7 -- INTERANNUAL WARMING OR COOLING OF SEA SURFACE

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.

1. It increases the stratification of the surface waters and the depth of the colder, nutrient-rich subsurface waters. Because of this and the decreased strength of the Trade Winds, upwelling of cold nutrient-rich waters and, consequently, the biological activity in the coastal regions of the eastern Pacific are significantly diminished.
2. Therefore, the SST distribution of the equatorial Pacific during an El Niño event consists of a more uniform east-west distribution, with warmer than normal temperatures in the eastern Pacific. Figure 1.17 illustrates this distribution.
3. The region of precipitation associated with the warm SSTs moves eastward with the Warm Pool. Therefore, an El Niño event is characterized by increased precipitation in the eastern Pacific and drought conditions in the western Pacific.