Download The Antarctic circumpolar current

DESSALE NILS
03123081
BSC MARINE SCIENCE
Coastal Oceanography
Essay
THE ANTARCTIC
CIRCUMPOLAR CURRENT
Subject : Discuss the oceanographic dynamics of the Antarctic
Circumpolar current and appraise its role in oceanic circulation
20 february 2004
SUMMARY
INTRODUCTION ............................................................................................................... 3
1)OCEANOGRAPHIC DYNAMICS OF THE ANTARCTIC CIRCUMPOLAR
CURRENT ......................................................................................................................... 4
LIMITS: ............................................................................................................................ 4
-The Subtropical convergence ................................................................................ 4
-The Antarctic convergence .................................................................................... 5
-The Antarctic divergence ....................................................................................... 5
GENERATION: .................................................................................................................. 6
-A wind driven current............................................................................................. 7
-a geostrophic current ............................................................................................. 7
HYDROLOGY: .................................................................................................................. 8
-Sub Antarctic Mode Water .................................................................................... 8
-Antarctic Intermediate Water ................................................................................ 8
-Circumpolar Deep Water ...................................................................................... 8
-Antarctic Bottom Water ......................................................................................... 9
2)ANTARCTIC CURRENT’S ROLE IN OCEANIC CIRCULATION................. 10
MIXING CURRENT ......................................................................................................... 10
ACC and Australia ................................................................................................. 11
ACC and the Drake Passage ................................................................................ 11
ACC and Africa ...................................................................................................... 12
deep circulation. ..................................................................................................... 12
ITS CLIMATOLOGIC IMPACT: ........................................................................................ 13
CONCLUSION................................................................................................................ 14
BIBLIOGRAPHY ........................................................................................................... 15
INTRODUCTION
.
There are two major currents in the Antarctic Ocean: the
anticlockwise Polar Current near the coasts, and the clockwise Antarctic
Circumpolar Current (ACC). This is a Westerly flowing current that
flows around Antarctica powered by Antarctic winds. It flows at the rate
of around 140 million cubic metres of water per second, or the equivalent
of 5000 Amazon rivers.
The Antarctic Circumpolar Current is the most important current in
the Southern Ocean, and the only current that flows completely around
the globe. The ACC, as it encircles the Antarctic continent, flows
eastward through the southern portions of the Atlantic, Indian, and Pacific
Oceans. Edmond Halley, the British astronomer, discovered the ACC
while surveying the region during the Paramore expedition in 1700. Later,
the famous mariners James Cook in 1772-1775, and James Clark Ross in
1839-1843 described the Atlantic Circumpolar Current in their journals.
Cook was the first person to use the term, Southern Ocean, to describe
this area.(DEACON, 1984).The Antarctic Circumpolar Current is very
important for understanding the deep circulation in all the other oceans.
In a first part we will interest in the oceanographic dynamics of the
ACC . Then in a second part we will explain ACC’s role in the global
oceanic circulation.
1)OCEANOGRAPHIC
DYNAMICS OF THE
ANTARCTIC
CIRCUMPOLAR CURRENT
LIMITS:
The ACC is a westerly flowing current that flows around Antarctic
powered by winds. It is made up of two major areas: the Sub Antarctic
area and Antarctic area. Both areas are delimited by hydrologic
boundaries called the Subtropical convergence, Antarctic convergence
and Antarctic divergence.
-THE SUBTROPICAL CONVERGENCE
The Subtropical convergence, located between 38°S and 42°S,
corresponds to a tightening up of the isotherms between 15°C and 12°C.
This limit, well marked in South Atlantic, Indian Ocean and West Pacific,
appears less obvious in the East part of Pacific Ocean.
This area surrounding the Antarctic Ocean is the result of the
meeting of superficial tropical waters, driven to Southeast by subtropical
anticyclones, and superficial Antarctic waters, driven to East-Northeast
by the ACC.
-THE ANTARCTIC CONVERGENCE
The Antarctic convergence corresponds to a second tightening up
of the isotherms, between 2°C and 5°C. More distinct than the
Subtropical convergence, its location varies according to seasons and
places.
This convergence is not only the result of a currents meeting, but
also follows from the uprising of deep Atlantic waters, in its Southward
movement, caused by Antarctic deepwater masses. Even if the Antarctic
convergence marks a hydrologic boundary, it doesn’t represent a barrier
for the ACC.
-THE ANTARCTIC DIVERGENCE
The Antarctic divergence, located between 63°S and 65°S, is the
result of the divergence between the ACC and Polar Current; it also
corresponds to thermohaline circulation caused by different water masses
meeting in this area (TCHERNIA, p65-66-67).
Figure 1: The ACC and its limits.
GENERATION:
Despite its relatively slow eastward flow of less than 20 cm s-1 in
regions between the fronts, the ACC transports more water than any other
current (KLINCK and NOWLIN, 2001).
The ACC extends from the sea surface to depths of 2000-4000 m
and can be as wide as 2000 km. This tremendous cross-sectional area
allows for the current's large volume transport.
The Antarctic Circumpolar Current's eastward flow is driven by
strong westerly winds. The average wind speed between 40°S and 60°S
is 15 to 24 knots with strongest winds typically bewteen 45°S and 55°S.
Historically, the ACC has been referred to as the 'West Wind Drift'
because the prevailing westerly wind and current are both eastward.
Drained by boundaries described above, the ACC is generated by
two phenomenons; it is a wind driven and a geostrophic current at once.
-A WIND DRIVEN CURRENT
Firstly, we must interest in wind blowing across the sea surface.
South part of each subtropical anticyclone generates
strong regular winds blowing eastward
in Antarctic Ocean. Moreover, some depressions
are drained by the low-pressure ditch from
Northwest to Southeast, and thus are added to
the winds quoted before.
Figure 2:Eckman Layer
These winds transfer horizontal momentum to the sea surface and
drag water in its direction and thus explain the movement of surface
waters (TCHERNIA, p54-55).
-A GEOSTROPHIC CURRENT
Secondly, below the wind driven layer, the density distribution is
such that, in general, the horizontal pressure gradient force and the
Coriolis force balance and geostrophic equilibrium is maintained.
As this is in the Southern Hemisphere, Ekman transport is to the
left of the wind, and the sea surface slopes down towards the Antarctic
continent. It generates a geostrophic slope current to the East. Flowing in
the same direction than the wind, it explains the extension of the ACC to
greater depths than the surface wind driven layer(Ocean Ciculation,
p151).
HYDROLOGY:
The ACC, having an effect on nearly the entire water column, is
made up of four different water masses:
-Sub Antarctic Mode Water (SAMW)
-Antarctic Intermediate Water (AAIW)
-Circumpolar Deep Water (CDW)
-Antarctic Bottom Water (AABW)
-SUB ANTARCTIC MODE WATER
The SAMW, located only in the Sub Antarctic area, corresponds to
a several hundreds meters thick layer resulting from the mixing of
Antarctic superficial waters and waters of tropical origin.
-ANTARCTIC INTERMEDIATE WATER
The surface waters, met in the Antarctic area, are relatively fresh but
cold. When they sink at the level of the Antarctic convergence, they
define characteristics of the AAIW. Its thickness is about 100 to 250
meters.
-CIRCUMPOLAR DEEP WATER
The CDW is a mixture of deep water from all oceans. Its upper
branch contains oxygen-poor water from all oceans. The lower (deeper)
branch contains a core of high salinity water from the Atlantic, including
contributions from the North Atlantic deep water mixed with salty
Mediterranean Sea water. The CDW can be met from hundreds meters to
3000 meters.
-ANTARCTIC BOTTOM WATER
The coldest, saltiest water is produced on the continental shelf
around Antarctica in winter, mostly from the shallow Weddell and Ross
seas. The cold salty water drains from the shelves, entrains some deep
water, and spreads out along the sea floor; it forms the AABW
(STEWART, s.p.).
This dense water then seeps into all the ocean basins.(see figure 3)
Figure 3: Antarctic schematic hydrology.
2)ANTARCTIC
CURRENT’S
ROLE
IN
OCEANIC
CIRCULATION
MIXING CURRENT
As the different water masses circulate around Antarctica, they mix
with other water masses with
similar density. In a sense, the
ACC is a giant “mix-master”
taking water from each ocean,
mixing it with water from other
oceans, and then redistributing it
backs to each ocean.
Figure 4: Water masses mixing
In surface layers, the direct effect of the wind stress, combined with
Coriolis force, leads to a northward component of flow feeding each
ocean. Moreover, the ACC meet some continental obstacles that remove a
part of it.(TCHERNIA, p 58)
Figure 5 : ACC and others
currents
ACC AND AUSTRALIA
The obstacle represented by Australia and Tasmania also sends
back a part of waters to the north. It mixes there with the Australian
current.(see figure 6)
Figure 6: The ACC and Australia.
ACC AND THE DRAKE PASSAGE
The west and east South American coasts send back a part of the
current to the north. It contributes to the Hambolt current all along
Chilean and Peruvian coasts, and it forms the Falkland current all along
Argentine coasts.(see figure 7)
Figure 7: The ACC and the Drake Passage.
ACC AND AFRICA
The peaks current, all along South African coasts, deflect a part of
the ACC to the north to form the Benguela current.(see figure 8)
Figure 8: The ACC and Africa.
DEEP CIRCULATION.
Because the ACC extend all the way in the water column,
topographic steering influences it. As the current crosses ridges such as
the Kerguelen Plateau, the Pacific-Antarctic ridge, and the Drake Passage,
the ridges deflect it. It explains that Antarctic bottom water, too dense to
cross through the Drake Passage, is not considered as real circumpolar
water. It also allows redistributing some circumpolar deep water, less
dense and thus less deep, in all oceans.(see figure 9)
Figure 9:Antarctic deep circulation.
ITS CLIMATOLOGIC IMPACT:
Because it connects the three great ocean basins, allowing
exchanges between each ocean, the ACC is an important factor in world’s
climate. It controls it in three ways:
-By connecting the world’s oceans, the ACC redistributes heat
and other properties influencing the patterns of temperature and rainfall.
-The vertical movement of water, caused by Antarctic freezing
during the winter and warming during summer, controls the renewal of
deep water in the world’s oceans.
-There is an exchange of gases, such as oxygen and carbon dioxide,
with the atmosphere at the sea surface. The ocean contains 50 times more
carbon than the atmosphere, so the rate at which the Southern Ocean
absorbs carbon dioxide can directly affect climate change.
(http://www.parks.tas.gov.au/fahan_mi_shipwrecks/infohut/acc.htm)
Satellites have detected some differences of more or less 2-3°C in
the ACC waters. It forms two warmer and two colder pool, each one
measuring several thousands of kilometres long and thousands of meters
deep. These regions, resulting from ocean/atmosphere exchanges, move
with the ACC, surrounding the globe in southern latitudes in 8-9 years.
This phenomenon has been called the Antarctic Circumpolar
Waves and is believed to have a considerable on influence the weather
patterns in southern Australia, South America and southern Africa.
(http://www.parks.tas.gov.au/fahan_mi_shipwrecks/infohut/acc.htm)
CONCLUSION
The ACC is the most important and the biggest current all around
the world. It links all the oceans, and mixes all waters to redistribute them
in each ocean.
In recent years, other areas such as sections of the ACC south of
Tasmania and New Zealand have also been examined closely.
Observations have revealed a mean ACC transport of 100-150
Sverdrups(SARUKHANYAN,1985).
Its size and the heat exchanges taking place in it have an important
influence on climates that is being studied by scientists.
The Antarctic Circumpolar Current is a critical component of the
'Great Ocean Conveyor Belt.'
BIBLIOGRAPHY
DEACON, G., 1984: The Anarctic circumpolar ocean. Cambridge
University Press, p 180.
Ocean Circulation, 1989. The Open University, p 60 & 151-154.
NOWLIN, W. D., Jr., and J. M. KLINCK, 1986: The physics of the
Antarctic Circumpolar Current. Rev. Geophys., 24, p469-491.
SARUKHANYAN, E.I., 1985: Structure and Variability of the Antarctic
Circumpolar Current, NSF translation of Russian text, Oxonian
Press/New Delhi, p 108.
STEWART R.H., s.d.. Introduction to Physical Oceanography, p 230232.
TCHERNIA P., s.d.. Oceanographie Regionale, Ecole nationale
superieure de techniques avancees, p 45-85.
http://www.ocean.tamu.edu/education/common/notes/PDF_Files/book_p
df.file.html
http://www.parks.tas.gov.au/fahan_mi_shipwrecks/infohut/acc.htm
http://topex-www.jpl.nasa.gov/science/invest-white.html
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