Easterly winds (red arrow) drag the surface water westward along the equator. The Earth's rotation deflects the western current toward the right in the Northern Hemisphere and toward teh left in the southern Hemisphere, driving the surface water away from the equator and bringing up water from below (upward arrows) In addition, the winds cause warm surface water to accumulate on the western side of the Pacific. Because of the lower density of the warmer water, sea level is about two feet higher on the western side of the basin than on the eastern side when the winds are blowing at full strength. The thermocline, which marks the boundary between warm surface water and cold, deep water (deeper blue) is tilted. It reaches almost up to the sea surface in the eastern equatorial Pacific.
Strong southeasterly winds (red arrow) prevail along the coast of southern Equador and Peru. These winds, which blow during both normal and El Nino years, drag the surface water northwestwad and cause cold, nutrient-rich water (dark blue) to upwell along the shore of the eastern Pacific.