El Niño / La Niña
The El Niño-Southern Oscillation (ENSO) is a recurring natural phenomenon characterised by fluctuating ocean temperatures in the equatorial Pacific, coupled with changes in the atmosphere, which have a major influence on climate patterns in various parts of the world.
Overview
El Niño and La Niña are the oceanic components while the Southern Oscillation is the atmospheric counterpart, thus giving rise to the term El Niño/Southern Oscillation. Though ENSO is a single climate phenomenon, it has three phases - El Niño, La Niña and Neutral.
El Niño, meaning “boy child” in Spanish, was first used some centuries ago by fishermen in Peru and Ecuador to refer to the unusually warm waters that reduced their catch just before Christmas. The opposite of El Niño is La Niña, which means “little girl” and refers to the large-scale cooling of the ocean surface temperatures in the same region, coupled with a reversal of the atmospheric conditions.
Monitoring and outlook of ENSO conditions is primarily based on Sea Surface Temperatures (SSTs) anomalies (computed relative to a base period of 30 years) exceeding pre-defined thresholds in four geographic regions of the equatorial Pacific. Positive SST anomalies, above a given threshold, are usually indicative of an El Niño (ENSO warm phase) leading to weakening of easterly winds. The negative anomalies are associated with La Niña (ENSO cool phase) and the easterly winds become stronger. During the Neutral phase tropical Pacific SSTs are generally close to average.
El Niño and La Niña typically develop in northern hemisphere’s spring-summer and peak in winter. The oscillation between ENSO warm phase (El Niño) to neutral or cold (La Niña) conditions occurs on average every three to five years, while also ranging from two to seven years. El Niño can last up to 18 months and La Niña up to three years. The last multi-year La Niña event, began in September 2020 and prolonged into early 2023 - first "triple dip" La Niña of the 21st century.
Impact
El Niño/La Niña can have widespread impacts on climate and weather patterns, with changes in temperature and rainfall in various parts of the world and it is the dominant feature of climate variability on inter-annual timescales. Scientific progress on the understanding and modelling ENSO has improved prediction skills within a range of one to six or more months in advance, helping society to prepare for the associated hazards such as heavy rains, floods, and drought. The value of these predictions can also translate into hundreds of millions of dollars in potential savings.
No two El Niño/La Niña events are alike. The effects of each El Niño/La Niña event vary depending on the intensity, duration, time of year when it develops, and the interaction with other modes of climate variability. Not all regions of the world are affected, and even within a region, the impacts can be different.
In many locations, especially in the tropics, La Niña produces the opposite climate variations to El Niño (typical impacts on rainfall patterns of El Niño and La Niña). During ENSO-neutral phases, atmospheric patterns are dominated by other climate drivers.
El Niño events can have the overall effect of increasing global average surface temperatures, whereas La Niña events have a cooling tendency. The strong El Niño event of 1997/1998 was followed by a prolonged La Niña phase that extended from mid-1998 to early 2001, with a clear impact on global temperatures. At the time, 1998 was the second hottest year on record.
There is yet no conclusive evidence of the impact of climate change on the frequency, intensity of El Niño/La Niña events. However, climate change is likely to affect the impacts related to El Niño and La Niña in terms of the intensity and frequency of extreme weather and climate events.
WMO's response
WMO issues quarterly El Niño/La Niña Updates prepared through a collaborative effort between WMO and the International Research Institute for Climate and Society (IRI) and based on contributions from the leading centres around the world, monitoring and predicting this phenomenon and expert consensus facilitated by WMO and IRI. These updates comprise the observational monitoring of the current situation in the equatorial Pacific and consensus-based outlook for the next season. WMO also issues regular Global Seasonal Climate Updates (GSCU), 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 Long-Range Forecast Multi-Model Ensemble (LC-LRFMME) using forecasts from WMO Global Producing Centres of Long-Range Forecasts (GPCs-LRF).
WMO El Niño/La Niña Updates and GSCU are available to support governments, United Nations partners, including the United Nations Inter-Agency Task Force on Natural Disaster Reduction, humanitarian organizations, decision-makers, and stakeholders in climate-sensitive sectors in mobilizing preparedness actions and protecting lives and livelihoods.
