Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
In each ocean basin, ocean surface currents move in a circular pattern or gyre. North of the equator, the gyres flow clockwise and south of the equator they flow counterclockwise. In the southern Pacific Ocean, the main current flows north along the coast of South America, along the countries of Chile and Peru. Then, at the equator it turns toward the west and heads for Indonesia and Australia. As the current travels along the equator, the water is warmed by the sun and, in turn, helps warm the air. The warm water and air promotes evaporation. Large rain clouds form and are carried west bringing monsoons to Indonesia and southeast Asia.
Also at the equator, tradewinds are blowing west, helping to push the currents even more westerly. This causes the ocean water to pile up on the western side of the Pacific Ocean. When the tradewinds blow especially hard for a number of years, a lot of water piles up near Australia.
An El Niño event begins when the tradewinds slacken. Once the wind that had been pushing the water toward the west eases up, water begins flowing back toward the east. The current begins flowing toward South America, an event termed "the southern oscillation." As warm ocean water heads toward South America, it brings with it the rains that normally fall on Indonesia, causing extreme rains and somewhat cooler temperatures in Peru, Ecuador, California and Gulf Coast states. It also brings warm air, which pushes the jet stream of North America further north and creates milder winter temperatures for the northern and eastern United States. Another climate change is an increase in hurricane formation in the Pacific and a decrease in hurricane formation in the Atlantic.
Climate isn't the only thing affected during El Niño. The changes in ocean circulation and weather can cause drastic changes in the biology of an area. During non-El Niño years, the waters off the coast of Peru are cold and nutrient-rich. This makes it an extremely productive area, especially for anchovetta, which in the 1970s was the world's largest single fishery. When the southern oscillation occurs, cold, nutrient-rich water is replaced with warm, nutrient-poor water which leads to no large phytoplankton blooms and in turn few anchovetta. The El Niño/southern oscillation (ENSO) event combined with drastic overfishing has cause the anchovetta fishery to crash. Being able to predict an El Niño event could help fishermen and farmers prepare for the extreme climate changes to come.
Print out an El Niño climate tracking chart for each student. Using the data links below, record in the first set of columns the monthly temperature anomalies, or the difference from the normal average temperature, for each area. To determine the temperature difference, look at each state or country and identify which color(s) dominate that area. Use the scale on the side of the image to determine the range of temperature difference. For instance, an area with the lightest shade of pink has a temperature anomaly range of +1º to +3ºF for the U.S. As an example, August's data have already been recorded.
U.S. monthly temperature anomalies:
The second set of columns is for precipitation anomalies recorded as the percent of normal precipitation. A value less than 100% (browns & yellow) means precipitation is below average, while a value above 100% (greens and blues) means precipitation is above average. Use the links below to access the monthly precipitation anomalies.
U.S. monthly precipitation anomalies:
Record the climate changes each month. Analyze the data and determine if, indeed, 2002-2003 was a typical El Niño year. From our background resources on El Niño, the following climate changes were expected:
Open up the composite graphic that illustrates El Niño and La Niña anomalies from 1950-1992. Keep in mind that for temperature, blue represents colder than normal temperatures and red represents warmer than normal temperatures. For precipitation, green and blue represent wetter than normal areas and brown and yellow represent drier than normal areas.
After examing the maps, how do you think the 2002 event compared to previous events?
Was this a strong El Niño event or a mild one?
What climate clues might you use to predict when the next El Niño will come?