Ocean Currents

What an Ocean Current Is

An ocean current is, in essence, a river flowing inside the ocean. It is a regular, identifiable volume of water moving along a definite path and in a definite direction. Currents do not drift randomly; they follow consistent routes across the ocean basins, driven by a combination of forces that act on the water.

These forces are divided into two groups. Primary forces are those that actually initiate the movement of water. Secondary forces are those that influence the direction and behaviour of the current once it is already flowing.

The Primary Forces

Four primary forces set ocean water in motion.

Heating by solar energy. Solar heat causes water to expand. Because of this, near the equator the ocean surface stands about 8 cm higher than in the middle latitudes. That small difference in level creates a very slight gradient, and water tends to flow down this slope, away from the equator.

Wind. Wind blowing across the ocean surface pushes the water along. The friction between the wind and the water surface affects how the water body moves along its course. This is the most obvious and visible driver of surface currents.

Gravity. Gravity pulls the water down from the pile that has accumulated, working to smooth out the gradient and create variation in the water level.

Coriolis force. This force intervenes and deflects the moving water — to the right in the northern hemisphere and to the left in the southern hemisphere.

The large accumulations of water produced by these forces, and the flow that develops around them, are called gyres. Gyres produce large circular currents in all the ocean basins.

Density and Vertical Movement

Differences in water density affect the vertical mobility of ocean currents. Water with high salinity is denser than water with low salinity; in the same way, cold water is denser than warm water. Denser water tends to sink, while relatively lighter water tends to rise.

This is what drives the great vertical circulation of the oceans. Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator. Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water.

Types of Ocean Currents

Ocean currents can be classified in two ways — by depth and by temperature.

Classification by Depth

• Surface currents constitute about 10 per cent of all the water in the ocean. These are the waters of the upper 400 m of the ocean.
• Deep water currents make up the other 90 per cent of the ocean water. These waters move around the ocean basins due to variations in density and gravity. Deep waters sink into the deep ocean basins at high latitudes, where the temperatures are cold enough to cause the density to increase.

Classification by Temperature

• Cold currents bring cold water into warm water areas. They are usually found on the west coast of the continents in the low and middle latitudes — this is true in both hemispheres — and on the east coast in the higher latitudes in the northern hemisphere.
• Warm currents bring warm water into cold water areas. They are usually observed on the east coast of continents in the low and middle latitudes, again true in both hemispheres. In the northern hemisphere they are also found on the west coasts of continents in high latitudes.

Characteristics of Ocean Currents

Currents are referred to by their drift, which is the term for the speed of a current. Drift is measured in knots. The strength of a current refers to its speed — a fast current is considered a strong current.

A current is usually strongest near the surface and decreases in strength with depth. Most currents have speeds less than or equal to 5 knots near the surface. At depth, currents are generally slow, with speeds less than 0.5 knots.

Major Ocean Currents and Their Driving Forces

Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and by the Coriolis force. The oceanic circulation pattern roughly corresponds to the earth's atmospheric circulation pattern.

The air circulation over the oceans in the middle latitudes is mainly anticyclonic — this is more pronounced in the southern hemisphere than in the northern hemisphere — and the oceanic circulation pattern corresponds with the same. At higher latitudes, where the wind flow is mostly cyclonic, the oceanic circulation follows that pattern as well. In regions of pronounced monsoonal flow, the monsoon winds influence the current movements.

Due to the Coriolis force, warm currents from low latitudes tend to move to the right in the northern hemisphere and to the left in the southern hemisphere.

The oceanic circulation transports heat from one latitude belt to another in a manner similar to the heat transported by the general circulation of the atmosphere. The cold waters of the Arctic and Antarctic circles move towards warmer water in tropical and equatorial regions, while the warm waters of the lower latitudes move polewards.

Effects of Ocean Currents

Ocean currents have a number of direct and indirect influences on human activities, most visibly on climate and on fisheries. …