El Niño/La Niña and the Southern Oscillation

El Niño is the result of the interaction between the ocean and the atmosphere in the Pacific and the resulting effect on global climate. It is caused when the trade winds that blow from east to west along the equator in the Pacific decrease in intensity (this is the Southern Oscillation) and bring about an increase in the ocean temperature. This in turn affects where storms occur along the equator and triggers ripple effects worldwide.


Expected sesonal effects of El Nino and La Nina
Expected seasonal effects of El Niño (warm episodes) across the globe during December− February (top) and expected seasonal effects of La Niña (cold episodes) during the same time period(bottom)
Source: ClimateDiagnosticsCenter, NOAA.

Under normal conditions, the westward blowing winds push the ocean’s water towards western Pacific. In the deeper eastern Pacific, cold water is pulled from the ocean’s depths to replace the water that is blown west. The normal ocean temperature balance is therefore warm water (30oC) in the west and cold water (22°C) in the east.


El Niño events occur about every two to eight years and are considered a normal phenomenon. In an El Niño event, the trade winds weaken and less water is pushed westwards. Cold water in the eastern Pacific is not pulled up, making the water in that part of the ocean warmer than usual. The warmer water contributes to the weakening of the winds in a self-perpetuating or positive feedback cycle. 


El Niño events generally cause droughts in several regions, including the Caribbean, the western Pacific, Southeast Asia and Southern Africa. They are also associated with coral bleaching, a phenomenon brought about by high water temperatures that cause corals to lose the symbiotic algae that boost their metabolism, respiration, waste excretion and growth rate.  Very severe coral bleaching occurred in the tropical Indian Ocean in 1998, the hottest year on record to date and the year of the strongest El Niño recorded. That year, ocean temperatures were between 3 and 5°C above normal .


El Niño events are generally followed by a period of opposite conditions called La Niña. In La Niña conditions, the water in the eastern Pacific is cooler and the winds that blow from east to west are stronger than normal. In most locations, the phenomenon is associated with increased rainfall. El Niño and La Niña are experienced worldwide. Preliminary findings suggest  that climate change will cause El Niño events to increase in frequency.  When the El Niño phenomena are superimposed on the upward climate change-induced temperature trends, phenomena such as coral bleaching will become more frequent. It has been suggested that bleaching may become an annual event by 2015 .