The ocean churns up various types of currents. Together, these larger and more permanent currents make up the systems of currents known as gyres. Wind, tides, and differences in temperature and salinity drive ocean currents.
The ocean churns up different types of currents , such as eddies , whirlpools , or deep ocean currents. Near Antarctica the circulation is somewhat different. Because there is little in the way of continental land masses between o south, the surface current created by the westerly winds can make its way completely around the Earth, creating the Antarctic Circumpolar Current ACC or West Wind Drift WWD that flows from west to east Figure 9.
The Antarctic Circumpolar Current is the only current that connects all of the major ocean basins, and in terms of the amount of water that it transports, it is the largest surface current on Earth. Above 60 o latitude the prevailing winds are the polar easterlies , which create a current flowing from east to west along the edge of the Antarctic continent, the East Wind Drift or the Antarctic Coastal Current.
The Antarctic Circumpolar Current creates the southern boundary for all of the Southern Hemisphere gyres. Not all of the equatorial water that is moved westward by the trade winds and reaches the continents gets transported to higher latitudes in the gyres, because the Coriolis Effect is weakest along the equator.
Instead, some of the water piles up along the western edge of the ocean, and then flows eastward due to gravity, creating narrow Equatorial Countercurrents between the North and South Equatorial Currents Figure 9. Some of this water also moves east as equatorial undercurrents that flow at depths between m, underneath the Equatorial Currents.
The Coriolis effect : The rotation of the Earth has an influence on the movement of the ocean and air. In the northern hemisphere, it causes the ocean or winds to rotate in an anti-clockwise direction. In the southern hemisphere, it causes them to move in a clockwise direction.
This video will give you a better visual explanation. Density: A difference in the density of the water in the ocean also causes the water to move. Water with a higher density is heavier and moves downwards. Water with lower density is lighter and rises. Density is also affected by the salinity. Landmass : These oceanic movements are restricted by landmasses, which determine the size of the gyre. The movements of gyres are responsible for the exchange of water between major oceans, creating a well-balanced system of water current trajectories that are collectively termed as the Ocean Conveyor Belt.
The resulting motion is not in line with the wind, however. Earth's rotation causes an apparent force known as the Coriolis effect to deflect straight-line movement across the surface about 45 degrees to the right in the Northern Hemisphere and 45 degrees to the left in the Southern Hemisphere.
In addition, each successive layer of water is slightly deflected from the motion of the one above, like a deck of cards fanned out. This forms a phenomenon called an Ekman spiral that was first described by Swedish mathematician Vagn Walfrid Ekman in , but it was not until the late s that a team from WHOI first observed it in the open ocean.
The net wind-driven movement of water, known as Ekman transport, creates a bulge in each ocean basin that is as much as three feet one meter higher than mean global sea level.
The force of gravity pulling on this large mass of water creates a pressure gradient similar to that in an atmospheric high pressure system which in turn leads to a stable, rotating mass of water. Five permanent subtropical gyres can be found in the major ocean basins—two each in the Atlantic and Pacific Oceans and one in the Indian Ocean—turning clockwise in the Northern Hemisphere and counterclockwise in the Southern.
Smaller counterclockwise gyres centered at around 60 degrees north latitude are created by the prevailing winds around permanent sub-Arctic low-pressure systems. Another subpolar gyre, the only one centered on a landmass, circles Antarctica driven by the near-constant westerly winds that blow over the Southern Ocean, unimpeded by land.
The subtropical gyres are surrounded by four linked currents: two boundary currents oriented roughly north-south at their eastern and western edges and two east-west currents at the northern and southern extent of the gyre. Western boundary currents are also among the fastest non-tidal ocean currents on Earth, reaching speeds of more than five miles per hour 2. As these warm western boundary currents slow and spread out, they turn east to form the most poleward currents of their associated gyre.
In the north, they also act as the southern boundary of the sub-polar gyres, permitting the exchange of water between the subtropics and the Arctic. In the south, the Antarctic Circumpolar Current connects to the southern subtropical gyres through these currents in a similar way. The colder eastern boundary currents, which flow from the high latitudes toward the equator, are the slowest and most diffuse currents around the gyre.
As they reach the equator, they turn west and pick up speed, driven by the trade winds and heat from the tropical sun. Eddies are relatively small, contained pockets of moving water that break off from the main body of a current and travel independently of their parent. They can form in almost any part of a current, but are especially pronounced in western boundary currents.
Once the fast-moving currents leave the confining influence of land, they become unstable and, like a fire hose with no one holding it, begin to meander and bend. If a current becomes so tightly bent that it doubles back on itself, that section of flow may "pinch off" and separate from the main body of the current like an oxbow bend in a river. These swirling features can take the shape of warm-core masses of warm water turning in colder ocean waters or cold-core masses of cold water in warm eddies and can travel for months across hundreds or thousands of miles of open ocean.
Eddies also form in the mid-ocean, far from boundary currents. Their genesis results from an instability process in which large-scale mean flows are constantly breaking down into smaller scale features.
The atmosphere behaves in much the same way: energy is put into the system on the planetary scale it is warm at the equator and cold at the poles , which creates large-scale flow that spawns the storms and fronts we know as weather.
In that sense, ocean eddies are analogous to atmospheric weather—although their spatial scales are smaller and temporal scales longer because of differences between air and water. Currents, gyres and eddies transport water and heat long distances and help promote large-scale mixing of the ocean. Strong currents and eddies also influence shipping routes and have been known to damage oil platforms.
Powerful offshore currents and weaker coastal currents shape the land by contributing to beach erosion and the movement of barrier islands. Knowledge of how and where these phenomena occur as well as how they might be changing is sought by fishing fleets to locate schools of fish, by the Coast Guard to respond to search-and-rescue emergencies or oil spills, and by policy makers to help formulate marine conservation plans.
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