El Niño : Characteristics

What happens during a typical El Niño?

1. The trade winds that blow from east-to-west cease, decrease in intensity, or reverse. Trade winds normally keep warm water pushed to the west side of the equatorial Pacific.



2. When the trade winds cease, warm waters of the western equatorial Pacific begin to slosh eastwards. Due to this sloshing, some people refer to the El Niño as a surface ocean current. This change can take place over the course of several months, allowing detection of the El Niño by sensors on buoys across the Pacific.



3. The trade winds cease because the relative atmospheric pressure between eastern and western sides of the Pacific changes. Typically winds blow from east-to-west because atmospheric pressure is higher in the east. Winds reverse when the relative pressure between west and east reverses. This alternation of atmospheric pressure is called the Southern oscillation. Since the El Niño (EN) and Southern Oscillation (SO) are often linked, some scientists prefer to call these events ENSOs.



4. During the El Niño, the thicker than normal layer of warm water in the eastern Pacific cuts off the delivery of nutrients by reducing upwelling from below. The warm water prevents vertical mixing of water and keeps the cooler nutrient-laden water away from the surface where most life is concentrated.



5. When less nutrients are made available to surface waters, not as much life can be supported because there is less biological productivity at the base of the marine food web. Since the photosynthetic organisms at the base of the food web need light, they live at or near the sea surface. When nutrients are cut off from the sea surface, they are less productive. This leads to the starvation of marine life at higher levels of the food web and a reduction of population. During an El Niño, there young marine mammals suffer. Sometimes many of next year's recruits do not make it. In the absence of other stresses, populations rebound after El Niños end.

What's a La Niña?
During a La Niña event, abnormally cool water develops along the eastern equatorial Pacific. The features of the La Niña are generally opposite to those of El Niño.