Ocean Currents: Definition, Causes & Types, Effects - UPSC Notes

Ocean currents are the persistent and foreseeable seawater flows influenced by gravity, wind, and water density. The movement of ocean water occurs in both horizontal and vertical directions, with vertical shifts labelled upwellings or downwellings and horizontal motions referred to as currents.

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Ocean currents are large-scale movements of water. This topic is relevant to the UPSC context under General Studies Paper I (India & World Geography) for UPSC mains and Paper I for UPSC prelims. The topic of ocean currents UPSC is important as it forms a part of physical geography and influences climate, trade, and marine life. It is an essential topic for UPSC Civil Services because it highlights the role of ocean movements in climate regulation, weather patterns, and marine navigation, which is frequently discussed in the exam. This article will cover the types of ocean currents. Join UPSC Coaching today to boost your preparation.

What are Ocean Currents?

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 the Earth's climate system.

Ocean currents are mainly classified into two groups - surface currents, which are influenced by wind, and deep water currents, which are influenced by differences in water density.

Ocean currents are caused by several natural forces together. The preliminary reasons for ocean currents are wind patterns, the rotation of the Earth (Coriolis effect), and disparities in water temperature and salinity levels. Surface currents are influenced mainly by prevailing breezes like the trade winds and westerlies. Deep ocean currents form due to thermohaline circulation. Gravity and the position of the mainlands also show the movement and force of currents. Warm water from the equator shifts toward the poles, and cold water rushes back toward the equator. This scenario creates a constant international circulation that controls climate and sustains aquatic energy.

There are two forces behind ocean currents: Primary and Secondary. Let's study each type in detail.

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Primary Forces

The primary forces of ocean currents are wind, the Coriolis effect, and gravity. These forces form water movement on the sea surface and support maintaining the direction and flow of currents. The primary forces that drive ocean currents can be classified into different types:

Wind

• Ocean currents are caused mainly by the wind, which generates surface currents and moves across the ocean's surface. 
• Winds blow across the ocean's surface. This friction between the atmosphere and the water causes the surface water to move toward the wind. 

Gravity

• Gravity also plays a significant role in causing ocean currents. It is the force that pulls objects towards the Earth's centre, and therefore, it affects the movement of water in the ocean.
• Gravity also causes ocean tides, the periodic rise and fall of the ocean's surface caused by the gravitational pull of the Moon and  Sun on the Earth.
• Tides create currents as water moves from high-tide areas to low-tide regions. However, the tides have relatively little effect on the large-scale ocean currents.

Coriolis force

• Due to the rotation of the Earth, objects, including water, appear to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 
• As water moves across the ocean's surface, it is deflected by the Coriolis effect, causing it to move in a circular motion, which is famously known as a gyre. 
• The Coriolis effect is one of the main forces that shape ocean currents on a large scale and plays a vital role in Earth's climatic system.

Heating by solar energy

• The Sun's energy heats the Earth's surface, including the ocean's surface. 
• Ocean currents are formed due to the temperature variations brought about by the ocean surface heating. Because the warmer water near the equator is less dense than colder water at higher latitudes, warm water rises and cold water sinks, resulting in a circular motion known as a convection cell.
• These cells help to drive ocean currents. It can transport warm water from the equator towards the poles and cold water from the poles towards the equator. 

Secondary Forces

Secondary forces of ocean currents have water density disparities driven by temperature and salinity variations, tides, and the shape of ocean basins. These forces control the depth, speed, and movement of currents. The secondary forces that drive ocean currents can be classified into different types:

Water Temperature 

• The Sun's rays heat the ocean surface. It causes warm surface water to rise and cooler water to sink. This process is known as convection. It drives the movement of deep ocean currents.
• Cold water at the poles sinks and slowly travels towards the equator, leading to cold-water ocean currents.
• Warm water currents move away from the equator to replace the sinking cold water and towards the poles.

Differences in water density

• The amount of salt in the water also affects the ocean water's density. When water evaporates from the ocean, it leaves behind salt, which increases the salinity of the water. The saltier the water, the denser it becomes, and this causes it to sink and drive the movement of deep ocean currents.

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Types of Ocean Currents

Ocean currents are primarily categorized into surface and deep ocean currents, and the Indian Ocean currents are highlighted in a detailed and straightforward manner. Surface currents rush in the upper 400 meters of the sea and are driven mainly by wind patterns, the Earth's orbit, and the formation of coastlines. These currents include warm currents like the Gulf Stream and cold currents like the California Current. Deep ocean currents are also known as thermohaline currents. There are two main types of ocean currents: Surface currents and Deep ocean currents.

Surface currents

• Large-scale surface ocean currents are driven by global wind systems that are fueled by energy from the sun. These currents transfer heat from the tropics to the polar regions, influencing local and global climate.
• Surface currents are affected by different factors, which include the wind, the Earth's rotation, the shape of the coastline, and the presence of land masses. Generally, surface currents flow in circular patterns called gyres. The prevailing winds in the region drive them.
• Surface currents can move nutrients and creatures throughout the ocean. They have a significant impact on marine ecosystems.

Deep ocean currents 

• These currents are mainly affected by differences in water density and are primarily caused by variations in temperature and salinity.
• Deep ocean currents travel slowly. They can complete a cycle in hundreds or even thousands of years and are in charge of the ocean's vertical water circulation, which helps transfer heat and nutrients from the surface to the deep sea.
• Thermohaline circulation is the process responsible for the motion of deep ocean currents. 
  • Warm and less dense water rises and flows back towards the poles. In contrast, cold, dense water sinks in high-latitude areas and flows towards the equator, resulting in a system that resembles a conveyor belt that helps in the global transportation of heat and nutrients.

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Classification of Ocean Currents Based on Temperature

Ocean currents can be categorized based on temperature into warm and cold. Warm currents form near the equator and rush toward the poles.

• It holds warm water and increases the temperature of the coastal regions it passes through. Instances contain the "Gulf Stream" and the "Kuroshio Current."
• Cold currents form in polar or high-latitude parts and rush toward the equator, obtaining cooler water that reduces the temperature of nearby coastal zones.
• Instances contain the "Labrador Current" and the "Peru Current". This temperature-based variety helps to understand how ocean currents impact global climate, aquatic biodiversity, and coastal climates worldwide.
• Ocean currents can also be classified based on their temperature. The list of warm and cold ocean currents is given below.

Warm Currents

Warm currents are one such classification. They are ocean currents that are comparatively warmer than the surrounding water. They typically originate in low-latitude regions where warm water is present.

• One such powerful warm ocean current, the Gulf Stream, rises in the Gulf of Mexico and moves northward along the East Coast of the United States before crossing the Atlantic Ocean to reach Europe. This Stream is crucial for moving heat and warm water from the tropics to higher latitudes, which can significantly affect the climate.

Cold Currents

Cold ocean currents are streams of water that carry cold water from higher latitudes (near the poles) towards the equator.

• One such cold current- the Labrador Current, which travels southward along the eastern coasts of Canada and the United States- illustrates this. The Labrador Current starts in the Arctic, where it gets cold from the icy and chilly air there. This makes the current very cold. The temperature and marine life in the places where the Labrador Current flows can be significantly affected by its cold water.
• Cold ocean currents can play an essential role in regulating the Earth's climate because they help to transport cold water and heat from the poles to lower latitudes. 

Prime Characteristics of Ocean Currents

North Equatorial Current

The North Equatorial Current is a significant surface current in the Pacific Ocean that flows east to west along the equator.

• It is a component of the larger North Pacific subtropical gyre, a ring-shaped system of ocean currents propelled by wind and the Coriolis force.
• The trade winds, which cross the equator from east to west and push the surface water with them, impact the North Equatorial Current.
• The warm current eventually merges with the Kuroshio Current in the western Pacific, transporting water and heat from the tropics.
• By transferring heat from the tropics to higher latitudes and influencing weather patterns, the North Equatorial Current contributes significantly to the global climate system's functioning.

South Equatorial Current 

The South Equatorial Current are ocean currents in the Pacific, Atlantic, and Indian Ocean that flow east-to-west between the equator and about 20 degrees south. In the Pacific and Atlantic Oceans, it extends across the equator to about 5 degrees north.

• It is a component of the larger South Pacific subtropical gyre, a ring-shaped system of ocean currents propelled by wind and the Coriolis force.
• Within the southern hemisphere, the South Equatorial Current is the westward limb of the very large-scale subtropical gyres.
• These gyres are driven by trade winds in the tropics and westerly winds south of about 30 degrees south, through a complicated process that includes western boundary current intensification.
• The South Equatorial Current is driven directly by the trade winds, which blow from east to west on the equator.
• In the Indian Ocean, the westward-flowing South Equatorial Current is well-developed only south of the equator. Directly on the equator, the winds reverse twice a year due to the monsoons, so that the surface current can be either eastward or westward.

Also, check out the Ocean Floor article and its UPSC preparation features!

Antarctic Circumpolar Current (ACC)

The Antarctic Circumpolar Current (ACC) is an ocean current that flows around the continent of Antarctica with an average water flow of 130 million cubic meters per second. It is the world's most significant ocean current and an essential part of the climate and ocean circulation systems.

• The Antarctic Circumpolar Current is also known as the West Wind Drift or the Southern Ocean Current.
• This current is driven by the westerly winds that circle Antarctica. It moves from west to east and completely circles the continent without landmass restrictions. It links the Atlantic, Indian, and Pacific Oceans. Although only 100 to 200 km. Broadly, the river is mighty and can reach speeds of over 2 mt/s.