Linda F. Malanson
(figure available in print form)
Warm and cold ocean currents can change the climate of coastal regions, but only when prevailing local winds blow from the sea to the land. Warm currents bring higher temperatures and more precipitation, while cold currents can lower temperatures and shorten growing seasons.
Most ocean currents are driven by prevailing winds, but the earth’s rotation deflects them into roughly circular paths, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
These circular currents, called gyres, are strongest in the largest oceans—the North and South Atlantic and the Pacific. The North Atlantic gyre moves from the warm north coast of South America toward Florida, then east across the ocean, and south off Portugal.
The following picture details the main ocean currents. Western currents such as the Gulf Stream and the Kuroshio flows faster than those on the eastern sides of oceans. Off the coast of Florida the Gulf Stream is 50-75 km (31-47 miles) wide and flows at 4-11 km/h (2.5-7 mph).
The Kuroshio is about 80 km (50 miles) wide and flows at a speed of about 11 km/h (7 mph). The Benguela current, flowing northward along the west coast of Africa, flows at about 1.0 km/h (0.6 mph). The most constant current is the Antarctic Circumpolar current, which flows through the southern oceans.
EXPERIMENT: DEEP-WATER CURRENTS
(figure available in print form)
The general circulation of water in the Atlantic and Pacific is driven by deep, slow-moving currents of cold water that flow from the poles toward the equator. When sea water freezes, its salt separates from it.
Sea ice is fresh, but the water close to it is saltier than other water because it contains the salt that was removed during freezing. This makes the water denser. Water close to the ice is also cold, and this dense water sinks all the way to the ocean floor, flows away from the ice and is replaced by warmer surface water which then cools and sinks.
These deep water currents move only 1.5-2.2 m (2-3 yards) a day. This experiment shows you how to make a deep-water current in a dish.
MATERIALS NEEDED:
1. ice
2. food coloring
3. eye dropper
4. water
5. glass dish
6. aluminum foil
DIRECTIONS:
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1. Fill the glass dish with warm water, and leave it until the water has become quite still. This represents a warm ocean such as the mid-Pacific.
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2. Wrap some ice cubes carefully in foil, making sure no melted water will be able to leak out. Put the foil package in one end of the glass dish and leave it until the water is still once again. The ice will start to cool the warm water.
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3. Place a few drops of food coloring over the foil so the color trickles into the water. What happens to the color as it sinks to the bottom of the dish? This is how deep-water currents carry water toward the equator from the poles.