Perhaps the best known example of interannual variations in SST is the El Niño Southern Oscillation (ENSO) that occurs in the equatorial Pacific. Normally, the winds from the equator to about 30°N are from the northeast and are steady (the Northeast Trade Winds), while they are from the southeast and steady south of the equator to about 30°S (the Southeast Trade Winds). Refer again to Figure 1.10. Since the Coriolis effect is very weak near the equator, and nonexistent at the equator, the surface waters move westerly across the equatorial Pacific Ocean because of the Northeast and Southeast Trade Winds. As the waters move along the equator, they are heated by the Sun. Coastal upwelling is also occurring in the eastern equatorial Pacific because of the wind driven divergence of the surface waters near the continents. As a result, the normal equatorial SST distribution consists of the warmest SSTs in the western equatorial Pacific and cooler SSTs in the eastern equatorial Pacific. Figure 1.16 shows the normal SST conditions in the equatorial Pacific Ocean.
The SSTs are normally about 8°C warmer in the western than eastern equatorial Pacific (NOAA El Niño Theme Page). Also, the height of the sea surface in the western Pacific is normally about half a meter higher than in the eastern Pacific (NOAA El Niño Theme Page). This is due to a combination of the trade winds physically piling up the water in the west and the increased volume of the warmer waters that result from thermal expansion.
Since warmer SSTs are associated with rising moist air, clouds and precipitation are closely associated with the pool of warm SSTs in the western Pacific (the Warm Pool). During an El Niño, the normally steady Northeast and Southeast Trade Winds weaken or shift direction. As a result, the pile of warm water in the western Pacific is allowed to propagate toward the east. The arrival of these anomalously warm waters in the eastern Pacific has multiple effects.