Surface Ocean Currents
The water at the ocean surface is moved primarily by winds. Large scale winds move in specific directions because they are affected by Earth’s spin and the Coriolis Effect. Because Earth spins constantly, the winds blow in predictable directions. The winds are able to move the top 400 meters of the ocean in a predictable pattern too.
Surface ocean currents form large circular patterns called gyres in the oceans. These gyres flow clockwise in Northern Hemisphere oceans and counterclockwise in Southern Hemisphere oceans because of the Coriolis Effect. Near the Earth’s poles, there is a tendency for the gyres to flow in the opposite direction.
While surface ocean currents flow in a regular pattern, they are not all the same. The amount of water flowing and how fast it is flowing differs from current to current. Some currents are deep and narrow, often affected by the shape of the ocean floor, while others are shallow and wide. Some move quickly, up to 3-4 km/hr (1.8-2.5 mph), while others move more slowly. A current can also change somewhat in depth and speed over time.
Surface ocean currents can be very large. The Gulf Stream, a surface ocean current that is part of the North Atlantic gyre, carries 4500 times more water than the Mississippi River. Each second, ninety million cubic meters of water is carried past Chesapeake Bay (US) in the Gulf Stream.
Surface ocean currents transport heat in the Earth system, which affects regional climates. The Sun warms water at the equator more than it does at Earth’s polar regions. The heat travels in surface currents to higher latitudes. A current that consistently brings warmth into a high latitude region will make that region’s climate less chilly.
Surface ocean currents can create eddies, swirling loops of water, as they flow. Surface ocean currents can also affect upwelling in many places. They are important for sailors planning routes through the ocean. Sailors have found that following the direction of a current is helpful, but trying to navigate against it can be difficult. Currents are also important for marine ecosystems because they transport ocean life around the world and affect the water temperature in ecosystems.