Antarctic Circumpolar Current - Dynamics

Dynamics

The Circumpolar Current is driven by the strong westerly winds which are found in the latitudes of the Southern Ocean.

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 offset 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 Ekman 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.