the overlying atmosphere.
Overview
ENSO has two opposite extreme phases: the warm phase, known as El Niño, and the cold phase, known as La Niña. Between these extremes lies a third state called ENSO-neutral. El Niño can typically last up to 18 months and La Niña up to three years.
El Niño is characterized by unusually warm ocean surface temperatures in the central and eastern Pacific – its name “boy child” was attributed by fishermen whose catch declined because of the suppression of the nutrient-rich cold water up-welling off the coast of Peru and Ecuador. In contrast, a La Niña event is characterized by unusually cool ocean surface temperatures in the same region and an intensification of the prevailing east to west surface winds.
In the Pacific Ocean, warmer-than-normal ocean temperatures usually mean El Niño conditions, which happen when easterly trade winds become weaker. Cooler-than-normal temperatures usually mean La Niña conditions, when easterly winds become stronger. During the Neutral phase, ocean temperatures in the tropical Pacific stay close to normal.
The transition between these extreme phases typically occurs every two to seven years. Both El Niño and La Niña generally begin developing between March and June and reach their peak intensity between November and February.
The effects of each El Niño/La Niña event vary depending on the intensity, duration, time of year when it develops, and how it interacts with other climate variability modes (such as the Indian Ocean Dipole). Not all regions of the world are affected, and even within a region, impacts can be different. Even when ENSO is neutral, extreme weather can still occur.
The strength of an ENSO event is highly significant – whether it is classed as weak, moderate, strong or very strong. WMO does not use the term Super El Niño or La Niña.
Improved seasonal forecast models can now predict climate patterns one to six months in advance. By identifying trends in heavy rainfall (potentially leading to floods) and drought conditions, this information provides early warning signals that support planning in climate-sensitive sectors such as agriculture and health, while also informing the activation of government emergency protocols and the pre-positioning of humanitarian supplies Together, these actions help to protect resources and save hundreds of millions of dollars and many thousands of lives.
A multiyear La Niña lasted from late 2020 to early 2023, followed by the strong El Niño of 2023–2024. ENSO-neutral conditions began in April–June 2024 and generally prevailed until September–November 2025, with a brief dip into weak La Niña conditions during late 2024 and early 2025. La Niña developed again in late 2025 and lasted until early 2026.
WMO’s Global Seasonal Climate Update issued in April 2026 signals a clear shift in the Equatorial Pacific: sea-surface temperatures are rising rapidly, pointing to a likely return of El Niño conditions as early as May–July 2026. Forecasts indicate there is a “nearly global dominance of above-normal land surface temperatures” in the upcoming three-month period, and regional variations in rainfall patterns.
Measurement
Since the 1980s, the dominant approach to measuring or identifying El Niño has been the use of the Oceanic Niño Index wherein scientists calculate the Sea Surface Temperature Anomaly in a specific region of the Pacific called the Niño 3.4 region. When computed over a three-month average, this becomes the Oceanic Niño Index (ONI). In recent years and with global warming, a newer metric of measurement called the Relative Oceanic Niño Index (RONI) has gained ground. This approach measures relative warming, not just absolute warming, helping scientists assess whether a warming event is ENSO-driven or mostly due to global ocean warming.
Impact
ENSO is a major influencer of climate and weather patterns. El Niño events affect temperature and rainfall patterns in different regions and typically have a warming effect on the global climate – the most pronounced impacts are in the year following development. Thus, 2024 was the hottest year on record because of the combination of the powerful 2023–2024 El Niño and human-induced climate change from greenhouse gases.
El Niño is associated with increased rainfall and flooding in parts of South America, East Africa, and the southern United States; drought conditions in eastern and northern Australia, Indonesia, southern Africa, and parts of South Asia due to suppressed monsoon activity; and reduced Atlantic hurricane activity.
The timing and intensity can make a difference. Thus, El Niño is associated with increased rainfall over East Africa, often resulting in wetter-than-normal and sometimes severe “short rains” between October and December. It is also typically linked to suppressed rainfall during the June–September season over the northern parts of the region, including Sudan, South Sudan, parts of Ethiopia and western Kenya.
There is no evidence that climate change increases the frequency or intensity of El Nino events. It can amplify associated impacts because a warmer ocean and atmosphere increase the availability of energy and moisture for extreme weather events such as heatwaves and heavy rainfall.
WMO's response
WMO works closely with Member States and Territories to enhance their capacity to respond to El Niño-related challenges including climate services delivery, monitoring of the climate system, forecasting and capacity development.
WMO issues quarterly El Niño/La Niña Updates prepared through a collaborative effort between WMO and the International Research Institute for Climate and Society. These consensus-based reports draw on contributions from leading global centres that monitor and forecast ENSO conditions.
WMO also issues monthly Global Seasonal Climate Updates which incorporate influences of the other major climate drivers such as the North Atlantic Oscillation, the Arctic Oscillation and the Indian Ocean Dipole. These updates are produced by the WMO Lead Centre for Seasonal Prediction Multi-Model Ensemble (LC-SPMME) using forecasts from WMO Global Producing Centres of Seasonal Prediction (GPCs-SP). It is important that early action be guided by seasonal forecasts which reflect the influence of El Niño and other factors.
WMO sponsors Regional Climate Outlook Forums around the world, which issue regional predictions. These are then used by National Meteorological and Hydrological Services to provide tailored products for government decision-makers and climate sensitive sectors.
WMO actively supports governments through its members United Nations partners, including the United Nations Inter-Agency Task Force on Natural Disaster Reduction, humanitarian organizations, and stakeholders in climate-sensitive sectors in mobilizing preparedness actions and protecting lives and livelihoods. For example, the WMO Coordination Mechanism delivers tailored solutions to support humanitarian preparedness and early action, curating authoritative weather, climate and water information and expert advice from WMO Members and Centres.
ENSO prediction and monitoring support the United Nations Secretary-General call for Early Warnings for All.
