The seas and oceans are dynamic ecosystems. The oceans are very large bodies of water that form an important part of the hydrosphere.
Wind blowing across the sea surface has the greatest influence on the movement of surface waters. Sunlight falling on the sea surface has a very significant impact on seawater temperature. These factors cause oceanic water masses to move and circulate.
Both vertical and horizontal movements of surface and deep water masses occur in the world’s oceans. They are called ocean currents. The circulation of seawater is an important aspect of the study of oceanography. We need to understand ocean currents, waves, and tides and their distribution around the world.
OCEAN WATER CURRENTS
Currents are large-scale water movements that occur in all parts of the world. They usually move in certain specific directions. Due to this circulation, moisture in the Earth’s atmosphere is replenished.
Because ocean currents circulate water around the globe, they have a significant impact on the movement of energy and moisture between the oceans and the atmosphere. This makes them important for the global climate.
Streams are named for their driving mechanisms. They can be wind-driven or thermohaline. Many marine life depends on these currents to move from one place to another, either to reproduce, feed, or adapt.
Ocean currents are also gaining importance today due to the possibility of using alternative energy sources. Because seawater is dense, it carries a tremendous amount of energy that could potentially be captured and converted into a usable form through the use of turbine generators.
CAUSES OF OCEAN CURRENTS
The uneven heating of the earth’s surface and the movement of water masses along the surface by winds are responsible for the formation of currents. Currents are reflections of seawater circulation. , density, temperature, morphology and relief of the oceans and rotation of the earth.
The mechanisms responsible for the creation of ocean currents are:
- Global movement of Winds & Coriolis Force
- Ekman flow
- Temperature and Density differences
- Geostrophic influences- Pressure gradient
- Upwelling and the
- Configuration of ocean floors.
An ocean current is a continuous and directional movement of seawater created by the forces acting on it. Ocean currents can travel great distances and together they create the great flow of water masses. Sea currents on the surface can be very large.
An ocean current is a continuous and directional movement of seawater created by the forces acting on it. Ocean currents can travel great distances and together they create the great flow of water masses. Sea currents on the surface can be very large.
MAJOR OCEAN CURRENTS
Currents are of two general categories as surface water currents and deep water currents. There are several factors and processes involved in the generation and movement of these two kinds of ocean currents. In addition, there are specific names given to the currents which are distributed at different parts of the globe.
They are:
1. Equatorial currents
2. Boundary currents
3. Circumpolar currents.
In addition to these, there are some terminologies used to denote the oceanic currents. They are: Periodic currents, seasonal current, coastal current, longshore current, offshore current, inshore current and drift currents.
SURFACE WINDS
The water masses present on the sea surface are mainly moved by winds. Winds blow in specific directions and patterns. Ocean water masses also move due to the Earth’s rotation and the Coriolis effect.
Wind-driven circulation is the large-scale ocean circulation resulting from the action of the wind. There are two components involved, a) a directly driven Ekman component and b) an indirect component, the geostrophic balance with pressure systems.
The speed of surface currents is greatest near the sea surface and decreases about 100 meters below the surface. This accounts for up to 10% of the water masses in the oceans.
Because surface currents travel great distances, the Coriolis force also plays a role in their movement and deflects them, further contributing to the formation of their circular pattern. Finally, since the upper part of the ocean is irregular, gravity plays a role in the movement of surface currents.
DEEP WATER CURRENTS
Deep sea currents are another category of ocean currents. They are also known as the thermohaline circulation. They are below 400 meters and make up about 90% of the ocean. Just as with surface currents, gravity plays a dominant role in the formation of deep water currents. This is mainly due to density differences in the ocean water masses.
TYPES OF CURRENTS
The Oceanic Currents are classified according to the force that makes them to flow. The various kinds of currents include:
- TidalCurrents
- Density Currents
- Wind Driven Currents
- Gyres
- Rip Currents and
- Cold surface currents.
TIDAL CURRENTS
Tidal currents, as the name suggests, are created by the tides. Tides are essentially long, slow waves created by the gravitational pull of the moon, and to a lesser extent the sun, on the earth’s surface. Closer to the earth than to the sun, its gravitational pull has more influence on the tides.
Tidal currents are caused by floods of water flowing from one place to another. Normally, the flow velocity of these tidal currents is low (typically less than 0.5 m/s). Tidal currents flow closer to the shores.
Tidal currents differ from normal ocean currents in that they do not flow as a continuous current. They also change direction when there are high and low tide transitions.
Although tides and tidal currents do not have a major impact on open oceans, they can create rapid currents of up to 25 kilometers per hour as they flow in and out of confined areas such as bays, estuaries and harbors.
DENSITY CURRENTS
A less dense substance always lies on top of a denser substance. When water of two different densities meet, the lower density (less dense) water moves onto the higher density (higher density) water.
The different densities actually cause the water to move and form a density current. In the oceans, deep currents are colder and saltier than surface currents. Density differences are a function of temperature and salinity.
Density currents are driven by gravity. Density differences in a liquid in a gravitational field result in pressure differences that drive flows.These flows are a few cm per second or less.
WIND DRIVEN CURRENTS
Wind driven currents are the strongest currents in the world. As their name suggests, they are mainly powered by the wind. Wind can nominally cause 2.5% of wind speed over land (wind speed of 10 m/s can cause => 25 cm/s overland current). This is called wind drift.
A current can also slightly change its course in depth and speed over time. Winds can move the top 400 meters of the ocean and create surface ocean currents. Surface currents are mainly caused by wind as it creates friction as it moves across the water. This friction forces the water to spiral, creating gyres.
In the northern hemisphere, gyres move clockwise and in the southern they spin counterclockwise.
GYRES
Gyres are another type of current in the oceans. These are large-scale, “circular” ocean current patterns. They result from the force of the wind, the buoyancy force and the Coriolis acceleration. Because the Coriolis acceleration varies with latitude, the eddy circulations are not symmetrical and the flow is stronger at the western limits.
Gyres flow clockwise in the Northern Hemisphere oceans and counterclockwise in the Southern Hemisphere oceans. This happens mainly due to the Coriolis effect, which creates ocean currents on the surface.Near the Earth’s poles, the vortices tend to flow in the opposite direction. Some of the ocean currents move quickly while others move more slowly.
Gyres are of various kinds as:
- Subtropical gyres
- Subpolar gyres and
- Recirculation gyres.
Subtropical gyres are found in all of the world’s oceans at mid-latitudes. The subpolar gyres have the opposite circulation and lie poleward of the subtropical gyres. Recirculating eddies are currents associated with large ocean currents and consist of water circulating around most of the ocean basin in a closed pattern.
Large-scale recirculation gyres are associated with fast western boundary currents. Mesoscale recirculations are associated with meandering currents.
RIP CURRENTS
Rip currents are currents seen closer to beaches when waves tend to carry water towards the shore. Changes in bottom topography (e.g. sandbanks) can cause water to move towards the beach without returning to the deeper sea. The return flow of water to the deep sea occurs suddenly in the form of a narrow current (and 5 m wide) that moves fairly quickly (up to 2 m/s). these are called rip currents.
COLD SURFACE CURRENTS
The next type of current is the cold surface current. These currents come from polar and temperate latitudes and tend to flow toward the equator. Like warm surface currents, cold surface currents are mainly driven by atmospheric forces and influenced by earth’s rotation.
HOW CURRENTS ARE FORMED
THERMOHALINE CIRCULATION
Hot water contains less salt than cold water. Therefore it is less dense. When it’s less dense, it rises to the surface of the oceans. When it’s cold, it gets denser and the salt water sinks. As the water rises, the cold water is forced to move away by buoyancy, filling the void left by the warm water.
Conversely, when cold water rises, it also leaves a void, and rising hot water is forced to fill the void by buoyancy. This is called the thermohaline circulation.
This thermohaline circulation is induced by differences in temperature (thermos) and salinity (haline). Thermohaline circulation is known as the Global Conveyor Belt because its circulation of warm and cold water acts as a submarine river and moves the water masses throughout the oceans. This is one of the very unique phenomena of oceanic water masses.
EKMAN TRANSPORT
Surface winds and ocean currents are closely related. Winds usually blow across the water and pull the surface. This causes a thin layer of water to move and try to pull the water underneath with it. This process continues downward, continuously transmitting impulses downward into the deep layers.
Energy is lost in the process. As a result, the current speed decreases with increasing depth. The earth’s rotation deflects surface water movements to the right of the wind in the northern hemisphere. The change in direction and speed of the current with increasing depth forms a spiral.
This is called the Ekman spiral. It is named after the pioneering work of Swedish oceanographer V. W. Ekman, who laid the foundations for dynamic theories of wind-driven ocean circulation.
Ekman transport is the total average flow integrated at depth, resulting in the flow that sets large-scale ocean gyres.
UPWELLING
There are places in the ocean where water from the deep sea rises to the surface. These are referred to as outcrop areas. Coastal upwelling is generally induced by the Ekman transport. The other major component of large-scale ocean circulation is driven by the wind and is known as Ekman flow.
Upwelling is a mechanism that brings cold, nutrient-rich water from the depths to the surface. The Earth’s rotation and strong seasonal winds push surface water away from some coasts, causing water to rise at the edges of the continents to replace it.
Upwelling also happens as a part of El Niño (ENSO) events in the Pacific Ocean off the coast of Ecuador and Peru. This has an impact on weather, changing the pattern of precipitation in many areas of the world.
DOWNWELLING
Downwelling is the opposite of upwelling – surfacewater moving to deeper parts of the ocean. This happens when the winds cause the Ekman transport to push the water towards the shore and then into the ocean depths.
GEOSTROPHIC CURRENTS
Geostrophic flows are controlled by a balance between a pressure gradient force and a Coriolis deflection. In addition, seafloor topography and the shape of ocean basins have a greater impact on surface and deep water currents.
These conditions can restrict flow between areas of the watershed where water can move and “direct” it into another area of the watershed. Ocean currents on the surface transport heat from one place to another in the Earth system. This also affects regional weather conditions.
DISTRIBUTION OF OCEANIC CURRENTS
Because of their flow patterns and geographic location, ocean currents are named differently in different regions. Because the currents in the northern hemisphere differ from those in the southern hemisphere, the global distribution of the main currents is listed according to their location north of the equator or south of the equator.
EQUATORIAL CURRENT
The North Equatorial Current is a main current in the Pacific and Atlantic Oceans that flows from east to west between about 10°N and 20°N.
The South Equatorial Current is a main current in the Pacific, Atlantic and Indian Oceans that flows in an east-west direction between the equator and about 20 degrees south. In the Pacific and Atlantic Oceans, it extends across the equator to about 5 degrees north.
At the equator, the South Equatorial Current is driven directly by trade winds blowing from east to west.
EQUATORIAL COUNTERCURRENT
Equatorial currents are often accompanied by countercurrents that flow at the surface in the opposite direction to the main current.
The equatorial countercurrent is a wind-driven, eastward, 1,015 m deep current found in the Atlantic, Indian, and Pacific Oceans. They exist on both sides of the equator. They are called north equatorial and south equatorial currents.
These currents circulate between 3 and 6 kilometers per day and typically extend 100 to 200 meters below the sea surface.In addition, there are three Equatorial Counter Currents, which flow towards the east.These are a partial return of the waters carried westward by the North and South Equatorial currents.This could be seen in all oceans.
NORTH EQUATORIAL COUNTERCURRENT
The North Equatorial Countercurrent (NECC), flows westtoeast at about 310°N in the Atlantic and Pacific basins.It is located between the North Equatorial Current (NEC) and the South Equatorial Current (SEC).
BOUNDARY CURRENTS
Boundary currents flow parallel to the continental margins, usually in north-south directions. Boundary currents on the western sides of the ocean basins will be strong. The border streams on the oriental sides of the sea basins are weak.
You are on the east side of the ocean basins, adjacent to the western coasts of the continents. The Southern Ocean has no southern borders and its waters are free to circulate around the world. It serves as a conveyor belt for the other oceans, which exchange water with each other.
WESTERN BOUNDARY CURRENTS
The western boundary currents are currents flowing from the equator to high latitudes regions in the northern and southern hemispheres. The Western boundary currents are warm, deep, narrow, and fast-flowing currents that form on the west side of ocean basins due to western intensification.
They carry warm water from the tropics poleward. They have specific names associated with their location as: North Atlantic Gulf Stream; North Pacific Kuroshio; South Atlantic Brazil; South Pacific East Australia; and Indian Ocean Agulhas.
All of these currents are generally narrow, jetlike flows that travel at speeds between 40 and 120 kilometers per day.The Western Boundary Currents are the deepest ocean surface flows, usually extending to 1000 meters below the ocean surface.
EASTERN BOUNDARY CURRENTS
Eastern boundary currents are relatively shallow, broad, and slow flowing. They are found on the eastern side of oceanic basins, adjacent to the western coasts of continents.
They flow from high latitudes towards the equator.They are streams of cold water. They also have specific names associated with their location, such as: North Atlantic Canary Islands; Pacific Northern California; South Atlantic Benguela; South Pacific Peru; and the Indian Ocean Western Australia.
All of these currents are generally broad, shallow rivers moving at speeds between 3 and 7 kilometers per day.
THE ANTARCTIC CIRCUMPOLAR CURRENT
The Antarctic Circumpolar Current(ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for it is the West Wind Drift. It is the dominant water circulation feature of the Southern Ocean.
The current is circumpolar because no land mass is connected to Antarctica. This condition keeps warm ocean water away from Antarctica and allows that continent to maintain its massive ice cap.
NORTH ATLANTIC REGIONS
The distribution of oceanic currents of the North Atlantic Ocean from north pole to south pole are:
- East Greenland Currents
- Irminger Current (flowing North +West)
- West Greenland Current (flowingNorth)
- Labrador Current (flowing South +East)
- Gulf Stream (flowingNorth)
- North Atlantic Current (flowing South west)
- Canaries Current (flowing Southwest)
- North Equatorial Current (flowingWest)
- Antilles Current (flowing NorthWest)
- Guiana Current (flowing NorthWest)
- Caribbean Current (flowing NorthWest)
- Equatorial Counter Current (flowing along the Equator towards East)
- Guinea Current (flowing East).
SOUTH ATLANTIC REGIONS
The distribution of oceanic currents of the South Atlantic are:
- South Equatorial Current(West)
- Brazil Current (South)
- Falkland Current (North)
- Antarctica Circumpolar Current (East)
- Benguela Current (North).
INDIAN OCEAN
The distribution of oceanic currents of the Indian Ocean are:
- Mozambique Current (South)
- Agulhas Current (South)
- West Australian Current (West)
- South Equatorial Current(West)
- Somali Current (North)
- Monsoon Drift (East)
SOUTH PACIFIC OCEAN
The distribution of oceanic currents of the South Pacific Ocean are :
- East Australian Current(South)
- Humboldt Current (North)
- Peru Current (North – NorthWest)
- Equatorial Current (West)
- South Equatorial Counter Current(E)
- South Equatorial Current(West).
NORTH PACIFIC OCEAN
The distribution of oceanic currents of the North Pacific Ocean are:
- Alaska Current (South West)
- Aleutian Current (SouthWest)
- Oyashio Current (South)
- Kuroshio Current (North East)
- Kuroshio Extension (North East)
- California Current (South)
- North Equatorial Current(West)
- North Equatorial Counter Current (East).
NAMES OF CURRENTS IN THE OCEAN
THE GULF STREAM
The Gulf Stream is a surface current that runs between the United States and Europe in the North Atlantic Ocean. Smaller spinning rings of water called eddies can form from surface ocean currents.
The Gulf Stream is a warm current. It originates in the Gulf of Mexico. It moves northward towards Europe. Since it is full of warm water, the sea surface temperatures are warm. Because of this, it keeps the places of Europe warmer than other areas located at similar latitudes.
The Gulf Stream carries 4500 times more water than the Mississippi River. Each second, ninety million cubic meters of water is carried past Chesapeake Bay (US) in the Gulf Stream.
THE HUMBOLDT CURRENT
The Humboldt Current is another example of a current that affects the weather. When this cold current is normally present off the coast of Chile and Peru, it creates extremely productive waters and keeps the coast cool and northern Chile arid.
THE LABRADOR CURRENT
The Labrador Current, which flows south out of the Arctic Ocean along the coasts of Newfoundland and Nova Scotia, is famous for moving icebergs into shipping lanes in the North Atlantic.
THE AGULHAS CURRENT
The Agulhas Current is a warm water current runs south along the east coast of southern Africa. The amount of water transported amounts to about 70 million cubic metres per second, making it one of the largest western Boundary Currents in the world.
The Agulhas’s has tributaries. The tributaries include the Mozambique Current, the East Madagascar Current, and anti-clockwise re-circulatory flow in the SW Indian Ocean.
BENGUELA CURRENT
Benguela Current is a cold, wide current that flows northwards along the west coast of southern Africa. It is an Eastern Boundary Current and forms the eastern flank of the South Atlantic Gyre.
THE INDIAN OCEAN GYRE
The Indian Ocean Gyre, is located in the Indian Ocean. It is one of the five major oceanic gyres. In the winter, it is reversed due to the South Asian Monsoon. Like the other gyres, it contains a garbage patch.
THE INDIAN MONSOON CURRENT
The Indian Monsoon Current refers to the seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean.
During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea. During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal.
These variations are due to changes in the wind stress associated with the Indian monsoon.
THE NORTH ATLANTIC GYRE
The North Atlantic Gyre, located in the Atlantic Ocean, is one of the five major oceanic gyres. It is a circular system of ocean currents that stretches across the North Atlantic from near the equator almost to Iceland, and from the east coast of North America to the west coasts of Europe and Africa.
THE BEAUFORT GYRE
The Beaufort Gyre is a wind-driven ocean current located in the Arctic Ocean polar region. The gyre contains both ice and water. It accumulates fresh water by the process of melting the ice floating on the surface of the water
OCEAN EDDIES AND RINGS
Eddies with horizontal diameters vary in size from 50-150km have their own pattern of surface currents. They create rings due to western boundary currents such as the gulf stream. At the equator and along the ocean boundaries, shallow undercurrents exist, flowing in a direction counter to that at the surface. These types of currents may affect the operation of submarines or trawlers. One good example is the equatorial undercurrent.
CONCLUSION
Currents are important for marine life because they transport most marine creatures all around the world. Currents affect the water temperature in marine ecosystems and play a major role in controlling the climate. Currents play an important role in the navigation of ships. Knowledge of ocean currents is essential to carry out the movement of ships, reduce shipping costs and fuel consumption.
Erzsebet Frey (Eli Frey) is an ecologist and online entrepreneur with a Master of Science in Ecology from the University of Belgrade. Originally from Serbia, she has lived in Sri Lanka since 2017. Eli has worked internationally in countries like Oman, Brazil, Germany, and Sri Lanka. In 2018, she expanded into SEO and blogging, completing courses from UC Davis and Edinburgh. Eli has founded multiple websites focused on biology, ecology, environmental science, sustainable and simple living, and outdoor activities. She enjoys creating nature and simple living videos on YouTube and participates in speleology, diving, and hiking.