El Niño
One of the dramatic manifestations of the interaction between the oceans and the atmosphere and its effects on both climate and weather is the Southern Oscillation, one of the consequences of which is El Niño. The Southern Oscillation is a back and forth variation in atmospheric pressure between a high pressure system normally located off the west coast of South America and a low pressure system normally located in the western Pacific near Indonesia and Australia. In the U.S. El Niño conditions result in heavy rains, flooding, landslides, and tornadoes in greater than normal amounts because El Niño conditions drive abnormal amounts of moist warm air across North America. El Niño causes flooding in Peru, as well as drought and fires in Indonesia and Australia.
The phenomena is manifested by the arrival of warm water off the coast of Peru around Christmas time, and thus is called El Niño (Spanish for the boy child) because it arrives at this time. An El Niño event occurs every 2 to 7 years with various degrees of strength. Some El Niño events are more intense than others, and the condition lasts from 18 to 24 months. The following table lists the years of El Niño events.
| El Niño Years | |||
|---|---|---|---|
| 1902-1903 | 1905-1906 | 1911-1912 | 1914-1915 |
| 1918-1919 | 1923-1924 | 1925-1926 | 1930-1931 |
| 1932-1933 | 1939-1940 | 1941-1942 | 1951-1952 |
| 1953-1954 | 1957-1958 | 1965-1966 | 1969-1970 |
| 1972-1973 | 1976-1977 | 1982-1983 | 1986-1987 |
| 1991-1992 | 1994-1995 | 1997-1998 | |
"Normal" conditions
Under "normal" conditions the easterly trade winds, driven by the pressure difference between the eastern Pacific high and the western Pacific low and blowing toward the equator, push warm water toward the equator and across the Pacific Ocean toward Australia and Indonesia. This causes a pool of warm water to form near the equator in the western Pacific. It also causes the thermocline (the boundary between warm waters in the upper layers of the ocean and the cold deep waters below) to move closer to the surface off the coast of South America, bringing nutrient-rich waters to surface by upwelling. Such nutrient-rich waters help sustain large fish populations. The upwelling cold water cools the atmosphere above, and prevents rain clouds from forming off the coast of Peru.
The warm water pushed to the west by the trade winds, heats as it flows along the equator, so that on arrival in the western Pacific heat is added to the overlying atmosphere causing it to rise, form clouds, and produce extensive rainfall. The moisture depleted upper atmosphere then circulates back to east where it descends off the coast of South America contributing to the dry conditions. During periods of exceptionally strong trade winds the upwelling of cold water off the South America cools the water even further creating a condition called La Niña (girl child).
El Niño Conditions - During El Niño periods there is a weakening of the easterly trade winds and the warm waters of the western Pacific are pushed toward the east. This causes the thermocline in the eastern Pacific to sink, preventing the upwelling of cold waters from below, depleting the waters in nutrients, and thus leading to starvation of fish populations. As the warm water shifts eastward so does the development of atmospheric disturbances that lead to upwelling of the atmosphere to form thunderstorms. Rising bodies of moist air thus occur closer to the coast of the Americas, leading to increased storminess, not only in South America, but in North America as well. These low pressure systems that develop in the eastern Pacific can move over the continent and cause severe weather as noted above.
In addition, they create upper level winds that tend to shear the tops off of developing tropical storms and hurricanes in the Atlantic Ocean and Gulf of Mexico, leading to a decrease in the number of intense tropical cyclones that develop in these regions.
La Niña conditions have about the opposite effect of El Niño conditions. i.e. better fishing harvests off the west coast of South America, drier conditions in North and South America, more hurricanes in the Atlantic, and wetter conditions in Australia and Indonesia.
Over the past 50 years, the oscillation of warm water back and forth across the tropical Pacific Ocean has created El Niño conditions 31% of the time, La Niña conditions 23% of the time, and "normal" conditions 46% of the time.
