All About Ocean Currents
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When used in association with water, the term "current" describes the motion of the water. Some currents you may be familiar with are the motion of rainwater as it flows down the street, or the motion of the water in a creek, stream, or river flowing from higher elevation to lower elevation. The ocean, like all liquids, is capable of flowing as a current, but the cause for those currents doesn’t solely lie with gravity as a river’s does. Oceanic currents are driven by several factors. One is the rise and fall of the tides, which is driven by the gravitational attraction of the sun and moon on Earth's oceans. The vertical motion of the tides near the shore causes the water to move horizontally, creating currents. Tidal currents are the only type of currents that change in a very regular pattern and can be predicted for future dates. A second factor that drives ocean currents is wind. Winds drive currents that are at or near the ocean's surface. These currents are generally measured in meters per second or in knots (1 knot = 1.15 miles per hour or 1.85 kilometers per hour). Winds drive currents in the open ocean on a global scale. The direction the wind blows is the direction that the current travels. T  he sun ultimately creates winds in the atmosphere and ocean currents. Because the Earth is heated unequally, the equator receives more direct rays from the sun throughout the year and the equatorial oceans "heat up" more than the oceans near the North and South Pole. This creates a temperature imbalance, causing ocean water near the equator to move toward the North or South Pole. In turn, the ocean water at the poles travels toward the equator.
A third factor that drives currents is thermohaline circulation - a process driven by density differences in water due to temperature (thermo) and salinity (haline) in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents. T  he next factor is known as continental deflection occurs when coastlines from landmasses such as continents act like dams and deflect the flow of currents. Currents bumping into the continents must change direction and flow toward the equator or the poles. Only the Antarctic circumpolar current can flow unimpeded around Earth.
T  he final factor is known as the Coriolis Effect is caused by the rotation of Earth on its axis. It is named after the French mathematician Gaspard Gustave de Coriolis (1792-1843), who studied the transfer of energy in rotating systems like waterwheels. If the Earth did not rotate and remained stationary, ocean currents would travel in a straight line. But because the Earth rotates, ocean currents travel in a curved manner. This curving is called the Coriolis Effect.
T  he Coriolis Effect curves major ocean currents to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a counter-clockwise spiral). These major spirals of ocean-circling currents are called “gyres” and occur north and south of the equator. There are 5 main gyres on Earth: 2 in the Atlantic Ocean, 2 in the Pacific Ocean, and 2 in the Indian Ocean. They do not occur at the equator, where the Coriolis effect is not present.
There are 2 types of currents found in the ocean: surface currents and deep currents. Just as their names suggest, surface currents are found on the top or surface of the ocean and deep currents are found far below the ocean’s surface. Surface Currents S 
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