Ocean currents

• Ocean currents are the most important ocean movements because of their influence on climatology of various regions. 
• Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction.

Ocean currents are influenced by two types of forces namely:

1. primary forces that initiate the movement of water;
2. secondary forces that influence the currents to flow.

The primary forces that influence the currents are:

1. heating by solar energy;
2. wind;
3. gravity;
4. Coriolis force.

The secondary forces that influence the currents are:

1. Temperature difference;
2. Salinity difference

Primary Forces Responsible For Ocean Currents

Influence of insolation

• Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes.
• This causes a very slight gradient and water tends to flow down the slope. The flow is normally from east to west.

Influence of wind (atmospheric circulation)

• Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course.
• Winds are responsible for both magnitude and direction [Coriolis force also affects direction] of the ocean currents. Example: Monsoon winds are responsible for the seasonal reversal of ocean currents in the Indian ocean.
• 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 [Sub-tropical High Pressure Belt] (more pronounced in the southern hemisphere than in the northern hemisphere due to differences in the extent of landmass). The oceanic circulation pattern also corresponds with the same.
• At higher latitudes, where the wind flow is mostly cyclonic [Sub-polar Low Pressure Belt], the oceanic circulation follows this pattern.
• In regions of pronounced monsoonal flow [Northern Indian Ocean], the monsoon winds influence the current movements which change directions according to seasons.

Influence of gravity

• Gravity tends to pull the water down to pile and create gradient variation.

Influence of Coriolis force

• The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere.
• These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins. One such circular current is the Sargasso Sea.

Secondary Forces Responsible For Ocean Currents

• Temperature difference and salinity difference are the secondary forces.
• Differences in water density affect vertical mobility of ocean currents (vertical currents).
• Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.
• Denser water tends to sink, while relatively lighter water tends to rise.
• 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

Based on depth

• The ocean currents may be classified based on their depth as surface currents and deep water currents:

1. surface currents constitute about 10 per cent of all the water in the ocean, these waters are the upper 400 m of the ocean;
2. 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 the 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.

Based on temperature

• Ocean currents are classified based on temperature: as cold currents and warm currents:

1. Cold currents bring cold water into warm water areas [from high latitudes to low latitudes]. These currents are usually found on the west coast of the continents (currents flow in clockwise direction in northern hemisphere and in anti-clockwise direction in southern hemisphere) in the low and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere;
2. Warm currents bring warm water into cold water areas[low to high latitudes] and are usually observed on the east coast of continents in the low and middle latitudes (true in both hemispheres). In the northern hemisphere they are found on the west coasts of continents in high latitudes.

General Characteristics of Ocean Currents

• Characteristics of Ocean Currents arise due to the interplay of the above-mentioned factors.

The general movement of the currents in the northern hemisphere is clockwise and in the southern hemisphere, anti-clockwise.

• This is due to the Coriolis force which is a deflective force and follows Ferrel’s law.
• A notable exception to this trend is seen in the northern part of the Indian Ocean where the current movement changes its direction in response to the seasonal change in the direction of monsoon winds.

The warm currents move towards the cold seas and cool currents towards the warm seas.

• In the lower latitudes, the warm currents flow on the eastern shores and cold on the western shores [food for imagination].
• The situation is reversed in the higher latitudes. The warm currents move along the western shores and the cold currents along the eastern shores.
• Convergence: warm and cold currents meet.
• Divergence: a single current splits into multiple currents flowing in different directions.

The shape and position of coasts play an important role in guiding the direction of currents.

• The currents flow not only at the surface but also below the sea surface (due to salinity and temperature difference).
• For instance, heavy surface water of the Mediterranean Sea sinks and flows westward past Gibraltar as a sub-surface current.

Effects of Ocean Currents

Ocean currents have a number of direct and indirect influences on human activities.

Desert formation

• Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
• There is fog and most of the areas are arid due to desiccating effect (loss of moisture).

Rains

• Warm ocean currents bring rain to coastal areas and even interiors. Example: Summer Rainfall in British Type climate.
• Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.

Moderating effect

• They are responsible for moderate temperatures at coasts. [North Atlantic Drift brings warmness to England. Canary cold current brings cooling effect to Spain, Portugal etc.]

Fishing

• Mixing of cold and warm ocean currents bear richest fishing grounds in the world.
• Example: Grand Banks around Newfoundland, Canada and North-Eastern Coast of Japan.
• The mixing of warm and cold currents help to replenish the oxygen and favor the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.

Drizzle

• Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].

Climate

Results in

• Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.],
• Cold and dry climates on the western margins in the sub-tropics due to desiccating effect,
• Foggy weather and drizzle in the mixing zones,
• Moderate clime along the western costs in the sub-tropics.

Tropical cyclones

• They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.

Navigation

• Currents are referred to by their “drift”. Usually, the currents are strongest near the surface and may attain speeds over five knots (1 knot = ~1.8 km). [At depths, currents are generally slow with speeds less than 0.5 knots].
• Ships usually follow routes which are aided by ocean currents and winds.
• Example: If a ship wants to travel from Mexico to Philippines, it can use the route along the North Equatorial Drift which flows from east to west.
• When it wants to travel from Philippines to Mexico, it can follow the route along the doldrums when there is counter equatorial current [we will study this in next post] flowing from west to east.