Linda F. Malanson
(figure available in print form)
The trade winds drive the South Pacific equatorial current which carries surface water west and leaves relatively cool water off South America and a much deeper layer off Indonesia. Every few years, however, this normal pattern changes.
The trade winds weaken and the temperature and rainfall patterns change over a huge area stretching from the Pacific coast of South America to the Indian ocean. This is called Southern Oscillation, and when it happens the equatorial current in the Pacific reverses. The oscillation is between the two states.
Warm water flows east and then south along the coast of Peru. This current is call El Nino. The two changes together are call an El Nino—Southern Oscillation (ENSO) event. This affects weather and climate over other parts of the world as well as the Pacific area.
An ENSO event raises the sea temperature off South America by 18 degrees Fahrenheit. Cold water, rich in nutrients, cannot penetrate the deeper layer of warm water. This reduces the amount of phytoplankton (tiny plants) in the surface water. The number of animals and fish that feed on the plants also dwindles, affecting the Peruvian fishing industry.
ENSO events usually happen in December. The strongest one in recent history occurred in 1982-1983. It generated a series of changes in the oceans that moved farther and farther from the equator, producing climatic changes that were still being felt in 1994. Another ENSO event lasted two years, in 1991-1993. El Nino brings rainstorms to Peru, unusual weather to North America and may affect crops in Zambia and elsewhere.
El Nino and the monsoons are driven by the same process.
EXPERIMENT: MAKING YOUR OWN EL NINO
(figure available in print form)
The trade winds blow warm surface water across the ocean, so it forms a pool on one side and only a thin layer on the other. During an El Nino, the flow reverses. This happens on a huge scale right across the Pacific, but the principle is simple. With the help of a friend you can easily see how it works.
MATERIALS NEEDED:
1. food coloring
2. water
3. ruler
4. wide, clear-sided container
DIRECTIONS:
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1. Fill the container with cold water to within about 10 cm (4 in.) of the brim. When the water is quite still, trickle colored, hot tap water into the container to form a surface layer.
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2. Use the ruler to measure the thickness of the warm, colored layer of water at either end of the container. This should be the same at both ends.
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3. Ask your friend to blow the water to make a “trade wind” that moves the colored water toward one end. To make your El Nino, ask your friend to stop blowing so that warm water flows to the other end. (Compare the depth of the colored layer to the level before you started to blow.)