El Nino Southern Oscillation (Enso)

El Nino Southern Oscillation (ENSO) is an artifact of the relationship between the atmosphere and the ocean. El Nino and La Nina represent opposite extremes in the ENSO cycle (Climate Prediction Center Internet Team, 2005). It is related to air pressure, wind, and ocean currents, and happens "every 3 to 7 years and alternate with the opposite phases of below-average temperatures in the eastern tropical Paci-c (La Nina)" (Trenberth, et al., 2007). The impacts are very strong in the northern winter months (November-March), though not limited to the Northern Hemisphere (Tropical Atmosphere Ocean Project, 1998).

(Tropical Atmosphere Ocean Project, 1998)

ENSO's warm phase (El Nino) involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America (Trenberth, et al., 2007). These waters are usually cooler off of South America, bringing an upwelling from the deep ocean (Tropical Atmosphere Ocean Project, 1998). The cold water is usually rich in nutrients, which supports lots of sea life, and so major fisheries (Tropical Atmosphere Ocean Project, 1998). Normally, there is significant rainfall in the eastern Pacific from the warm, humid water and western South America is dry (Tropical Atmosphere Ocean Project, 1998).

ENSO's warm phase is the result of a weakening of the usually strong Sea Surface Temperature (SST) differential across the equatorial Pacific (Trenberth, et al., 2007). During ENSO's nine to twelve month warm phase (Climate Prediction Center Internet Team, 2005) (though it lasted for 3 years from 1939 to 1941 (Trenberth, et al., 2007)), the normally constant trade winds in the central and western Pacific slow (Tropical Atmosphere Ocean Project, 1998). The air pressure changes from higher in the eastern Pacific and lower in the western Pacific to the opposite and this is called the Southern Oscillation (Climate Prediction Center Internet Team, 2005). The cold waters in the eastern Pacific stop coming up, while the western Pacific waters get colder (Tropical Atmosphere Ocean Project, 1998). Because the nutrients are no longer there, there are no more fish, and the fisheries collapse during ENSO's warm phase (Tropical Atmosphere Ocean Project, 1998).

Because the warm water slips towards the east and South America, the warm humid rain goes with it, and Peru, Ecuador, southern Brazil, central Argentina, and equatorial eastern Africa get flooded (Tropical Atmosphere Ocean Project, 1998), leading to disease, particularly bacterial diseases like cholera (Pascual, Xavier, Ellner, Colwell, & Bouma, 2000). Southeastern Asia (including India), southeastern Africa, Japan, southern Alaska, and western/central Canada, southeastern Brazil, south-central Africa, and southeastern Australia get a dry heatwave from December to February, instead of their monsoons (Tropical Atmosphere Ocean Project, 1998). In the meantime, it is cold in December through February along the Gulf coast (Climate Prediction Center Internet Team, 2005).

Because the ocean affects the atmosphere, global air circulation changes (Tropical Atmosphere Ocean Project, 1998). In fact, the trade winds weaken (Tropical Atmosphere Ocean Project, 1998). In North America, the temperatures are warmer than usual in the north central states because the jet stream shifts towards the southeast, leading to major snowstorms on the East Coast (Tropical Atmosphere Ocean Project, 1998). Correspondingly, it's cooler than normal in the Southeast and Southwest United States (Climate Prediction Center Internet Team, 2005). Because the upper westerly winds are more vertical, the tropical North Atlantic has fewer hurricanes, while the eastern tropical North Pacific has more (Climate Prediction Center Internet Team, 2005).