Questions and answers about the climate phenomenon and its consequences
This satellite image from the National Oceanic and Atmospheric Administration (NOAA) shows Hurricane Hilary (right) off the Mexican Pacific coast. Scientists believe that a natural El Niño phenomenon, human-caused climate change, a persistent heat dome over the central United States, and other factors combined to drive Hilary's forceful passage across California and Nevada in August 2023.
© AP / picture alliance
How do El Niño and La Niña develop, and what happens during these events?
El Niño is a natural climate phenomenon in which the surface waters of the central and eastern Pacific become unusually warm. It occurs every two to seven years, typically towards the end of the year (hence the name, Spanish for "Christ child")
El Niño is triggered when the trade winds, which normally blow from east to west across the Pacific, are weakened, for example by tropical storms. Under normal conditions, the trade winds push the Pacific's warm surface water westward. In the east, off the coast of Peru, cold, nutrient-rich water then rises from the depths, sustaining abundant fish stocks. When the trade winds weaken, warm water instead accumulates at the surface off the west coast of South America. Once this water is more than 0.5 °C warmer than average, meteorologists speak of an El Niño. The warm water can also spread from the tropics into the subtropics.
El Niño is part of the El Niño-Southern Oscillation (ENSO), whose counterpart is La Niña. During a La Niña event, the trade winds intensify: warm water is driven further west across the Pacific, allowing more cold water to well up along the west coast of South America. When surface water is more than 0.5 °C cooler than average, this is classed as La Niña.
What are the regional and global consequences of El Niño - climatic, health-related, ecological and economic?
El Niño and La Niña affect weather patterns worldwide, but their impact is especially pronounced in South America, Indonesia and Australia. The most serious consequences of an El Niño event include the following:
- Extremely heavy rainfall along the west coast of South America, which can cause flooding with numerous casualties and extensive damage.
- Off the Peruvian coast, the cool, nutrient-rich water fails to surface, causing fish stocks to decline and disrupting the fishing industry.
- Severe droughts occur in Indonesia and Australia. These hamper rice production and favour wildfires, particularly in Indonesia, where fires are often started deliberately to clear land but then run out of control, also causing heavy smoke pollution.
- The likelihood of intense hurricanes in the Atlantic decreases, although climate change is warming the Atlantic, which in turn raises hurricane risk.
- The likelihood of intense tropical cyclones rises in the Pacific (known as typhoons in the western Pacific and hurricanes in the eastern Pacific).
- In Europe, El Niño can bring cold late winters because it disturbs the polar vortex, a band of strong winds that encircles the cold air masses over the Arctic. This disturbance allows cold polar air to flow south and warm air to flow north.
- The monsoon weakens across South Asia. It was through this connection that the global impacts of El Niño and its underlying causes were first discovered: several El Niño events occurring in close succession in the early 20th century caused extreme droughts and devastating famines. This prompted the British meteorologist Sir Gilbert Walker to investigate the underlying causes, leading him to identify the El Niño-Southern Oscillation (ENSO), along with its causes and its link to the monsoon.
In some respects, La Niña has the opposite effects. As it represents an intensification of the climatological norm, its consequences are less pronounced:
- Less rainfall along the west coast of South America.
- Heavy, widespread rainfall across Southeast Asia and north-eastern Australia.
- The likelihood of intense hurricanes in the Atlantic increases.
Under what conditions does El Niño become especially strong?
An El Niño is considered strong when the surface water temperature off the west coast of South America is more than 1.5 °C above the long-term average. This is favoured not only by warm water accumulating in the western Pacific, but also by the formation of a reservoir of warm water in the depths of the central Pacific. If this reservoir migrates eastward and rises to the surface off South America, a strong El Niño becomes likely.
Climate models forecast, as of June 2026, an El Niño in which water temperatures in the eastern Pacific are elevated by around 1.3 to roughly 3.5 °C compared with the long-term average. The graph begins with the observed monthly mean value for May; the forecast temperatures likewise each refer to a three-month mean. Dynamic models capture climate processes comprehensively, incorporating as many relevant physical relationships as possible by means of complex mathematical equations. Statistical models, by contrast, derive forecasts using statistical methods from simplified physical relationships, drawing on individual indicator variables and historical data.
© Columbia Climate School - IRI
Can it already be assessed whether El Niño will be especially strong by the end of 2026?
There are already signs that a strong El Niño will develop by the end of the year. In particular, meteorologists have observed a pronounced pool of warm water deep in the central Pacific, migrating eastward. The US National Oceanic and Atmospheric Administration (NOAA) currently puts the probability of a strong El Niño at just over 60 per cent. To date, El Niño events with a temperature anomaly of just over 2 °C have been observed. The strength of an El Niño event can only be reliably forecast towards the end of summer.
How does the strength of an El Niño event affect its impacts?
"The intensity of rainfall depends heavily on the temperature of the sea surface over which it forms, since a warmer atmosphere holds more moisture and higher temperatures drive the water cycle more vigorously. Rainfall along the west coast of South America is therefore particularly heavy during a strong El Niño.
The intensity of tropical cyclones likewise depends on the sea surface temperature over which they form, since cyclones draw their energy from the ocean. This is why typhoons in the Pacific also become more destructive during a strong El Niño.
In many cases, a strong El Niño does not so much intensify its effects as make them more likely. This applies, for example, to the cold late winters that El Niño can bring to Europe. As with many meteorological developments, whether this actually happens depends on numerous factors, of which El Niño is only one. A strong El Niño, however, makes it more likely that its influence will prevail.
What is known about how El Niño will change under climate change?
Studies using high-resolution climate models that have examined this question predict that human-caused climate change could generally intensify El Niño events in the second half of this century. However, this question remains the subject of ongoing research, in which the Max Planck Institute for Meteorology is also involved.
