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HOME > El Niño/La Niña > The ENSO Cycle > Evolution: Low-Level Winds
 
 
Low-level Winds
Ocean Temperatures and Zonal Winds

The evolution of the oceanic temperatures and thermocline structure during El Niño and La Niña depends greatly on the structure and evolution of the low-level wind field. The low-level easterlies are weaker than average (westerly anomalies - indicated by positive departures in the figure) across the central and eastern equatorial Pacific during an El Niño episode, and stronger than average during a La Niña episode. During El Niño episodes, the reduced easterly winds act to diminish the build-up of warm water across the western tropical Pacific, resulting in below-normal ocean temperatures at thermocline depth in that region. Farther east, they act to suppress oceanic upwelling across the central and eastern Pacific, contributing to a deep layer of abnormally warm ocean waters and an increased depth of the oceanic thermocline. These conditions result in a flattening of the thermocline across the equatorial Pacific.

In contrast, La Niña episodes feature increased low-level easterly winds, which contribute to a build-up of warm water across the western tropical Pacific and to an increased thermocline depth in that region. Farther east, they act to increase oceanic upwelling across the central and eastern Pacific, contributing to abnormally cold ocean waters and to a reduced depth of the oceanic thermocline. These conditions result in an increased slope of the thermocline across the equatorial Pacific.

The character of the transition between the extreme phases of the ENSO cycle is greatly influenced by the variability of the low-level winds. During the latter stages of the El Niño, the equatorial easterly winds become re-established across the central and eastern equatorial Pacific, resulting in an increase in oceanic upwelling which can sometimes rapidly bring the oceanic thermocline toward the ocean surface. This evolution results in a drop in sea surface temperatures and an end to the El Niño. If this drop in temperatures is sufficiently large, it can signal the onset of La Niña conditions.

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