Currents, Waves, and Tides

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.

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.

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.

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.

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.

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.

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.

It is easiest to understand this phenomenon when thinking about travel in a northern or southern direction. Since Earth is essentially a sphere and it spins around an axis, anything near Earth’s equator will travel the fastest—since Earth is rotating at a constant rate and the equator runs along the widest part of the sphere, any object there must travel the entirety of Earth’s circumference in one rotation. As you get closer and closer to the poles, the distance traveled in one rotation gradually shrinks until it reaches zero at either pole. Therefore, an object on the surface will gradually spin slower the closer it gets to a pole. 

But leave the surface of the planet, and the anchor keeping you in sync with the land beneath you disappears. Any moving object (plane, boat, hot air balloon, water) will begin its travels at the rotating speed of the location where it took off from. If it should travel north or south, the ground beneath it will be traveling at a different speed. Travel North from the Equator, and the ground will gradually spin slower beneath you. This causes an object attempting to travel in a straight line to veer to the right in the Northern Hemisphere and veer to the left in the Southern Hemisphere relative to the direction traveling. 

Understanding how the rotating Earth affects movement to the west or east is a bit trickier. Envision an elastic string attached to a ball on one end and an anchored point at the other. The faster the ball is spun around the anchor, the more the elastic stretches and the farther the ball travels from the center point. An object traveling on Earth behaves the same way. If the object moves east, in the direction that Earth is spinning, it is now traveling around the axis of Earth faster than it was when it was anchored—and so, the object wants to move out and away from the axis. Still tethered by gravity, the object does so by moving toward the equator, the place on Earth that is the greatest distance from the axis. Travel west, the opposite direction that Earth is spinning, and now the object is spinning slower than Earth’s surface and so it wants to move toward the axis. It does so by moving toward the pole. This again appears as a bend to the right in the Northern hemisphere and to the left in the Southern hemisphere.

Water moving along Earth’s surface is also subject to the Coriolis effect which causes moving water to curve in the same directions described above. In the Northern Hemisphere, surface water curves to the right and in the Southern Hemisphere it curves to the left of the direction it is forced to move.

Earth’s rotation is also responsible for the circular motion of ocean currents. There are 5 major gyres—expansive currents that span entire oceans—on Earth. There are gyres in the Northern Atlantic, the Southern Atlantic, the Northern Pacific, the Southern Pacific, and the Indian Ocean. Similar to surface waters, Northern gyres spin clockwise (to the right) while gyres in the south spin counterclockwise (to the left). 

The center of the gyres are relatively calm areas of the ocean. The Sargasso Sea, known for its vast expanses of floating Sargassum seaweed, exists in the North Atlantic gyre and is the only sea without land boundaries. Today, gyres are also areas where marine plastic and debris congregate. The most famous one is known as the Great Pacific Garbage Patch, but all five gyres are centers of plastic accumulation.

Wind moving across the ocean moves the water beneath it, but not in the way you might expect. The Coriolis Effect, the apparent force created by the spinning of Earth on its axis, affects water movement, including movement instigated by wind. Recall that Coriolis causes the trajectory of a moving object to veer to the right or the left depending upon the hemisphere it is located in. But in this case, the three-dimensional nature of the ocean plays into the direction of the water’s overall movement. Wind blowing over water will move the ocean water underneath it in an average direction perpendicular to the direction the wind is traveling.

As wind blows over the surface layer of water, friction between the two pulls the water forward. As we know, when water (and other objects) moves across Earth’s surface it bends due to the Coriolis Effect. The top most layer of water will bend away from the direction of the wind at about 45 degrees. For simplicity, we will assume that this scenario is in the Northern Hemisphere and all movement bends to the right. As the top layer of water begins to travel, it in turn pulls on the water layer beneath it, just as the wind had. Now this second water layer begins to move, and it travels in a direction slightly to the right of the layer above it. This effect continues layer by layer as you move down from the surface, creating a spiral effect in the moving water.

In addition to a change in direction, each sequential layer down loses energy and moves at a slower speed. Friction causes the water to move, but drag resists that movement, so as we travel from the top layer to the next, some of the energy is lost. When all the layers down the spiral are accounted for, the net direction of the water is perpendicular to the direction of the wind.