TOPIC 4 - Ocean Circulation
Surface Ocean Circulation - This model for surface ocean circulation was first developed by Ekman and is termed the Ekman model.
a. Atmospheric Circulation and Coriolis Force
Prevailing winds develop waves and push them in one predominant direction forming surface ocean currents. This produces strong Equatorial Currents due to the trade winds and mid-latitude Easterly Currents due to the prevailing westerly winds. Ocean currents moving north or south in the northern hemisphere are deflected to the right by the Coriolis force forming Ekman spirals. Ekman spiraling and the Coriolis force also cause ocean currents to spiral toward the right with increasing water depth while the current velocities decrease. At depth, surface ocean currents die out just like particle motion associated with wave motion dies out. As a result of the Coriolis force, the Trade winds (tropical easterly winds) push surface water to the west and north while the mid-latitude westerlies push water to the east and south. This forms a dome of water near 30° latitude (on average) between the two wind belts. As the dome builds, it sets up a pressure gradient and water attempts to flow away from the dome. As it flows away, it is deflected by Coriolis force and ends up flowing in the same direction as the prevailing winds on either side of the bulge.
b. Continental Effects
As currents flow west under the trade winds, they strike continental barriers. The currents are then deflected mostly northward (southward) in the northern (southern) hemisphere by the Coriolis effect forming Western Boundary Currents. The Gulf Stream in the North Atlantic Ocean is a prime example of this process.
Some water, however, is deflected southward towards the equator where it meets some water deflected northward in the southern hemisphere. These currents meet and slowly flow eastward in the Doldrums forming Equatorial Counter-Currents.
At mid latitudes, currents moving eastward under the influence of the prevailing westerlies may also strike a continental barrier where they are primarily deflected toward the right (left) in the northern hemisphere. These currents as a group are called Eastern Boundary Currents.
c. Currents and Upwelling
As currents move north as western boundary currents or south as eastern
boundary currents (in the northern hemisphere), they are deflected by the
Coriolis force toward the open ocean. This allows deeper water to rise to
the ocean surface to replace it producing upwelling. This can also occur
if there are strong prevailing winds blowing from the continent toward the
ocean.
Large-scale upwelling occurs near the equator where northern and southern hemisphere eastern boundary currents meet and turn westward as equatorial currents.
Large-scale upwelling can also occur when two currents pass one another going in opposite directions. As the currents pass, Coriolis forces will cause them to either converge (causing downwelling), or diverge (causing upwelling).
2. Distribution of Surface Currents in the World's Oceans
A. Westerly Flowing Surface Currents: In all of the Equator-crossing oceans (Atlantic, Pacific, Indian), there are strong westerly flowing currents in the Tropics driven by the Trade winds. In each ocean basin, these currents are called North or South Equatorial Currents depending on whether they are north or south of the equator. These currents usually flow westward to continental boundaries and form easterly flowing Equatorial Counter currents which separate the North And South Equatorial Currents.
B. Western Boundary Currents: When the Equatorial Currents become blocked by continents which sit astride the equator, the currents are deflected northward and southward along the western boundaries of the ocean basins. The best known western boundary surface currents are the Gulf Stream in the western North Atlantic Ocean, the Kuroshio Current in the western North Pacific, and the Brazil Current in the South Atlantic Ocean.
C. Easterly Surface Currents: As western boundary currents reach latitudes of the prevailing westerly winds, they become easterly currents. The best known examples are the continuation of the Gulf Stream as it moves toward Europe and the North Pacific Current.
D. Eastern Boundary Currents: As westerly currents are blocked by continents, they are deflected toward the equator forming eastern boundary currents on the western margins of continents. The best examples are the California Current off North America, the (Pacific) Peru or Humbolt Current off South America, and the (Atlantic) Benguela Current off Africa.
E. The Antarctic Circumpolar Current flows easterly all the way around Antarctica.
F. All of these surface current systems form continental-scale gyres,
or zones of coherent cyclonic circulation (either clockwise or counter-clockwise)
in all of the major ocean basins. At the centers of these gyres, are saline
subtropical surface waters that underlie atmospheric high pressure systems.
These zones of surface water are not as well mixed as the rest of the surface
ocean. The best known of these regions in the Sargasso Sea within the North
Atlantic Ocean gyre.
3. Thermohaline Circulation
A. Changes in water temperature and salinity alter water mass density and cause them to move vertically to reach water masses of comparable density.
B. Water masses become more dense at the ocean's surface due to cooling or increased salinity. These masses sink so that the ocean maintains a stable density stratification (less dense water above more dense water).
C. Thermohaline circulation primarily affects water in the deep oceans below the zone of surface ocean circulation. It produces nutrient-rich waters that rise to the surface during upwelling.
D. Source Water Masses for Deep Ocean Circulation.
North Atlantic Deep Water (NADW)
The Gulf Stream and North Atlantic Current carry relatively saline surface water from the equatorial Atlantic Ocean into the high-latitude North Atlantic Ocean. NADW is formed by cooling relatively saline surface water in the Greenland Sea between Greenland and Great Britain. High salinity and cold temps yield a very dense water mass. (open ocean process of deep water formation). NADW is created in large volumes and as it sinks to the ocean bottom it slowly flows southward underneath the Gulf Stream and North Atlantic Current.
Antarctic Bottom Water (AABW)
AABW forms along the continental shelf of Antarctica by the process of freezing new ice onto the margins of the Antarctic ice sheet. The water is already quite cold and the increase in density which occurs as salts are expelled from the freezing ice produce an even denser water mass. This process of deep water formation is called the near-boundary process. The AABW water mass is the densest water mass in the world's oceans and its large volume fills most of the southern Atlantic, Pacific, and Indian oceans.
Mediterranean Intermediate Water (MIW)
MIW forms by evaporation of relatively-warm water in the semi-enclosed Mediterranean Sea. Increased salinity results and the water sinks to the bottom of the Sea. Less saline surficial water from the Atlantic Ocean flows into the Mediterranean Sea to replace the lost surface water while the deep warm and salty MIW flows over the sill at Gibraltar and sinks into the eastern North Atlantic Ocean. MIW then continues to flow westward at depths of 1-2 km reaching as far as the island of Bermuda in the western North Atlantic Ocean.
Antarctic Intermediate Water (AAIW)
As ice flows out from the Antarctic continent it melts forming a low-salinity very cold surface current. As that cold low-salinity water mixes with AABW that is sinking to the ocean bottom, AAIW is formed. This water mass is less dense than AABW and NADW which comes to the surface in the Antarctic, but denser than surface water and flows underneath it.
4. The Overall Thermohaline Circulation System
a. Atlantic Ocean
NADW forms in the high north Atlantic Ocean and flows southward underneath surface water mass and intermediate water masses until it is diverted to the surface in the Antarctic. AABW forms in coastal Antarctica, sinks to the bottom, and flows northward underneath all other water masses. It reaches the mid-latitude North Atlantic Ocean where the water is entrained with NADW and swings back southward. THE AABW is so dense that it underlies NADW and deflects NADW to the surface near Antarctica. MIW flows westward above NADW and AABW and reaches the western North Atlantic Ocean. AAIW flow northward above NADW and AABW but underneath surface water masses.
The surface water is thickest in the central ocean gyres like the Sargasso Sea. The central gyres are warm, saline, low-nutrient waters with low biological productivity.
b. Pacific Ocean
AABW flows northward into the Pacific Ocean basin. Since there is no other source of deep water, AABW flows far into the north Pacific. AAIW flows northward into the Pacific Ocean where it meets North Pacific IW near the equator. The cores of the northern and southern gyres have warm, saline, nutrient-poor pools of surface water.
c. Indian Ocean
as a whole, the Indian Ocean deep circulation looks very much like the Pacific Ocean. Here too, there is no northern source of deep water and so AABW and AAIW penetrate far into the basin. There is a local source of intermediate water around India which forms as a result of monsoonal heating and evaporation forming a more saline intermediate water mass which flows southward away from the sub-continent.
