The El Niño and La Niña phenomena are the oceanic components and the two opposite phases of the same system: El Niño-Southern Oscillation (ENSO). The latter is a large-scale phenomenon that will simultaneously lead to changes in oceanic and atmospheric circulation in the tropical Pacific, leading to disruptions in the trade winds, atmospheric pressure fields, ocean surface temperatures and thus to alter temperatures and areas of precipitation in many parts of the globe, both from the tropical belt and around the Pacific Basin, but also beyond.
El Niño is a phenomenon that has been known for a long time and was so named by South American fishermen about Jesus, as it appears around Christmas. Normally, the Humboldt current runs along the coasts of Chili, Peru, and Ecuador. This sea surface current is cold and rich in plankton, making its waters very fishy. But from December to April, a weak reverse current starts moving, replacing the cold waters with warmer waters and therefore less fish. At regular intervals (every two to seven years), this reverse current is stronger, impacting the activity of the fishermen. Conversely, the years of the strong El Niño, the coastal regions of northern Peru and Ecuador, experience abundant rainfall favoring agriculture in these regions.
Figure 1: The 3 ENSO phases
(Source Australian Bureau of Meteorology)
It was in the sixties that meteorologist Jacob Bjerknes established the link between El Niño and the Southern Oscillation. The latter is an atmospheric phenomenon discovered by Gilbert Walker in the 1920s, which explains the variations in pressure between the east and west of the Pacific, and thus the intensity of the trade winds (regular winds blowing from east to west).
This variation in the strength of the trade winds determines, among other things, the intensity of the El Niño current. Indeed, in normal times, the trade winds drive the hot surface waters to the west.
This shift of warm water causes the deep, cold water to rise in the east of the equatorial Pacific. In an El Niño episode, the high pressure of the South Pacific decreases, the trade winds weaken and even reverse, which has the effect of slowing considerably this rise of cold water.
The El Niño years are therefore marked by a warmer oceanic surface temperature in the eastern equatorial Pacific basin, and vice versa for the La Niña years.
In general, the El Niño and La Niña events develop during the April-June period and reach their maximum between December and February.