El Niño is a large scale oceanographic /

meteorological phenomenon that develops in the

Pacific Ocean, which is associated with extreme

climatic variability. It is the migration, from time to

time, of warm surface waters from the western

equatorial Pacific Basin to the eastern equatorial

Pacific region, along the coasts of Peru and Ecuador.

The warm current (El Niño) temporarily

displaces nutrient-rich up welling cold water

resulting in the heavy harvest of anchovies. The

abundant catch, however, is short-lived. What

follows is a sharp decline in the fish population,

resulting in a lesser catch. At times, warming is

exceptionally strong and ruins the anchovy harvest.

During an El Niño, the physical relationships

between wind, ocean currents, temperature, and

biosphere break down into destructive patterns.

El Niño: A temperature anomaly

The trade winds tend to lose strength with the onset of

springtime in the northern hemisphere. Less water is pushed

westward and, consequently, waters in the central and eastern

Pacific begin to heat up (usually several degrees Fahrenheit)

and the thermo cline tilt diminishes. But the trade winds are

usually replenished by the Asian summer monsoon, and the

delicate balance of the thermo cline tilt is again maintained.

Moreover, the natural spring warming in the central

Pacific is allowed to continue and also spread eastward

through the summer and fall. Beneath the surface, the thermo

cline along the equator flattens as the warm waters at the

surface effectively act as a 300-foot-deep cap preventing the

colder, deeper waters from upwelling. On average, these

waters warm by 3° to 5°F, but in some places the waters can

peak at more than 10°F higher than normal (up from

temperatures in the low 70s Fahrenheit, to the high 80s).

This animation shows the relationship between the direction and intensity of the Pacific trade winds, and the formation of El Niño. The arrows show surface wind dynamics, while the colors represent sea surface temperature. Notice how the warmer water expands, while cooler water contracts.

This series of images shows how the thermo cline tilt changes during the onset of an El Niño. Notice how in January 1997 (left) there is a steep temperature gradient between the western and eastern Pacific Ocean. In April 1997 (center), much of the deep pool of warm water in the west has migrated eastward, lessening the temperature gradient. By July 1997 (right), there is a full-blown El Niño and the thermocline is flat.

Historical Observations

Sir Gilbert Walker

Sir Gilbert Walker

empirically identified that

some notable climate

anomalies—changes in

atmospheric pressure and

circulation—happen around

the world every few years

Professor Jacob Bjerknes

Jacob Bjerknes proposed

that El Niño was just the oceanic

expression of a large-scale

interaction between the ocean

and the atmosphere, and that the

climate anomalies could be

understood as atmospheric

"teleconnections" spreading from

the warm-water regions along the

equator in the mid-Pacific.