Schematic Diagram

In normal, non-El Niño conditions (top panel), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about one-half meter higher at Indonesia than at Ecuador. During El Niño conditions (bottom panel), the winds pushing that water get weaker. As a result, some of the warm water piled up in the west slumps back down to the east, and not as much cold water gets pulled up from below. Both these tend to make the water in the eastern Pacific warmer, which is one of the hallmarks of an El Niño.