At the surface, ocean currents transport heat from the warm equatorial regions to the cold polar regions, much as the atmosphere moves warm air toward the poles. Near the poles, cold water sinks and flows toward the equator in deep ocean currents. Ocean currents often follow paths similar to the major wind systems. For instance, in the Indian Ocean, you'll see that change in wind direction off of East Africa causes a reversal in ocean currents.
The regions where cold-water currents rise again represent some of the most fertile waters in the world. The waters off of Monterey, California and northern Chile, Peru, and Ecuador reap the benefits of this flow as the cold currents bring nutrients up from the ocean floor. The abundant nutrients╤as well as increased amounts of dissolved gases such as carbon dioxide and oxygen╤promote tremendous blooms of microscopic plankton, which in turn provide food for fish and marine animals that become food for still larger fish and mammals.
Although tropical waters hold fewer gases than the polar seas, their warmth leads to faster rates of growth and reproduction in most species. Many animals come to spawn where warm currents run parallel to the continental coasts.
The major difference between the circulation of the atmosphere and the oceans is the presence of the continents. Winds, although altered by the land mass, still blow over the continents. Seas, with no where to go, exert their energy on the coasts. When land masses obstruct the flow of water, strong currents form along the edge of the continent. For example, the Gulf Stream brings warm water from the Gulf of Mexico and the Caribbean to the East Coast of North America.
Strong currents from different directions can give rise to rapid temperature differences, or ocean fronts. While the southwestern tip of Cape Cod may be lapped by the warm waters of the Gulf Stream, the northern tip is often cooled by the tail end of the Labrador Current from the Arctic. Similarly, the eastern side of the Cape of Good Hope at the southern tip of Africa receives warm water from the Indian Ocean, while barely twenty miles away from Cape Town, the Benguelas Current draws cold waters up from the Southern Ocean.
