Several weeks to several months before El Niño begins to manifest in the eastern Pacific, a dramatic change occurs in the atmosphere over the Pacific. The pressure over the western Pacific increases while the pressure in the east decreases. This causes a change in the wind pattern and the convection, as shown in the figure below. This atmospheric change is called the Southern Oscillation. The trade winds decrease or even reverse and blow west to east. The equatorial surface slows, and a subsurface current that always exists strengthens. This undercurrent has a core at about 100 meters depth and always flows west to east. Most of the time it dies out in the central Pacific. During El Niño , though, the current is strong all the way to the Galapagos Islands off the coast of South America; it sometimes surfaces.

Thus, there is a change in the ocean associated with the Southern Oscillation. In the west, the thermocline rises and warm water flows east. In the east, months after the initial wind changes, the thermocline gets deeper. The upper water column warms, and the sea-level rises.

This is a simplified model. The ocean takes some time to adjust to the wind changes. One of the primary ways it adjusts during El Niño is through the creation of Kelvin waves. These waves are very different from the surface waves you are probably familiar with and are discussed more in the following slides.

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Slide 10 of 27


Last Modified: Fri, Jul 2, 1998