This name was used for the tendency of the phenomenon to arrive around Christmas. In normal, non-El Nino conditions, 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 half a metre higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting diverse marine ecosystems, and major fisheries.
Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.
During El Nino, the trade winds relax in the central and western Pacific. The result was a rise in sea surface temperature. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific. During the other extreme of the Oscillation, the eastern tropical Pacific is cold and heavy rainfall and flooding is observed over the areas usually affected by drought during El Nino events.