El Niño and La Niña events are caused by, and contribute to, naturally occurring climate variability. They disrupt the normal patterns of tropical precipitation and atmospheric circulation and are considered to be the opposite phases of air-
sea interactions collectively referred to as the El Niño/Southern Oscillation (ENSO).
They occur every two to seven years and typically last for 9 to 12 months, and have widespread impacts on weather around the world. The 2015/2016 El Niño was one of the strongest on record.
They are not the only drivers of our climate, but are the best known because research has increased our understanding of these phenomena and their worldwide impacts.
El Niño/La Niña do not affect all regions, and even in a given region, the impacts are not the same. For the most accurate information at national or local level, it is important to consult National Meteorological and Hydrological Services.
El Niño, meaning “boy child” in Spanish, was first used in the nineteenth century by fishermen in Peru and Ecuador to refer to the unusually warm waters that reduced their catch just before Christmas. El Niño events often begin in the middle of the year with large-scale warming of surface water in the central and eastern equatorial Pacific Ocean and changes in the tropical atmospheric circulation (i.e. winds, pressure and rainfall).
In general, El Niño reaches a peak during November–January and then decays over the first half of the following year. Strong and moderate El Niño events have a warming effect on average global surface temperatures.
The opposite of El Niño is known as La Niña, which means “little girl” and refers to the large-scale cooling of the ocean surface temperatures in the same region in the equatorial Pacific, coupled with a reversal of the overlying atmospheric conditions. In many locations, La Niña cold episodes produces the opposite climate effects to El Niño.
The third phase is the ENSO (El Niño Southern Oscillation) – neutral phase, when the atmosphere and oceans are influenced by other climate drivers.
The outcomes of each El Niño event are never exactly the same: they depend on the intensity of the event, the time of year when it develops and the interaction with other climate patterns.
El Niño is often associated with warm and dry conditions in southern and eastern inland areas of Australia, as well as Indonesia, the Philippines, Malaysia and central Pacific islands such as Fiji, Tonga and Papua New Guinea. During the northern hemisphere summer season, the Indian monsoon rainfall generally tends to be less than normal. In the northern hemisphere winter, drier than normal conditions are typically observed over south-eastern Africa and northern Brazil.
Wetter than normal conditions are typically observed along the Gulf Coast of the United States, the west coast of tropical South America (Colombia, Ecuador and Peru) and from southern Brazil to central Argentina. Parts of eastern Africa (Kenya, Uganda) also usually receive above-normal rainfall. El Niño is associated with milder winters in north-western Canada and Alaska due to fewer cold air surges from the Arctic – a result of a large-scale region of lower pressure centred on the Gulf of Alaska/North Pacific Ocean.
The outcomes of each La Niña are never exactly the same: they depend on the intensity of the event, the time of year when it develops and the interaction with other climate patterns.
La Niña is often associated with wet conditions in eastern Australia, and with heavy rainfall in Indonesia, the Philippines and Thailand.
La Niña usually leads to increased rainfall in North Eastern Brazil, Colombia and other northern parts of South America and is associated with rainfall deficiency in Uruguay and parts of Argentina.
Drier than normal conditions are generally observed along coastal Ecuador and North Western Peru.
La Niña episodes feature a very wave-like jet stream flow over the United States and Canada. in the northern winter, with colder and stormier than average conditions across the North, and warmer and less stormy conditions across the South.
La Niña events are generally associated with increased rainfall in southern Africa, although they are not the only contributing factors. La Niña is associated with rainfall deficiency in equatorial eastern Africa – for instance Somalia and eastern Kenya.
A transition between El Niño and La Niña is not unusual as both are part of the same phenomenon called the El Niño/Southern Oscillation or ENSO, which swings between unusually warm and cold conditions over eastern and central tropical Pacific.
However, the warm and cold phases need not necessarily follow each other in quick succession, and are quite often separated by extended periods (from a few months to a couple of years) of neutral situations, i.e., neither El Niño nor La Niña. There have also been instances when an El Niño was followed by another El Niño, and La Niña followed by another La Niña, separated by neutral conditions of a few months.
There is no evidence to suggest that climate change is increasing the frequency of El Niño and La Niña events. But it is likely that it is increasing the impacts, especially of El Niño, in terms of more intense heat and heavier precipitation.
2015 was the hottest year on record because of the combined effects of climate change and El Niño. Even more records were broken in early 2016.
Even the relative cooling effect of La Niña will no longer likely to be sufficient to contain rising temperatures as a result of greenhouse gases in the atmosphere.
Further research is being conducted to conclusively establish the relationship between El Niño and/or La Niña events.
It is important to stress that other factors (such as the Indian Ocean Dipole or the North Atlantic Oscillation/Arctic Oscillation) can also have an important influence on seasonal climate.
The meteorological and oceanographic data that allow El Niño and La Niña episodes to be monitored and forecast are drawn from national and international observing systems. The exchange and processing of the data are carried out under programmes coordinated by the World Meteorological Organization.
The forecasting of Pacific Ocean developments is undertaken in a number of ways. Complex dynamical models project the evolution of the tropical Pacific Ocean from its currently observed state. Statistical forecast models can also capture some of the precursors of such developments. Expert analysis adds further value.
WMO facilitates consensus among the various centres and prepares an El Niño/La Niña Update, issued on a quasi-regular basis (approximately once in three months) through a collaborative effort with the International Research Institute for Climate and Society (IRI) and based on contributions from the leading centres around the world.
Scientific advances mean that the development of El Niño and La Niña events can be forecast months in advance. Regional Climate Outlook Forums in different parts of the world factor the likely development of El Niño or La Niña events into their seasonal predictions for users in the sectors of agriculture and food security, water management, health, disaster risk reduction and energy. The 2015/2016 El Niño drew on lessons learnt during the strong 1997/1998 event. Unprecedented preparation and collaboration between meteorological and climate services and decision-makers in affected sectors helped manage the risks and limit the loss of life.
Despite predictions, and disaster risk management, the humanitarian impact was considerable – for instance of El Niño-induced drought in Southern Africa. In terms of environmental impacts, El Niño and high ocean temperatures as a result of global warming led to a massive coral bleaching episode.