Marine Mammal Care Center
Los Angeles

Science Corner - El Niño

El Niño is generating a lot of press these days. Simply put, an El Niño condition brings unusually warm temperatures to the equatorial Pacific, changing normal weather patterns. It’s opposite, La Niña, brings unusually cool temperatures to the same region.

El Niño’s formal name is the El Niño Southern Oscillation or ENSO, for short. The “oscillation” portion of this phenomenon’s name stems from the oscillation, or patterned change in surface air pressure between the Tropical Eastern Pacific and the western Pacific Ocean waters. California is on the west coast of North America, but it borders the eastern edge of the Pacific Ocean. The Eastern Tropical Pacific is generally considered to extend as far north as San Diego and as far south as Peru.

From an oceanographic perspective, a detailed description of how the ENSO works can become complicated. However, a relatively simple explanation can still provide an understanding of the processes involved.

The trade winds blow from east to west in the Pacific, moving warmer surface waters toward Indonesia. These warm waters actually pile up in the western Pacific resulting in the ocean surface being about one-half meter higher in Indonesia than off the west coast of South America. As this warm surface water is blown by the wind to the west, cold water upwells from the deep ocean off the coast of South America, bringing nutrient-rich water to the surface, water that supports a significant amount of marine biodiversity.

Rainfall develops over warmer waters, making the western Pacific region (Indonesia, Australia, Southeast Asia, etc.,) a very wet region and leaving the eastern Pacific relatively dry. Enter El Niño. As the trade winds subside, the warm water piled up in the western Pacific runs downhill back to the eastern Pacific, keeping the upwelling cold water from reaching the surface. Productivity is seriously reduced due to the lack of nutrients brought to the surface by the cold water; food webs suffer and serious disruptions occur in the associated marine ecosystems.

Since rain follows warm water, land areas that are normally dry, such as Southern California, get higher than normal rainfall which may result in flooding, depending on the intensity of the El Niño. Those regions in the western Pacific that are normally wet may suffer drought and sometimes devastating brush fires. An ENSO condition basically results in a reversal of normal weather patterns in the Tropical Eastern Pacific.

The National Oceanic and Atmospheric Administration (NOAA) Tropical Atmosphere Ocean (TAO) project measures real-time sea surface temperatures, winds, and currents from moored buoys in the Tropical Eastern Pacific. Recent ENSO conditions occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, 2002-2003, 2004-2005, 2006-2007 and 2009-2010. The last strong El Niño occurred in 1997-1998. Stronger El Niños occur about every 7-10 years. It is highly irregular for them to occur in successive years as they did in 1991-1994 and 2002-2007.

The current prediction for this year is for a 95% chance of a strong El Niño this winter, bringing heavy rains to eastern Pacific regions, including Southern California. For those of you interested in the science behind these predictions, visit NOAA’s El Niño Theme Page at or the NOAA’s Pacific Marine Environmental Laboratory TAO project page at

To learn more about the possible effects of El Niño on pinnipeds, please visit

Photo Courtesy of the National Oceanic and Atmospheric




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