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. It keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.
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.
The current creates two Antarctic gyres.
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, which is defined as the latitude at which a subsurface salinity minimum or a thick layer of unstratified Subantarctic Mode Water first appears. Still further south 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.
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 directly, 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. Such theories link the magnitude of the Circumpolar Current with the global thermohaline circulation, particularly the properties of the North Atlantic.
Alternatively, ocean eddies, the oceanic equivalent of atmospheric storms, or the large scale meanders of the Circumpolar Current may directly transport momentum downwards in the water column. This is because such flows can produce a net southward flow in the troughs and a net northward flow over the ridges without requiring any transformation of density. In practice both the thermohaline and the eddy/meander mechanisms are likely to be important.
The current flows at a rate of about four km per hour. Recent studies have indicated that the Antarctic Circumpolar Current varies 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.