Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ocean acidification wikipedia , lookup
Indian Ocean wikipedia , lookup
Anoxic event wikipedia , lookup
Marine pollution wikipedia , lookup
Marine habitats wikipedia , lookup
Arctic Ocean wikipedia , lookup
Effects of global warming on oceans wikipedia , lookup
Ecosystem of the North Pacific Subtropical Gyre wikipedia , lookup
History of research ships wikipedia , lookup
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 area