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Ocean circulation

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This article gives an overview of the main circulation patterns in the ocean. The two main subjects are surface ocean currents and deep-ocean circulation.


Introduction

Although tidal currents such as waves and tides are the most obvious water movements to us, their main effects are restricted to very shallow layers. They also can't be detected at the bottom of the deepest oceans. But in these deeper waters the more important water movements are those of the global system of oceanic currents. They are crucial in controlling the Earth’s climate.


Surface currents

Surface currents are mainly driven by the winds. This results in a complicated system dominated by a series of gyres moving clockwise in the northern parts of each ocean and anticlockwise in the south. There are six major gyres: the North Atlantic, the South Atlantic, the North Pacific, the South Pacific, the West Wind Drift and the Indian Ocean gyre. The West Wind Drift or Antarctic Circumpolar Current is situated in the Southern Ocean and constantly circles around Antarctica because there are no land masses to interrupt the currents. It is an eastward-flowing current. In the North Atlantic, the Gulf Stream transports warm water from the Caribbean along the eastern coast of North America and then flows across the ocean to Europe. This gyre is completed by the Canaries Current in the Eastern Atlantic which transports relatively cold water south and west. This water goes back to the Caribbean. The Gulf Stream is a vast moving mass of water, which gives rise to complex eddies and whirls on either side. It transports an enormous amount of heat. This helps to make the climate in North-western Europe much warmer than that at similar latitudes on the other side of the ocean. If this system stops, Europe will be much colder than it is today. In the Pacific, the eastern part of the gyre consists of the Peru Current. This current normally flows northwards along the coast of South America and then westwards across the Pacific towards Australia. It is associated with very important fisheries. The anticlockwise gyre in the southern part of the ocean is included in the phenomenon called El Niño. This occurs every few years when the system fails. Then the rich Peru Current is displaced by a very poor southward-flowing current. This causes dramatic changes in the marine conditions at the coast of South America and has important effects on the weather in many parts of the world. [1]


Surface currents [2]


In the Northern hemisphere, warm air around the equator rises and flows north toward the pole. As the air moves away from the equator, the coriolis effect deflects it toward the right. It cools and descends near 30° N. This descending wind blows from the northeast to the southwest back toward the equator. A similar pattern occurs in the Southern Hemisphere. These prevailing winds are known as the trade winds. The westward directed trade winds (easterlies) drive important transverse currents (east-west) near the equator. These are the North and South Equatorial Currents. Countercurrents return part of the water piled up at the western margins of the ocean basins by the convergent equatorial currents. This is the Equatorial Counter Current. They are slightly below the the original currents. The western boundary currents are among the largest and strongest ocean currents. They occur at the western side of an ocean basin or the eastern side of a continent. They are deep and moving fast and transport water and heat to the poles. An example is the Gulf Stream. The eastern boundary currents transport water from the poles to the equator. They return the water from the western boundary currents, driven by westerly winds. They occur at the eastern side of an ocean basin and are shallower, broader and slower than the western boundary currents. They are also associated with upwelling. The winds from the equator toward the poles are known as westerlies. They are called westerlies because the wind comes from the west. The global winds drag on the water’s surface, causing it to move and build up in the direction that the wind is blowing. Because of the coriolis effect, the wind-induced surface currents are deflected to the right in the Northern hemisphere (clockwise spiral) and to the left in the Southern hemisphere (counterclockwise spiral). These spirals are called gyres and are not present at the equator. This is because of the absence of the coriolis effect. [3] [4]


Western and Eastern boundary currents that form gyres [3]


Deep circulation

At greater depths, a slowly current system stirs the water of the ocean. This is much more important for the animals in the deep sea. It is called the thermohaline system or Conveyor belt. These currents are driven the sinking and rising of waters of different density caused by differences in salt content and particularly temperature. The water in the deep sea is cold with a potential temperature less than 4°C. The water mass is formed when cold, dense water sinks from the surface to great depths at high latitudes (North Atlantic and Antarctica). From these regions, the water fills the ocean basins. The whole trip takes approximately 1,000 years to complete. The deep water eventually is pulled up by deep mixing and is called upwelling. This upwelling drives the deep circulation. The vast deep ocean bottom is usually referred to as the abyssal plain and the circulation as the abyssal circulation. The circulation is important because it mixes the water and keeps its chemistry more or less uniform. It also carries oxygen into the deeper layers. In this way, organisms can survive. The dense water is formed when frigid air blows across the ocean at high latitudes in winter in the Atlantic, between Norway and Greenland, and in Antarctica. The wind cools and evaporates water. If the wind is cool enough, sea ice can form and this increases the salinity of the water because of salt expulsion. Deep water is only formed in the Atlantic Ocean and around Antarctica (North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW)), but in other polar regions dense water is formed. This water will not sink because it is not salty enough. The only place where water is salty enough to sink is the Mediterranean Sea. The salty water sinks to intermediate depths and spreads out in the Atlantic Ocean. This is called the Mediterranean Intermediate water (MIW). [5] [6]


Thermohaline circulation with cold, dense water in blue and warm, less dense surface water in red. It is a simplified scheme. [7]
Water masses [8]


Upwelling

Upwelling is a phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface. This water replaces the warmer, usually nutrient-poor surface water. There are several types of upwelling: coastal, equatorial, large scale upwelling in the Southern Ocean, tropical cyclone and non-oceanic (in other fluid environments such as magma) upwelling. Coastal upwelling is the best known type of upwelling. The wind blows parallel to the western coasts of continents and drive surface waters away from the coast (Ekman effect). This surface water is replaces by deep, cold water, which is rich in nutrients and CO_2. This causes a high primary production. Upwelling creates more phytoplankton, which creates in their turn rich fish stocks. The most productive areas of the ocean are characterized by upwelling. [9] At the opposite coast, downwelling is present. The thermocline also rises at the time upwelling is appearing and it is decreasing at the time downwelling is appearing. Downwelling causes water to sink and the water takes the nutrients with it.


Coastal upwelling [3]


Equatorial upwelling is another type of upwelling. At the Equator, cool and dense water rises and replaces the surface water. It also creates a high density of phytoplankton. At both side of the upwelling, downwelling is caused. This downwelling is the opposite phenomenon of upwelling. When the wind blows more strongly than about 3.5 meters per second, water flows parallel to the wind. This flow is called Langmuir circulation. Each convection cell is 10 to 15 meters broad, 5 to 6 meters deep and hundreds of meters to several kilometers long. Because of rotation in opposite directions, boundaries between adjoining Langmuir cells alternate between convergent and divergent flow. Floating material (bubbles, oil slicks, floating debris, seaweed) aggregates at the zones of flow convergence and forms a streaked appearance. It is a short-term response to wind drag.


Equatorial upwelling with the rising thermocline [10]
Langmuir circulation [3]


Further reading

Coriolis effect


References

  1. Rice T. 2000. Deep Ocean. The natural history museum, London. p. 96
  2. currents.gif
  3. 3,0 3,1 3,2 3,3 NOAA
  4. Course Paleoclimatology – Marc De Batist
  5. http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html
  6. Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508
  7. http://earthobservatory.nasa.gov/Study/Paleoclimatology_Evidence/paleoclimatology_evidence_2.html
  8. NADWcrossection.jpg
  9. http://en.wikipedia.org/wiki/Upwelling
  10. http://www-das.uwyo.edu/~geerts/cwx/notes/chap11/equat_upwel.html


The main author of this article is TÖPKE, Katrien
Please note that others may also have edited the contents of this article.