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.