Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 149

Figure 5.10 The Antarctic Circumpolar Current as derived

from a numerical model (FRAM). (Credit: Southampton
Oceanography Centre)

branches which are related to the oceanic fronts.
The most important branches are the Subtropical,
the Subantarctic and the Polar Fronts which
comprise 80% of the total transport.
The Antarctic Circumpolar Current is forced
along by the West Wind Drift which accelerates
the surface water particles by friction. Due to the
west wind increasing from north to south and
decreasing then towards the Antarctic coast, near
surface water transport is driven in the north by the West Wind Drift but in the
south by the east wind belt. This change in transport with latitude gives rise to
convergences and divergences. Convergence produces elevation of sea level,
divergences a depression. Therefore in the Southern Ocean, the sea surface is
inclined from north to south forming again a pressure gradient which drives, under
the in uence of the rotating Earth, a current to the east which spreads over the

complete water column. However, ocean strati cation acts through the pressure

gradient related to isotherms and isohalines ascending to the south and decreases the
currents towards the bottom.
The transfer of energy from the directly wind driven surface layers to the deep
ocean is achieved by an intense eddy field. These eddies which are essential for the

Antarctic Circumpolar Current dynamics, provide a heat transport from the north
to the south across the Antarctic Circumpolar Current. This cross-frontal heat
transport is an essential part of the heat budget of the Southern Ocean and plays an
important role in determining the sea ice thickness.

Subpolar gyres

South of the Antarctic Circumpolar Current clockwise currents in the subpolar
gyres transport Circumpolar Deep Water to the south in the vicinity of the water
mass formation areas. The gyres are mainly wind-driven current systems related
to the transition from the west winds in the north to the east winds in the south.
The main subpolar gyres are the Weddell gyre, the Ross gyre and the Kerguelen gyre.
The gyres carry source water masses to the south and newly formed water masses
to the north. Their zonal structures are strongly determined by underwater