El Nino

10 Pages 2408 Words

Pacific. He noticed that when pressure rises in the west it usually falls in the east and vice versa. He coined this term Southern Oscillation. When it is on its high index state pressure is high on the eastern side of the Pacific and low on the western side (figure 1). The east west pressure contrast drives easterly surface winds. When it switches into low index (figure 2) the easterly surface winds weaken. The biggest changes are over the western Pacific. West of the dateline the easterlies usually completely disappear during low index years, but east of the dateline they only weaken.
The surface winds that move the ocean currents are a major controller in weather. The easterly winds that blow along the Ecuador and the southeasterly winds that blow along the Peru and Ecuador coasts both tend to drag surface water with them. The Earth’s rotation then deflects the resulting surface currents northward in the Northern Hemisphere and southward in the Southern Atmosphere. The surface waters are therefore deflected away from the equator in both directions and away from the coastline. When the surface water moves away, colder nutrient-rich water comes up from below to replace it. This is known as upwelling. This nutrient rich water is the beginning of the food chain as phytoplankton establish themselves in these waters.
When the easterlies are blowing at full strength, the upwelling of cold water along the equatorial Pacific chills the air above it, making it too dense to rise high enough for water vapor to condense and form clouds. As a result, this strip of ocean stays free of clouds during normal years and rain in the equatorial belt is largely confined to the extreme western Pacific near Indonesia.
But when the easterlies weaken and retreat eastward during early stages of an El Nino event, the upwelling slows and the ocean warms. The moist air above the ocean also warms. It becomes light enough to form deep clouds ...