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El Niño in December 1997
El Niño in December 1997. Visualisation: GEOMAR

How does El Niño Change in a Changing Climate?

The natural climate fluctuation El Niño / Southern Oscillation (ENSO) is caused by a complex interaction between the ocean and the atmosphere. Every two to seven years on average, the eastern tropical Pacific warms up, usually around Christmas, hence the name El Niño (in Spanish "the Christ child"). One of the strongest El Niños, which occurred in 1997, claimed more than 23,000 lives and caused US$33 billion in damage due to its extreme weather events. It is therefore of great interest to know when such events occur and whether they change in intensity and frequency in the context of global warming.

Illustration El Niño: Normal weather conditions
Normal weather conditions: Higher air pressure over the eastern Pacific and lower pressure near Indonesia create easterly trade winds along the equator. These winds promote upwelling and cooler water in the eastern Pacific, while warmer water prevails in the western Pacific. The trade winds are part of the Walker circulation, which leads to heavy rainfall over the western Pacific and generally dry weather over the eastern Pacific.
Illustration El Niño: El Niño conditions
During an El Niño event, the atmospheric Walker circulation over the tropical Pacific changes. This results in extreme precipitation in the otherwise dry eastern Pacific and South America, while the Indonesian-Australian region suffers from drought. Illustration: Christoph Kersten / GEOMAR

At GEOMAR, observational data and climate model experiments are used to investigate what influence climate change will have on El Niño. It was found that the atmospheric part of ENSO, the so-called Walker circulation, is very important for the strength of El Niño events, as it has a major influence on the course of El Niño via the shifting of the atmospheric convection regions in the tropics.

In addition, current questions being researched in the Maritime Meteorology unit in collaboration with scientists from around the world: Will the Walker circulation become stronger or weaker in climate change, how does the complex interaction of ocean and atmosphere change in the ENSO phenomenon and what influence does this in turn have on the strength and frequency of El Niño events?