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HOME > El Niño/La Niña > The ENSO Cycle > Evolution: Subsurface Temperatures
Subsurface Temperatures
Ocean Temperatures and Thermocline

The evolution of the El Niño and La Niña, as well as the transition between the extreme phases of the ENSO cycle, depends greatly on the subsurface ocean temperature structure and the variability of the low-level winds. As an El Niño episode evolves, significant changes occur in both the subsurface temperatures and in the depth of the oceanic thermocline (The thermocline separates the warm upper ocean from the cold deep ocean waters.). In the early stages of El Niño episodes the oceanic thermocline is deeper than normal in the western and central equatorial Pacific, in association with an abnormally deep pool of warm ocean water throughout the region.

As El Niño episodes progress to the mature phase, the depth of the thermocline gradually decreases in the central and western equatorial Pacific and increases in the eastern equatorial Pacific, in response to weaker-than-average low-level easterly winds. As a result, subsurface temperatures become cooler than normal in the western equatorial Pacific, and warmer than normal across the eastern equatorial Pacific. In the latter stages of El Niño episodes, both the depth of the thermocline and subsurface temperatures become less than normal throughout most of the equatorial Pacific as the heat in the upper ocean is gradually depleted.  Thus, the warmer than normal temperatures become increasingly confined to a shallow layer near the ocean surface in the eastern equatorial Pacific, setting the stage for a transition to either a neutral state or to a La Niña episode. This transition process is critically dependent on the evolution of the low-level atmospheric winds. For example, if the easterly winds strengthen sufficiently, they can produce upwelling over the eastern equatorial Pacific, bringing the cold ocean waters to the surface. If the drop in sea surface temperatures is sufficiently large, it can lead to the onset of La Niña conditions.

Conversely, in the early stages of La Niña episodes the thermocline is generally shallower than normal across the equatorial Pacific. The thermocline gradually deepens in the western Pacific during the mature phase of La Niña episodes, and in the central Pacific during the latter stages of the episode. As a result, the subsurface temperatures become warmer than normal in these regions, while the ocean surface temperatures remain colder than normal. This decrease in the overall volume of abnormally cold ocean waters indicates an increase in the upper ocean heat content, and results in conditions more favorable for a transition to either a neutral state or to an El Niño episode. Once again the critical factors in the transition are the low-level winds and the subsurface temperature structure.

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