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Currents are driven by a combination of forces, including the temperature of the water. Close to the poles, the ocean water is extremely cold—cold enough to freeze at the surface. Since sea ice is pure water, the cold water beneath the ice becomes more salty and dense, and sinks to the bottom. In the north Atlantic this cold “bottom water” flows south toward the tropics, while at the surface, warmer water flows in from the Gulf of Mexico. The warm surface current is known as the Gulf Stream. It is the Gulf Stream that gives northern Europe its mild climate. Ocean currents are also driven by the turning of the Earth. The rotation creates huge circular currents, called eddies, at the surface. In the northern hemisphere, these currents move clockwise. They flow westward along the Equator, alongside similar currents that swirl counterclockwise in the southern hemisphere. The movement of these huge masses of water from the trop- ics toward the poles and back again has a powerful effect on air temperatures above the oceans. Warm water carried toward the poles warms up the air above, making the climate milder. Warm air rises, and this causes low pressure zones that suck in air from areas where colder water is cooling the air and making it sink. The moving air masses create winds that blow across the oceans, and these then help drive the currents. So the winds and currents are intimately connected, and the world’s weath- er is partly controlled by the movements of the oceans.

El Niño and La Niña One of the most powerful ocean currents is the Humboldt

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