GEOSTROPHIC CIRCULATION

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Horizontal Circulation

Surface currents are wind-generated. The major wind patterns we looked at in the last chapter are reflected in the currents. The dominant features are the gyres and the boundary currents, both eastern and western boundary currents.

 

 

WBC EBC

 

Direction poleward equatorward

 

Speed fast (2-4kts) slow (1-2kts)

 

Width narrow wide

 

Depth deep shallow

 

Other eddies, rings upwelling

 

warm, salty cold, fresher

The gyres rotate in a clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

Ekman Spiral

When Nansen set out on the Fram in 1893 with 12 other men and provisions for five years, he intended to become frozen into the ice and be carried across the North Pole.

They missed the pole by about 400km, but showed that no continent existed in the Arctic Sea. This means that icebergs in the Arctic are not of glacial origin as they are in the Antarctic. Nansen observed that the ice moved at an angle of 20o to 40o to the right of the winds blowing across the ice. A physist by the name of Ekman was given this information and he developed a circulation model to explain the observations. The model shows that the surface current moves at a 45 degree angle to the wind and at a speed of 3% or less of the wind speed. As this energy is transmitted to the next deeper layer, the direction of motion is to the right and the speed is slower. Finally the depth of no motion is reached at about 100m. The net transport of this layer is at a 90o angle to the wind direction and the projected vectors for a logrithmic spiral.

Geostrophic Currents

Let's look at the subtropical gyre in the North Atlantic. It rotates in a clockwise direction. The Coriolis effect causes a flow to the right of the direction of movement or inwards toward the center. This causes the water to form a hill in the middle. If the rotation were stopped, the sea surface would flatten out due to gravity. The net result of this is an elevation in the sea surface such that there is a balance between the coriolis effect and gravity. In this gyre this elevation is on the order of 2m, but the horizontal distance is quite large so it can only be detected by remote sensing. This type of current is called a geostrophic current.

Westward Intensification

If we look closely at these sea surface elevations in an XY plane, we see that the apex of the mound is asymmetrical. The center is closer to the western boundary of the gyre. The major reson for this is the Coriolis effect. Since this effect varies with latitude and these gyres cover a large range of latitude, we would expect the net effect to vary from north to south.

The Coriolis effect is greater for the eastern currents since they are on the northern boundary. Assuming an equal volume of water flowing around the gyre, this means slower currents on the eastern boundary and faster currents on the western boundary. This is called westward intensification.

Dynamic Topography

A technique that uses TS data from a number of stations to compute the vertical distribution of density and to infer current velocities. Critical to this approach is selection of the depth of zero motion. Warmer, less dense water takes up greater volume, producing a higher sea surface. Current speed and direction can be calculated by the steepness of the slope and the hemisphere. Vertical Circulation

Wind-induced Circulation

       Upwelling - found in eastern boundary and coastal currents. Caused by the net effect of the wind and Ekman transport (Coriolis effect). Also found in equatorial current systems.

       Langmuir circulation - caused by constant winds, act on much smaller scales than upwelling systems. Described in 1938 based on the accumulation of Sargassum in the convergent regions. Typically the scale is 20-30m in width, but it is dependent on wind speed. The convergent regions not only concentrate Sargassum, but any floating objects such as tar balls, plastics etc.

Antarctic Circulation - the largest surface current in the world is found in the Antarctic and is called the Antarctic Circumpolar Current because it flows around the continent of Antarctica. It flows in an eastward direction. Another, lesser current flows westward closer to the continent. This current is called the East Wind Drift. It is driven by easterly polar winds and is in the Wedell and Ross seas. As the Circumpolar Current passes from the Pacific sector to the Atlantic sector, it must narrow to go through the Drake Passage. Its speed is about 1 knot and the mass transport is about 130sv. A mixture of circumpolar water and shelf water forms the Antarctic Bottom Water (which has a sigma t of about 27.8 in the Wedell Sea where it sinks and flows north.

Atlantic Ocean Circulation

There are two large subtropical gyres, clockwise in the north and counterclockwise in the south. These gyres are separated by the Equatorial Counter Current and are driven by the northeast and southwest trade winds.

In the South Atlantic the major currents of the gyre are the South Equatorial Current which splits into a north and south component, the Brazil Current and the Bengula Current on the eastern bpundary. This current is characterized by upwelling.

In the northern gyre the North Equatorial Current splits into the Antilles Current on the eastern side of the West Indies and the Caribbean Current which passes through the Yucatan Straits and into the Gulf of Mexico. After forming the Loop Current, its passes between Florida and Cuba as the Florida Current and merges with the Antilles Current to form the Gulf Stream. At times the mass transport can exceed 35sv. By the time it reaches Chesapeake Bay, the mass transport can reach 90sv indicating a large amount of entrainment of Sargasso Sea water. Rings are common features once the Stream turns out to sea off Hatteras. Give additional explanation. The major deep water masses are the Antarctic Intermediate Water, the North Atlantic Deep Water and the Antarctic Deep Water. In midlatitudes, the outflow of the Mediterranean can be seen based on its TS characteristics. The residence time is estimated to be about 275 years.

A study of deep-ocean water in 1981 showed slightly lower salinity and temperature values (.02ppt and .15oC) between 50 and 60 degrees N in the Atlantic than in 1972. It was previously thought changes of this magnitude occurred only on time scales of thousands of years. It is now thought that this may be due to an increase in the sinking rate of North Atlantic Deep Water. The forcing factors are presently unknown.

Pacific Ocean Circulation

The surface circulation of the Pacific is very similar to that of the Atlantic with two gyres separated by the Equatorial Countercurrent which is better developed. In the southern gyre we have the South Equatorial current, the East Australian current, the Antarctic Circumpolar current and the Peru current.

In the northern gyre we have the North Equatorial current, the Kurshio Current, the Kurshio Extension and the California Current. To the north of this gyre is the Alaskan gyre.

The Pacific Ocean bottom water is a mixture of NADW and ABW which is called the Oceanic Common Water (OCW). It moves slowly to the north with a volume transport of about 25sv. The dissolved oxygen of these water is below that found in the deep Atlantic which suggests slower movement. The residence time is estimated at about 500 years. Excess helium produced by hydrothermal vents along the East Pacific Rise have been traced 2000km to the west suggesting that the excess heat produced from these vents is forcing deep circulation in the opposite direction to what one would have predicted.

Indian Ocean Circulation

The Indian Ocean only extends to only about 20 degrees N. The currents in this ocean are strongly influenced by seasonally changing winds systems driven by the differences in specific heat of the continent and the ocean. In the northern hemisphere winter the rapid cooling of the continent creates a high pressure cell producing northeast winds and surface ciculation similar to that in the other oceans. There is and equatorial current system, the Agulhas Current on the western boundary and the West Australia Current as the eastern boundary current. When the winds shift, the Southwest Monsoon causes the development of the Somali Current and a northern gyre.

In the deep water we find the Red Sea Water which is characterized by high salinity. The remainder is the OCW. The residence time is estimated at about 250 years.

World Ocean Circulation Experiment (WOCE)

A large and expensive program headquartered at TAMU to look at the large-scale flow of heat, fresh water and chemicals by both the ocean and the atmosphere to provide data for modeling decadal climate change. North of about 30 degrees, the atmosphere becomes more important than the ocean in heat transport.