Easterly winds (red arrow) drag the surface water westward along the equator. The Earth's rotation deflects the westward current toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, driving the surface water away from the equator and bringing up the 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 then 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 (darker blue), is tilted. It reaches almost up to the sea surface in the eastern equatorial Pacific.

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