The Science of the El Niño Southern Oscillation


El Niño and La Niña are the warm and cool phases of a recurring climate pattern across the tropical Pacific—the El Niño-Southern Oscillation, or “ENSO” for short.

The pattern can shift back and forth irregularly every two to seven years, and each phase triggers predictable disruptions of temperature, precipitation, and winds.

These changes disrupt the large-scale air movements in the tropics, triggering a cascade of global side effects.



The El Niño/La Niña are shifts in ocean temperature in the Pacific Ocean, shown in the red to blue contours on the surface in the diagram above. The Southern Oscillation is the pressure change between Tahiti and Darwin, Australia, which affects the strength and direction of the winds. Together they are referred to as ENSO.

In normal, non-El Niño conditions (left, top diagram) the trade winds blow to the west along the equator from South America towards Asia in the tropical Pacific Ocean. These winds pile up warm surface water off Asia, so that the sea surface is about 1/2 meter (1 1/2 feet) higher at Indonesia than at Ecuador in South America.

During El Niño (bottom schematic diagram), the trade winds relax in the central and western Pacific leading to a flattening of the thermocline (blue band) due to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110ºW show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region.

The El Niño phenomenon can have an impact on weather conditions around the globe, for both the winter and summer periods, as seen in the maps below. The key shows which areas will be wetter, drier, warmer and cooler than normal.

Source of diagrams and some text: and

ClimateBits: El Niño

NASA: Observing the 2015 El Niño