Oceans Currents

Currents in the world oceans are classified according to the force that makes them flow:

Tidal Currents

During ebb en flood there is water flowing from one location to another. Normally, the flow speed of these tidal currents is small (typically less than 0.5 m/s).

 However, close to the coast (i.e. in shallow areas), or in channels and other topographical constrictions, the flow speeds can be greater.

To a stationary observer the water will moving in one direction during flood tide, and in the opposite direction during ebb.

Density Currents

Imagine a rectangular fish tank with a removable partition in the middle. One side of the tank contains fresh water (=less dense) and the other section contains salt water (=more dense). What happens when the partition is removed?

The more dense, saline water will start flowing under the less dense water. This is called a density current, i.e. driven by the difference in densities between the two water masses.

In nature this occurs when the dense, cold Antarctic water comes in contact with the less dense, subtropical water and sinks to the bottom and spreads northward. These flows are normally a few cm per second or less.

Wind Driven Currents

The strongest currents in the world are wind driven.

The wind can cause surface flow of nominally 2.5% of the wind speed (10 m/s wind => 25 cm/s surface flow). This is known as wind drift.

In the open ocean the surface flow would be at 45 deg to the left of the wind direction (in the S. Hemisphere), progressively turning further with depth. This movement does not extend deeper than a few hundred metres (the so-called Ekman depth).

 While the wind drift speed is relatively small, the large wind fields of the world operate over huge ocean basins, and the wind drift is ultimately driven towards continents where the resultant flow speed can be fast (well over 1 m/s), deep (more than a km deep), wide (about 100 km) and transport huge amounts of water.

The large wind fields such as the southeast trades (the main wind field over, e.g. the South Indian Ocean) operate between the equator and about 30° S (centre of the South Atlantic High pressure and the South Indian High Pressure).

These trade winds will carry water northwestward and westward. Similarly, the westerlies drive the flow towards the southeast and east.

The combined effect of these two systems is to create a large anti-clockwise gyre in the South Indian and the South Atlantic.


http://uregina.ca/~sauchyn/geog221/global_circulation.bm


Western boundary currents, earth.usc.edu

However, the circulation is not symmetrical. Under the influence of the Coriolis force varying with latitude, the basin-scale gyre shows higher speeds in the western part of the basin than in die eastern part.

This is referred to as “westward intensification”. The western boundary currents are therefore significantly stronger than the eastern boundary currents.


http://www.coconutstudio.com/Coconut%20Origins_files/curentts_winds_marin

Rip Currents

Close to the beach, the waves tend to carry water toward the coast.

Changes in the bottom topography (e.g. sand banks) can cause water to be moved toward the beach without allowing the water to return to the deeper sea.

The flow back towards the deeper sea then occurs quite suddenly in the form of a narrow (<5 m wide) current moving quite fast (up 2 m/s).

A bather caught in a rip current will be rapidly carried away from the coast, and should not try to swim back toward the beach (the current will be too strong). Rather stay afloat until the flow of the current reduces (within about 50m from the beach), or swim parallel to the beach into calmer water.

Rip currents are dangerous and can lead to drowning.

http://www.seagrant.umn.edu/coastal_communities/img/r