WATER CURRENT

Ads by Eonads >A large movement of water in one general direction is a current. Currents can be temporary or long-lasting. They can be near the surface or in the deep ocean. The strongest currents shape Earth’s global climate patterns (and even local weather conditions) by moving heat around the world.

Surface Currents

This is a map ocean surface currents from 1877. (John James Wild, 1877)

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At the surface, currents are mainly driven by four factors—wind, the Sun’s radiation, gravity, and Earth’s rotation. All of these factors are interconnected. The Sun’s radiation creates prevailing wind patterns, which push ocean water to bunch in hills and valleys. Gravity pulls the water away from hills and toward valleys and Earth’s rotation steers the moving water.

Sun and Wind

Wind is a major force in propelling water across the globe in surface currents. When air moves across the ocean’s surface, it pulls the top layers of water with it through friction, the force of resistance between two touching materials moving over one another. Surface ocean currents are driven by consistent wind patterns that persist throughout time over the entire globe, such as the jet stream. These wind patterns (convection cells) are created by radiation from the Sun beating down on Earth  and generating heat. Ads by Eonads

The Sun’s radiation is strongest at the equator and dissipates the closer you get to the poles. This uneven distribution of heat causes air to move. The hot air over the equator rises and moves away from the equator. Likewise, cold air from the poles sinks and moves towards the equator. The clashing of hot air originating at the equator and cold air originating at the poles creates regions of high atmospheric pressure and low atmospheric pressure along specific latitude lines. It would make intuitive sense that the hot air and cool air would meet in the middle of the equator and the North or South pole, however, in reality it is much more complicated. A combination of Earth’s rotation, the fact that Earth is tilted on an axis, and the placement of most continents in the Northern Hemisphere, create pressure systems that divide each hemisphere into three distinct wind patterns or circulation cells.

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The major wind patterns drive oceanic currents. (NASA)

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In the Northern Hemisphere, the most northern system, the polar cell, blows air in a consistent southwestern direction toward a pocket of low pressure along the 60-degree latitude line. The middle system, the Ferrel cell, blows in a consistent northeastern direction toward the same 60-degree low. And the most southern system, the Hadley cell, blows air in a consistent southwestern direction toward a region of low pressure along the equator. The result is a global pattern of prevailing wind, and it is this consistent wind that impacts the ocean.

While it may appear that the ocean is a flat surface, the reality is that it is a series of hills and valleys in the water. At the places where the wind generated currents converge into each other, the ocean water is pushed to build a slight hill. Likewise, where the winds diverge, the ocean water dips in a slight depression.

Gravity and Earth's Rotation

Wind pushes water into hills of high pressure which leave behind valleys of low pressure. Since water is a liquid that prefers to stay at a level height, this creates an unstable situation. Following the pull of gravity, ocean water moves from the built-up areas of high pressure down to the valleys of low pressure.

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But as the water moves from hills to valleys, it does so in a curved trajectory, not a straight line. This curving is a result of Earth’s spin on its axis.

Ads by Eonads >On Earth, movement in a straight line over long distances is harder than it may seem. That’s because Earth is constantly rotating, meaning every object on its surface is moving at the speed at which the Earth is spinning on its axis. From our perspective, stationary objects are just that, unmoving. In reality, they are whipping around at a speed of roughly 1,000 miles per hour (1600 km/hr) at Earth’s equator. It is that whipping, rotating motion that influences the movement of any object not in direct contact with the planet’s surface, making straight appearing trajectories actually bend. It also influences the movement of ocean currents. Scientists refer to this bending as the Coriolis Effect.