Several weeks to several months before El Niņo 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 from west to east. The equatorial surface current 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 Niņo, 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 Niņo 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.
Figure courtesy of NOAA PMEL