Ocean Currents

Learning Objectives

Know what is meant by an ocean current.

Distinguish between surface and deep currents.

Be able to use ocean currents to explain why our Pacific Coast is cool and dry and our Atlantic Coast is warm and wet.

Ocean currents are streams of water that flow through the ocean. There are two types of ocean currents; surface and deep currents.

Surface Currents

Surface currents are powered by the sun and driven by the wind. The sun drives the global wind systems on Earth. When this wind blows over the surface of the ocean, it causes waves to form and the water at the surface to move. The force of the wind pushing on the water results in what are known as surface currents.

As the wind hits the surface of a body of water, it forms a surface current, taking the water with in the direction of the wind.
Fig. 1 Surface Currents

The global winds that drive surface currents are affected by a number of things, one of which is the rotation of the Earth. The Earth’s rotation causes the ocean currents to turn in a process known as the Coriolis effect. Because of the Coriolis effect, ocean currents deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

A sphere, representing the earth, is depicted. The equator is marked across the center. Arrows pointing north and south away from the equator show the curve of the coriolis effect.
Fig. 2 The Coriolis Effect

Additionally, while the wind only pushes the water at the very surface of the ocean, the water molecules moved by the wind current drag along the water molecules just below them. The result is that the layer of water just below the surface also moves (though slightly slower). That second layer, in turn, pulls on the water molecules below it. This continues down to a depth of about 100 m (330 feet) with each increase in depth resulting in a slowing down of the water.

Current speeds decrease as the depth increases. The top wavelength shows greater peaks and troughs. These shrink moving to the bottom wavelength.
Fig. 3 Deep ocean currents

Deep Ocean Currents

Deep ocean currents (currents that occur at depths below those of surface currents) are driven by differences in seawater density which are the result of differences in temperature and salinity (saltiness). The system of deep ocean currents that is the result of these changes in density is known as the global conveyor belt

The Global Conveyor Belt:

Near the pole of the North Atlantic, water near the surface of the ocean begins to freeze due to cold temperatures. When this water freezes, however, it leaves salt behind, which increases both the salinity and density of water left behind (not frozen). This denser water begins to sink to the bottom of the ocean, while surface water moves in to replace it, generating a current. This deep water then moves south in a complex pattern around the globe, moving at speeds much slower than those of the surface currents.

Global currents are depicted through arrow indicating the temperature of the current on the map. Cold currents tend to run north while warm currents run south.
Fig. 4 Global Conveyor belt

How Ocean Currents Affect the Coast:

Ocean currents can affect local coasts by transferring heat (in the form of warm water) from warmer (tropical) to colder (polar) regions.

For example, currents flowing from the poles (think Alaskan and northern ocean currents; these currents are coming from cold regions) travel southward along the west coast (Pacific Coast) of the United States, causing cooler ocean temperatures and a cooler climate. The cold current along the west coast of the US is known as the California current. On the other hand, currents from locations near the equator (think the Caribbean and tropical Atlantic; these currents are coming from warm regions) move along the east coast (Atlantic Coast), resulting in a warm and wet climate.

Original source: Ocean Currents. Provided by: National Oceanic and Atmospheric Administration. Located at: https://www.noaa.gov/education/resource-collections/ocean-coasts- education-resources/ocean-currents
Original source: Ocean Circulations. Provided by: National Weather Service. Located at: https://www.weather.gov/jetstream/circulation
Original source: Surface Ocean Currents: Ekman Spiral. Provided by: National Oceanic and Atmospheric Administration. Located at: https://oceanservice.noaa.gov/educa tion/kits/currents/05currents4.html
Original source: Coriolis Effect. Provided by: National Oceanic and Atmospheric Administration. Located at: https://oceanservice.noaa.gov/education/kits/currents/media/supp_ cur05b.html


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