Hundreds of years ago fishermen off the west coast of Peru noted how periodically around Christmas time the waters grew unusually warm and fish became scarce: a phenomenon they called 'the Christ Child' - El Nio. It begins when a mass of warmer water from the western Pacific moves east, displacing cooler, nutrient-rich waters in the vicinity. This warmer water adds moisture to the atmosphere, raises rainfall levels and disrupts atmospheric circulation on a global basis.

La Nia is an equivalent cooling event during which the warm waters shift westwards to induce upwelling of cold water, reducing rainfall in the eastern equatorial Pacific but increasing it in the west. Researchers now recognise that these twin extremes of El Nio and La Nia are ocean components of a larger phenomenon that extends to the atmosphere, called the El Nio Southern Oscillation (ENSO).

Back in the 1920s meteorologist Sir Gilbert Walker noticed seasonal fluctuations in the air pressure difference across the equatorial Pacific, which he called the Southern Oscillation, and in the 1960s the realisation came this was linked to El Nio and La Nia events. Inter-annual ENSO variations can influence weather patterns worldwide, and researcher seek to combine all available data for enhanced understanding and forecasting.

So today, as the Pacific warm pool shifts westward and La Nia's 'cold tongue' of cool water extends across the eastern Pacific, it is being monitored via a global ocean observing system that includes an important space element.

Sea surface height (SSH) is not constant but varies across the global ocean, with vertical expansion due to increased water temperature being on
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Contact: Mariangela D'Acunto
mariangela.dacunto@esa.int
39-069-418-0856
European Space Agency
3-Mar-2006