page_title: El Niño–Southern Oscillation
section_title: Sea surface temperature oscillation
hierarchical_section_title: El Niño–Southern Oscillation / Sea surface temperature oscillation
caption_attribution_description: English: This image defines the various "Nino" regions where sea surface temperatures are monitored to determine the current phase of ENSO (warm or cold)
context_page_description: El Niño–Southern Oscillation is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and typically occur every few years with varying intensity per period.
  The two phases relate to the Walker circulation, which was discovered by Gilbert Walker during the early twentieth century. The Walker circulation is caused by the pressure gradient force that results from a High-pressure area over the eastern Pacific Ocean, and a low-pressure system over Indonesia. Weakening or reversal of the Walker circulation decreases or eliminates the upwelling of cold deep sea water, thus creating an El Niño by causing the ocean surface to reach above average temperatures.
context_section_description: Within the National Oceanic and Atmospheric Administration in the United States, sea surface temperatures in the Niño 3.4 region, which stretches from the 120th to 170th meridians west longitude astride the equator five degrees of latitude on either side, are monitored.  This region is approximately 3,000 kilometres (1,900 mi) to the southeast of Hawaii.  The most recent three-month average for the area is computed, and if the region is more than 0.5 °C (0.9 °F) above (or below) normal for that period, then an El Niño (or La Niña) is considered in progress.  The United Kingdom's Met Office also uses a several month period to determine ENSO state.  When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".
caption: The various "Niño regions" where sea surface temperatures are monitored to determine the current ENSO phase (warm or cold)
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