Ocean Currents
Surface Currents
Deep Currents
Oceanic Conveyor System
Surface Currents:Atmospheric winds drive
the circulation of the upper 1000 meters of the ocean. The wind exerts a stress
on the surface of the ocean, generating waves and surface currents. The wave
motion tends to keep the surface layer well-mixed; the surface currents
transport water long distances.
The tropical easterly trade winds drive the westward-flowing equatorial currents
found in all tropical oceans. When blocked by landmasses, these currents are
deflected North and South. Deflected currents travel poleward along the western
parts of the oceans. These are called western boundary currents and are the
strongest of all currents in the oceans. Examples are the Gulf Stream in the
western North Atlantic and the Kuro Shio Current in the western North Pacific.
As they move into the mid-latitudes, these boundary currents are turned eastward
by the prevailing westerly winds. These currents cross the oceans at mid- to
high latitudes and then turn equatorward down the west coasts of the continents,
completing the convection cell in a manner similar to what happens in the
atmosphere. These great cells or gyres occur in the tropical and subtropical
regions of the North and South Atlantic, the North and South Pacific, and the
Southern Indian oceans.
The North Indian Ocean is a special case. While a gyre similar to those in
the other oceans is found, it changes direction every six months, reflecting
the reversals in the atmospheric circulation associated with the monsoons.
Weak gyres are found in the subpolar regions of the Northern Hemisphere.
In the Southern Hemisphere, there is open ocean completely around Antarctica.
Consequently, the Antarctic circumpolar current encircles the continent.
Deep Currents: Complementing the surface
currents is a series of currents in the deep ocean. These tend to be slow moving
and generally from high latitude toward the equator. The water flowing in these
deep currents -- North Atlantic Deep Water and Antarctic Bottom Water -- are
easily identified by temperature, oxygen content, and salinity.
The Oceanic Conveyor System: The surface and
deep currents mentioned above are components of a global system that continually
exchanges water, heat, and salt among all the oceans and between the surface
layer and the deep water.
The warm, poleward flowing surface currents cool as they loose heat to the air
and water evaporates (additional cooling, plus it leaves behind the dissolved
salt). Via thermohaline
convection, at high latitudes the resulting cold, relatively salty
near-surface water sinks into the deep ocean, then flows slowly toward the
equator in the deep current.
Two regions in the North Atlantic, to the north and to the south of Iceland, are
at present the locations in which most of this sinking occurs. The resulting
deep current of cold, salty (i.e., dense) water flows southward, then
around the southern tip of Africa, across the southern Indian Ocean, passes east
of Australia, and ultimately into the North Pacific, where it upwells to the
surface. Surface water from the North Pacific moves south, passing between Asia
and Australia, around southern Africa (this time heading west), and then across
the South Atlantic, finally connecting to the Gulf Stream to flow north once
more. For most of this journey, the surface water is heated by the Sun and
collects fresh water from river outflow to become progressively more buoyant.
Only when it starts up the Gulf Stream does loss of heat to the air and
evaporation lead to increasing density and lack of buoyancy.
The oceanic conveyor appears to susceptible to "mode switching", that
is, it can exist in more than one configuration.