El Niño and La Niña are two opposite phases of a large-scale oscillatory pattern in the equatorial Pacific Ocean that is known to have dramatic consequences on the weather patterns across the whole ocean and surrounding landmasses. This oscillation is actually coupled to an oscillation pattern in the atmosphere, the so-called Southern Oscillation, and the phenomenon has therefore been named El Niño-Southern Oscillation, shortened in ENSO.
ENSO is a loosely periodic oscillation whose period varies between two to seven years, and which can last for nine months to two years. El Niño is defined as a prolonged warming of the sea surface temperature by at least 0.5°C in the east-central tropical Pacific Ocean. La Niña is the opposite situation characterized by a cooling of the same area.
The exact causes of this oscillation and its triggering events are still under research. During an El Niño situation, winds above the western equatorial Pacific Ocean weaken, whereby the low-pressure area that is usually located over Southeast Asia moves eastward. Winds over the eastern part of the basin also weaken, thereby reducing (and sometimes almost shutting down) coastal and equatorial upwelling along the eastern oceanic boundary. As a result, the tilting of the thermocline (the layer in which temperatures vary the greatest with depth) also changes across the whole basin, whereby subsurface temperatures in the western (resp. eastern) Pacific decrease (resp. increase) compared to the normal situation (Figure 3.6). In such conditions, subsurface temperatures may vary with up to 2°C or more compared to normal conditions. A very strong El Niño was recorded in 1997-1998.
La Niña is a reinforced state of the normal conditions, whereby the Trade winds intensify, thereby reinforcing the coastal and equatorial upwelling in the eastern part of the basin (Figure 3.6, below). A strong La Niña was recorded in 1988-1989.
El Niño’s and La Niña’s are associated with dramatic meteorological changes in all areas around the tropical Pacific Ocean with numerous extreme events (droughts, floods, cyclones, etc). They are believed to influence other regions throughout the world through so-called teleconnections, although some links remain tenuous.