SEA CURRENTS: HOW THEY ARE PRODUCED, TYPES, CONSEQUENCES, IMPORTANCE - ENVIRONMENT

The currents are massive displacement of both surface and deep water, caused by winds, the earth 's rotation, differences in temperature and salinity. They can be shallow and deep, with shallow ones appearing in the first 200 to 400 m depth. For its part, the deep currents in greater depths.

The superficial marine currents are produced due to the push of the water by the winds and the deep ones due to differences in temperature and salinity.

Main marine currents in the world. Source: Dr. Michael Pidwirny (see http://www.physicalgeography.net) / Public domain

Both shallow and deep currents complement each other forming a large oceanic conveyor belt. Thus, water masses move in surface currents that go from the equator to the polar circle and return in deep currents.

In the case of deep currents, they return to the equator and continue to Antarctica through all the oceans. In Antarctica they head east, crossing the Indian Ocean and from there to the Pacific, where warm surface currents move north and return to the Atlantic.

The systems of marine currents form the so-called ocean gyres, through which water circulates in the planet's oceans. There are 5 main gyres, two in the Atlantic Ocean, two in the Pacific and one in the Indian Ocean.

Among the most prominent currents are the Gulf of Mexico, Las Agujas, Eastern Australia, Humboldt and Mediterranean currents. All ocean currents fulfill important functions in the planetary system by regulating the climate, distributing nutrients and biodiversity, as well as facilitating navigation.

How are ocean currents produced?

- General ocean conditions

In the oceans there is a surface temperature gradient, where the maximum temperature is located in the Red Sea with 36 ºC and the minimum in the Weddell Sea (Antarctica) with -2 ºC. Likewise, there is a vertical temperature gradient, with warm waters in the first 400 m and a very cold zone below 1,800 m.

There is also a salinity gradient, with saltier waters in areas with less rainfall such as the Atlantic and less salty where it rains more (Pacific). On the other hand, there is less salinity on the coasts where rivers that supply fresh water flow in relation to the offshore.

In turn, both temperature and salinity affect the density of the water; the higher the temperature, the lower the density and the higher the salinity, the higher the density. However, when seawater freezes and forms ice, its density is greater than that of liquid water.

- Coriolis effect

The Earth rotates on its axis to the east, causing an apparent deflection in any object that moves across its surface. For example, a projectile launched from the equator toward a site in Alaska (north) will land slightly to the right of the target.

This same phenomenon affects winds and ocean currents and is known as the Coriolis effect.

- Development of currents

Surface currents

Due to the differential heating of the Earth, there are warm temperatures near the equator and cold at the poles. The hot air masses rise creating a vacuum, that is, a low pressure area.

Thus, the space left by the hot air is filled with air from a cold region (high pressure zone), which moves there due to the action of the winds. In addition, the Earth in its rotational movement causes a centrifugal force at the equator, causing the water to move north and south in this area.

Likewise, the waters near the equator are less salty due to the fact that there are more rains that provide fresh water and dilute the salts. While towards the poles it rains less and a large percentage of the water is frozen, so the concentration of salts in liquid water is higher.

On the other hand, at the equator the waters are warmer due to the higher incidence of solar radiation. This causes the water in this area to expand and raise its level or height.

The surface currents of the North Atlantic Gyre

When analyzing the effect of these factors in the North Atlantic, it is observed that a large system of closed circulation of marine currents is generated. It begins with the winds that come from the northeast (trade winds) causing superficial sea currents.

These northeast currents, when reaching the equator, move westward due to rotation, starting from the western coast of Africa. Then upon reaching America, the equatorial current encounters continuous ground obstacles to the north.

North Atlantic current. Source: Goddard Space Flight CenterDerivative work MagentaGreen (SVG Version) / Public domain

The presence of the obstacles, plus the centrifugal force of the equator and the difference in temperature between the equatorial and polar waters, direct the current to the northeast. The current increases its speed when it circulates in the narrow channels between the Caribbean islands and the Yucatan channel.

Then, from the Gulf of Mexico, it continues through the Straits of Florida, strengthening by joining the current of the Antilles. From here it continues its course north along the east coast of North America and later northeast.

Deep currents of the North Atlantic Gyre

On its journey north, the Gulf Stream loses heat and the water evaporates, becomes saltier and denser, sinking to become a deep current. Later, upon reaching the North Western European ground obstacle, it splits and one branch continues north, then turns west, while the other continues south and returns to the equator.

Closing of the North Atlantic Gyre

The branch of currents of the North Atlantic Giro that collides with Western Europe heads south and forms the Canary Current. In this process the currents of the Mediterranean Sea in a west direction are incorporated, which contribute a large amount of salts to the Atlantic Ocean.

Similarly, the trade winds push the waters of the African coast to the west, completing the North Atlantic Turn.

North Atlantic Subpolar Gyre

The current heading north forms the North Atlantic Subpolar Gyre, going west meets North America. Here the cold and deep Labrador current is formed, which heads south.

This Labrador Ocean Stream passes under the Gulf Stream in the opposite direction. The movement of these currents is given by differences in temperature and saline concentration (thermohaline currents).

Large ocean conveyor belt

The set of thermohaline currents form the system of currents that circulates below the surface currents, forming the great oceanic conveyor belt. It is a system of cold and deep currents that goes from the North Atlantic to Antarctica.

Ocean conveyor belt. Source: Avsa / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

In Antarctica the currents go east and when passing Australia it heads towards the North Pacific. In this process, the waters are warming, so they rise when they reach the North Pacific. Then they return to the Atlantic in the form of a warm surface current, passing through the Indian Ocean and connecting with the oceanic gyres.

Types of ocean currents

There are two basic types of ocean currents defined by the factors that give rise to them and the oceanic level through which they circulate.

Shallow and deep sea currents. Source: Thomas Splettstoesser / Public domain

Surface sea currents

These currents occur in the first 400-600 m depth of the sea and are originated by the winds and the rotation of the Earth. They comprise 10% of the mass of water in the oceans.

Deep sea currents

Deep currents occur below 600 m depth and displace 90% of the seawater mass. These currents are called thermohaline circulation, since they are caused by differences in water temperature ("thermo") and salt concentration ("haline").

Main ocean currents

Main marine currents in the world. Mariiana QM / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The ocean gyres

According to the pattern of the winds and by action of the Earth's rotation, the marine currents form circular systems of currents called oceanic gyres. There are 6 main turns:

North Atlantic gyre
South Atlantic gyre
North Pacific Gyre
South Pacific Giro
Turn of the Indian Ocean
Antarctica spin

Each turn is formed by different currents, of which the current of the western limit of each turn is directed towards the corresponding pole. In other words, the North Atlantic and North Pacific gyres go to the North Pole and the South Atlantic, South Pacific and Indian gyres go to the South Pole.

Ocean gyres. Source: NOAA / Public domain

The currents of the western limit of each gyre are the strongest and thus the Gulf of Mexico current corresponds to the North Atlantic Gyre and the Kuroshio current to the North Pacific Gyre.

In the South Atlantic Gyre, the strongest current is that of Brazil and in the South Pacific that of Eastern Australia. For its part, in the Giro del Indico is the Las Agujas current, which runs along the eastern coast of Africa from north to south.

Taking the North Atlantic Gyre as an example, we find that the entire system is made up of four currents. In this Giro, in addition to the Gulf Stream in the west, there is the North Atlantic Stream going northeast.

Then, in the east, the Las Canarias current is located, which heads southeast, and the circuit closes with the North Equatorial current to the west.

The Gulf of Mexico Stream

This current is part of the North Atlantic gyre and is so named because it is born in the Gulf of Mexico. Here the surface waters heat up and expand, raising the sea level relative to the colder northern waters.

Therefore, the current is generated from the Gulf to the north, where the water will lose heat sinking and forming the North Atlantic current.

Western European Climate

The Gulf Stream contributes greatly to regulating the climate of Western Europe, thanks to the heat it carries from the Gulf of Mexico. This heat released off Greenland is blown towards the continent by westerly winds, moderating continental temperatures.

The Mediterranean current

The Mediterranean Sea is an almost closed basin, except for the 14.24 km wide connection with the Atlantic Ocean through the Strait of Gibraltar. This sea loses about 1 m of water annually by evaporation in its warm summers.

The connection with the Atlantic and the currents that are generated, allow the lost water to be renewed and oxygenated. The currents that leave the Mediterranean help to form the Gulf Stream.

Salinity gradient

Salinity and temperature are the fundamental factors that act to produce the current between the Mediterranean and the Atlantic. By losing water through evaporation in a closed area, the salinity in the Mediterranean is higher than in the Atlantic Ocean beyond the strait.

Water with a higher salt content is denser and goes to the bottom, forming a deep current towards the Atlantic with a lower concentration of salts. On the other hand, the surface water layer of the Atlantic is warmer than that of the Mediterranean and generates a surface current from the Atlantic to the Mediterranean.

The Humboldt current

It is a superficial stream of cold water that travels from Antarctica to the equator along the South American Pacific coast. It comes from the rise or rise of part of the cold waters of the deep current of the South Pacific when colliding with the South American coast.

It is part of the subtropical Giro of the South Pacific and is responsible for providing a large amount of nutrients to the coasts of Chile, Peru and Ecuador.

Consequences

Heat and salinity distribution

Ocean currents flow from places with warmer and saltier waters to colder regions with less saline concentration. In this process they help to distribute the ambient heat and the salt content in the oceans.

Impact on climate

By moving masses of warm water to cold areas, currents participate in the regulation of the Earth's climate. An example of this is the moderating effect of the ambient temperature of the Gulf of Mexico current in Western Europe.

Thus, if the Gulf Stream were to stop flowing, the temperature of Western Europe would drop by an average of 6 ° C.

Hurricanes

Sea currents, by transporting heat, provide moisture by evaporation and generate a circular movement in close relationship with the winds, which are the cause of hurricanes.

Gas exchange

Seawater maintains a constant gaseous exchange with the atmosphere, including water vapor, oxygen, nitrogen and CO _2. This exchange is made possible due to the movement of water by sea currents that contributes to breaking the surface tension.

Coastal modeling

Ocean currents exert a wear and drag force (erosion) on the surface of the seabed and the coasts through which they pass. This erosive effect over thousands of years shapes the seabed, seamounts and coastlines.

Nutrient distribution and biodiversity

On the other hand, the marine currents carry with them the nutrients as well as the plankton that feed on them. This conditions the distribution of marine fauna, since it is concentrated where there is more food available.

Plankton is passively carried away by surface currents, and part of the nutrients precipitate to the bottom, where they are displaced by deep currents. Later, these nutrients return to the surface in the so-called upwellings or marine outcrops of waters.

Surge or outcrops of marine waters

Deep currents give rise to the so-called upwellings or outcrops of marine waters. It is the rise of cold deep waters to the surface, which carry nutrients deposited in the deep ocean.

Rising sea currents. Source: NASA / Public domain

In the areas where this occurs, there is a greater development of the populations of phytoplankton and therefore of fish. These areas become important fishing zones, such as the Peruvian Pacific coast.

Pollutant concentration

The oceans suffer serious pollution problems due to human action, which incorporates large amounts of waste, especially plastic. The marine currents carry this debris and due to the circular pattern of the surface, these are concentrated in defined areas.

This is where so-called plastic islands arise, which are formed by concentrating plastic fragments in large areas in the center of ocean gyres.

In the same way, the combination of the superficial marine currents with the waves and the shape of the coastline, concentrate the waste in certain areas.

Importance for ecosystems and life on Earth

Marine migrations

Many marine species, such as turtles, cetaceans (whales, dolphins), and fish, use ocean currents for their long-distance ocean migrations. These currents help define the route, reduce travel energy and provide food.

Nutrient availability

The distribution of nutrients both horizontally and vertically in the oceans depends on marine currents. This in turn affects the phytoplankton populations that are the primary producers and base of the food webs.

Where there are nutrients there are plankton and fish that feed on it, as well as other species that feed on fish like seabirds.

Fishing

The distribution of nutrients through marine currents affects the availability of fishing for humans.

Oxygen availability

Marine currents, by mobilizing water, contribute to its oxygenation, which is essential for the development of aquatic life.

Terrestrial ecosystems

Coastal and inland ecosystems are influenced by marine currents to the extent that they regulate the continental climate.

The navegation

The marine currents have allowed the development of navigation by the human being, allowing marine travel to distant destinations. This has made possible the exploration of the Earth, the dispersal of the human species, trade and economic development in general.

Factors influencing the direction of currents

The direction that ocean currents take is expressed in a regular pattern in the world's oceans. This pattern of directions is determined by multiple factors whose forces are solar energy and the gravity of the Earth and the Moon.

Solar radiation, atmospheric pressure and the direction of the winds

Solar radiation influences the direction of ocean currents by being the cause of the winds. These are the main cause of formation of surface currents that follow the direction of the winds.

The temperature gradient and gravity

Solar radiation also affects the direction of ocean currents by heating the water and causing it to expand. Due to this the water increases in volume and raises the sea level; with higher areas of the ocean (hot) than others (cold).

This forms a level difference, that is, a slope, moving the water towards the lower part. For example, at the equator the temperatures are high and therefore the water expands, determining a sea level 8 cm higher than in other areas.

The salinity gradient

Another factor that affects the direction of ocean currents is the difference in salinity between different areas of the ocean. As water is saltier, its density increases and sinks, and deep currents move as a function of temperature and salinity gradients.

The marine and coastal relief

The shape of the continental shelf and the coastline also influence the direction of the marine currents. In the case of surface currents that run along the coasts, landforms influence their direction.

On the other hand, deep currents when impacting with the continental shelf can suffer both horizontal and vertical deviations.

The rotation of the Earth and the Coriolis Effect

The rotation of the Earth affects the direction of the winds by generating a centrifugal force at the equator, pushing the currents towards the poles. Furthermore, the Coriolis effect diverts currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

References

Campbell, N. and Reece, J. (2009). Biology. 8th edition Pearson Benjamin / Cummings.
Castro, P. and Huber, ME (2007). Marine Biology. 6th edition McGraw- Hill.
Kelly, KA, Dickinson, S., McPhaden, MJ and Johnson, GC (2001). Ocean currents evident in satellite wind data. Geophysical Research Letter.
Neumann, G. (1968). Ocean currents. Elsevier Publishing Company.
Pineda, V. (2004). Chapter 7: Morphology of the ocean floor and characteristics of the coastline. In: Werlinger, C (Ed.). Marine Biology and Oceanography: Concepts and Processes. Volume I.
Prager, EJ And Earle, SS (2001). The Oceans. McGraw-Hill.
Ulanski, S. (2012). The Gulf Stream. The incredible story of the river that crosses the sea. Turner Publications SL

SEA CURRENTS: HOW THEY ARE PRODUCED, TYPES, CONSEQUENCES, IMPORTANCE - ENVIRONMENT - 2025