Figure 1. A schematic representation of conditions at and beneath the surface of the tropical Pacific Ocean, extending from the longitude of Australia on the left to the Americas on the right. Contours are isotherms of sea-surface temperature, with warmer surface temperature in darkest blue. The layer beneath the surface, labeled Thermocline, separates the Sun-warmed water near the ocean surface from the much colder waters of the deeper ocean below. Wide arrows along the equator indicate the direction of prevailing surface winds. Dashed lines depict the closed loop of vertical and horizontal circulation of air above the ocean, driven by convection and marked by towering tropical cumulus clouds. The upper diagram (a) is for "normal" conditions. When an El Niño is in progress (diagram b), the westward-blowing winds are reversed, warmer surface waters migrate eastward, and the thermocline becomes shallower in the west and deeper in the east. Regions of heavy tropical rain are also shifted eastward. (Based on a diagram by M. McPhaden, NOAA/PMEL.)