Figure 4: Schematic cross sections of the Pacific Ocean under normal conditions (top) and El Niño conditions (bottom). Shown are the sea-surface temperature (SST) patterns, with orange corresponding to SSTs greater than 29°C and with a 1°C contour interval; the thermocline, which divides the cold deep ocean from the mixed surface layers; and the change in tropical atmospheric convection in the form of organized thunderstorm activity. Steady trade winds blowing east to west along the equator as shown by the small arrows in the top panel induce the accumulation of warm surface waters and enhanced precipitation in the Indonesian archipelago. Note the depression of the thermocline in this region. Under El Niño conditions, surface pressure is higher than normal over the Indonesian archipelago, the trade winds weaken and may actually become westerlies between Indonesia and the Internaitional Date Line, and the warm pool migrates eastward towards the Pacific coast of equatorial South America. Note the enhanced precipitation over the central Pacific and the depression of the thermocline in the eastern Pacific during El Niño.
Last revised: Wed May 17 11:28:21 MDT 2000