Antarctic Circumpolar Current
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The Antarctic Circumpolar Current (ACC) is an ocean current that flows from west to east around Antarctica. An alternate name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean. The ACC has been known to sailors for many years; Jack London's story "Make Westing" poignantly illustrated the difficulty it caused for mariners seeking to round Cape Horn on the clipper ship route between New York and California.
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[edit] Structure
The ACC connects the Atlantic, Pacific and Indian Ocean basins, and as such serves as a principal pathway of exchange between these basins. The current is strongly constrained by landform and bathymetric features. Starting at South America, it flows through the Drake Passage between South America and the Antarctic Peninsula and then is split by the Scotia Arc to the east, with a shallow warm branch flowing to the north in the Falklands Current and a deeper branch passing through the Arc more to the east before also turning to the north. Passing through the Indian Ocean, the current is split by the Kerguelen Plateau in the Indian Ocean, with most of the transport passing to the north. South of New Zealand, it follows the contours of the Campbell Plateau, first deflecting far to the south and then moving northward again. Deflection is also seen as it passes over the mid-ocean ridge in the Southeast Pacific.
The current consists of a number of fronts. The northern boundary of the ACC is defined by the Subtropical Front. This marks the boundary between warm, salty subtropical waters (generally with a salinity of greater than 34.9 parts per thousand) and fresher, cooler subpolar waters. Moving southward we find the Subantarctic Front, along which much of the ACC transport is carried. Further south still lies the Polar Front, which is marked by a transition to very cold, relatively fresh, Antarctic Surface Water at the surface. Further south still is the Southern Boundary front, which is determined as the point where very dense abyssal waters upwell to within a few hundred meters of the surface. The bulk of the transport is carried in the middle two fronts. The total transport of the ACC at Drake Passage is estimated to be around 135 Sverdrups (135,000,000 m³/s), or about 135 times the transport of all the world's rivers combined. There is a relatively small addition of flow in the Indian Ocean, with the transport south of Tasmania reaching around 147 Sv, at which point the current is probably the largest on the planet.
[edit] Dynamics
There is general agreement that the large transport of the Circumpolar Current is linked to the strong westerly winds which are found in the Southern Ocean and that these winds blow over a band of open latitudes. In latitudes where there are continents, winds blowing on light surface water can simply pile up light water against these continents. But in the Southern Ocean, the momentum imparted to the surface waters cannot be balanced in this way.
Different theories of the Circumpolar Current balance the momentum imparted by the winds in different ways. The increasing eastward momentum imparted by the winds causes water parcels to drift outwards from the axis of the earth's rotation (in other words, northward) as a result of the Coriolis force. This northward transport is balanced by a southward, pressure-driven flow below the depths of the major ridge systems. Some theories connect these flows, implying that there is significant upwelling of dense deep waters within the Southern Ocean, transformation of these waters into light surface waters, and a transformation of waters in the opposite direction to the north. This theory links the magnitude of the Circumpolar Current with the global thermohaline circulation, particularly the properties of the North Atlantic. A contrary view, however, suggests that eddies, the oceanic equivalent of atmospheric storms, transport momentum downwards in the water column, so that while the deep waters are set into motion, this motion simply involves southward flow in troughs and northward flow over ridges with no transformation into light surface waters required.
Recent study has indicated that the Antarctic Circumpolar Current may vary with time. Evidence of this is the Antarctic Circumpolar Wave, a periodic oscillation that affects the climate of much of the southern hemisphere. There is also the Antarctic oscillation, which involves changes in the location and strength of Antarctic winds. Trends in the Antarctic Oscillation have been hypothesized to account for an increase in the transport of the Circumpolar Current over the past two decades.
[edit] Formation
The Antarctic Circumpolar Current formed during the Miocene epoch, when the pieces of the former supercontinent Gondwana which would become Antarctica and South America finally separated enough for the Drake Passage to form about 23Mya. As Antarctica was isolated from warmer waters it became progressively cooler, and glaciers began to form on the formerly forested continent.
[edit] References
Orsi, A.H., T. Whitworth and W.D. Nowling, 1995, On the meridional extent and fronts of the Antarctic Circumpolar Current, Deep Sea Research, Series I, 42, 641-673.
