Ocean Currents: Meaning and Types

Ocean current is the general movement of a mass of oceanic water in a definite direction, which is more or less similar to water streams flowing on the land surface of the earth. Ocean currents are most powerful of all the dynamics of oceanic waters because these drive oceanic waters for thousands of kilometers away.

Origin of Ocean Currents

The currents in the oceans are originated due to combined effects of several internal as well as external factors, which control the origin and other characteristics of ocean current. They are related to different characteristics of ocean waters, rotational mechanism of the earth, external factors or atmospheric factors, topographic characteristics of the coasts and ocean basins.

Each of the above factors can also significantly affect / modify the ocean currents.

Difference between Drifts, Current and Streams

On the basis of velocity, dimension and direction, they can be divided into drifts, currents and streams. The forward movement of surface water of the oceans under the influence of prevailing winds is called drift whereas the ocean current involves the movement of Oceanic water in a definite direction with greater velocity.  Ocean stream involves movement of larger mass of ocean water like big rivers of the continent in a definite direction with greater velocity than the drifts and currents such as in Gulf Stream.

Difference between Warm Currents and Cold Currents

Ocean currents are divided on the basis of temperature into warm currents and cold currents.  Those currents that flow from the Equator towards the poles are warmer than the surrounding water and so they are called warm currents. The ocean currents that flow from the polar areas towards the Equator are cooler compared to the surrounding water, so they are called cold currents. The actual difference in temperature of warm and cold currents is only a few degrees.

• The cold currents are usually found on the west coast of the continents in the low and middle latitudes in both the hemispheres and on the east coast in the middle latitudes in the Northern Hemisphere.
• The warm currents are usually observed on the east coast of the continents in the low and middle latitudes in both the hemispheres. In the Northern Hemisphere they are found on the west coasts of the continents in the high latitudes.

Difference between Surface Currents and Deep Currents

Ocean currents can also be divided into Surface Currents and Deep Currents. Surface currents affect surface water above the pycnocline (<10% of ocean water). These currents are primarily driven by major wind belts. The Deep currents affect deep water below pycnocline (90% of ocean water) and are primarily driven by density differences. The deep currents are larger and slower than surface currents.

Ekman Transport

The stress of wind blowing across the sea causes a surface layer of water to move. Due to the low viscosity of water, this stress is not directly communicated to the ocean interior, but is balanced by the Coriolis force within a relatively thin surface layer, 10-200m thick. This layer is called the Ekman layer and the motion of this layer is called the Ekman transport. Because of the deflection by the Coriolis force, the Ekman transport is not in the direction of the wind, but is 90° to the right in the Northern Hemisphere and 90° toward the left in the Southern Hemisphere. The amount of water flowing in this layer depends only upon the wind and the Coriolis force and is independent of the depth of the Ekman layer and the viscosity of the water.

The major surface currents are shown below: