Ocean currents are the continuous, predictable, directional movement of seawater driven by gravity, wind (Coriolis Effect), and water density. Ocean water moves in two directions: horizontally and vertically. Horizontal movements are referred to as currents, while vertical changes are called upwellings or downwellings. This abiotic system is responsible for the transfer of heat, variations in biodiversity, and Earth’s climate system.

The heat budget Is different in the latitudes with heat surplus in the tropics and heat deficit in the arctic region (Beyond 40 degrees N and S). All the weather phenomenon and circulatory phenomenon is there to transfer heat from the tropics towards the poleand maintain the heat balance. The ocean currents follow the same phenomenon.

Ocean currents are categorized as warm ocean currents and cold ocean currents. You need to keep this in mind that warm and cold are not because of their absolute temperature. It is because of their impact on the destination region.

A current that moves towards the pole in both hemispheres is a warm current because it is carrying warm water from lower latitudes to the upper latitudes. On contrary, a current coming from upper latitudes towards the tropics is a cold current.

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 percent 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 percent of the ocean water. These waters move around the ocean basins due to variations in density and gravity.
     • The density difference is a function of different temperatures and salinity
     • These 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 the clockwise direction in the northern hemisphere and in anti-clockwise direction in the 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.

Forces Responsible For Ocean Currents

Primary Forces

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

• Temperature differences and salinity differences are secondary forces.
• Differences in water density affect the 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.

Causes of Ocean Currents

Planetary Winds

Planetary winds play a vital role in the formation and sustenance of ocean currents. Since planetary winds blow with consistency, over the surface of the ocean, they tend to push the water in one direction because of friction. This is the main cause of the flow of water.

Because of the Coriolis Effect, in the Northern Hemisphere currents flow to the right of the wind direction while in the Southern Hemisphere, winds blow to the left. Intervening continents and basin topography often block the continuous flow of the water and often deflect the moving water in a circular pattern. Thiscircular motion of water along the periphery of the ocean basin is called Gyre. The following Map will broadly show the formation of different gyres in Oceans.

Effect of the temperature

There are marked variations in the horizontal and vertical distribution of the temperatures in the ocean. In general, the temperature decreases as we move towards the pole from the equator.

There is an inverse relationship between temperature and density of the water i.e. higher the temperature, the lower will be its density. As a result, the warm and low-density water from the equatorial region moves towards the colder polar waters. Contrary to this there is a movement of ocean water below the water surface in the form of subsurface current from colder polar areas to warmer equatorial area. The Gulf Stream and Kuroshio Current (warm) are very good examples of this.

Salinity

The salinity of the Ocean varies from place to place. Water with high salinity is denser than the one with low salinity. Ocean currents on the water surface are generated from the areas of low salinity to the areas of high salinity. For example, there are ocean currents moving from the ocean to inland seas i.e. ocean current flows from the Atlantic to the Mediterranean Sea. A similar ocean current is seen from the Indian Ocean to the Red Sea via Bab Al Mandab. Peru Current also originates because of the difference in density.

Rotation of Earth

The rotation of the earth from west to east on its axis is the cause of a deflective force called Coriolis force.

Similar to winds, it deflects the ocean current in the Northern Hemisphere towards its right and in the Southern Hemisphere towards its left. Because of this at the periphery of the ocean, ocean currents form a clockwise circulation in the Northern Hemisphere and Counterclockwise circulation in the Southern Hemisphere. This giant loop is called Gyre.

A gyre formed in Northern Atlantic is of special importance as it traps the inner water of the ocean and makes it stagnant. This stagnant body of water is known as the Sargasso sea named after the Sargassum weed found in it. Sargassum is unique vegetation endemic to it and is an internationally protected area. The Sargasso Sea is the only sea totally inside an ocean.

Configuration of the coastline

Coastline plays an important role in governing the direction of the flow of the ocean current. For example, the equatorial current after being obstructed by the Brazilian coast is bifurcated into two branches. The Northern Branch is called the Caribbean current while the Southern branch is called the Brazilian current.

Note: After hitting a coastline, apart from moving towards North and South, some of the water also moves downward, this is called Downwelling. This water penetrates deep in the ocean and moves parallel to the surface current as an undercurrent and comes out on the other side of the ocean as upwelling. Since this upwelling water comes out from the depth, it is relatively cold and brings a lot of nutrients to the surface. Regions, where upwelling is present, are rich fishing grounds e.g. Peru coast.

Desert Formation and Ocean Currents

Major hot deserts are located between 20-30 degree latitudes and on the western side of the continents. Why?

• The aridity of the hot deserts is mainly due to the effects of off-shore Trade Winds, hence they are also called Trade Wind Deserts.
• The major hot deserts of the world are located on the western coasts of continents between latitudes 15° and 30°N. and S .They include the biggest Sahara Desert (3.5 million square miles). The next biggest desert is the Great Australian Desert. The other hot deserts are the Arabian Desert, Iranian Desert, Thar Desert, Kalahari, and Namib Deserts.
• The hot deserts lie along the Horse Latitudes or the Sub-Tropical High-Pressure Belts where the air is descending, a condition least favorable for precipitation of any kind to take place.
• The rain-bearing Trade Winds blow off-shore and the Westerlies that are on-shore blow outside the desert limits.
• Whatever winds reach the deserts blow from cooler to warmer regions, and their relative humidity is lowered, making condensation almost impossible.
• There is scarcely any cloud in the continuous blue sky. The relative humidity is extremely low, decreasing from 60 percent in coastal districts to less than 30 percent in the desert interiors. Under such conditions, every bit of moisture is evaporated and the deserts are thus regions of permanent drought. Precipitation is both scarce and most unreliable.
• On the western coasts, the presence of cold currents gives rise to mists and fogs by chilling the on-coming air. This air is later warmed by contact with the hot land, and little rainfalls.
• The desiccating effect of the cold Peruvian Current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama Desert is not more than 1.3 cm.

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