Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 153

the heat loss to the atmosphere initiates ice growth again. This freeze and melt
cycle limits the thermodynamic ice growth and consequently the ice thickness of
non-ridged ice to less than a metre.
Under certain conditions polynyas can be formed in the open ocean. By a
combination of wind forcing in the Antarctic Divergence, vertical motion due to
topographic features and vertical mixing by eddies and tides, the vertical heat
transport from the Circumpolar Deep Water into the surface mixed layer can be
enhanced to such an extent that the ice melts completely and open water appears
away from the coast. Such conditions occurred in the Atlantic sector of the Southern
Ocean at Maud Rise where an open ocean polynya was observed by satellite images
around 40 years ago. The large

existed for several years and
caused anomalous open ocean deep convection. However, since then, neither a large
Weddell Polynya nor deep convection has been observed again in the vicinity of
Maud Rise, but reduced sea ice conditions do occur quite often in that area.

' Weddell Polynya '

Bottom water formation

The most important contribution to the global vertical overturning circulation
from the Southern Ocean is the bottom water formation. Antarctic Bottom Water is
the deepest water mass in the global ocean. It spreads up into the North Atlantic
beyond 50 N.
Bottom Water formation occurs in the Weddell Sea, the Ross Sea and near
Adélie Land. The Weddell Sea is the most important source since 60% of bottom
water is produced there. Bottom Water is formed by dense water sinking at the
continental slope into the deep sea. This dense water forms initially on the shelf
where, in polynyas, huge amounts of ice form, increasing the local salinity and thus
the water density. This high salinity shelf water is actually dense enough to descend
the continental slope into the deep ocean, but the action of the rotating Earth tends
to keep it on the shelf. Only topographic features such as canyons or eddy-related
cross-frontal processes can force the dense water flow to leave the shelf and descend.

Once it has reached the continental slope, the bottom water is forced to flow along

the slope and sink rather slowly. In the Weddell Sea the water spilling down the
slope from the southern shelf off the Filchner/Ronne Ice shelves follows the
continental slope of the Antarctic Peninsula as a plume towards the northern tip,
where it reaches depths below 4500m and fills up the Weddell Basin. However, this

is too deep to leave the basin since the gaps in the northern ridges are at shallower
depths. Therefore, the newly formed water has either to mix with adjacent water
when flowing along the slope or mix vertically in to Weddell Sea Deep Water in the

basin. Weddell Sea Deep Water layers are shallow enough to leave the Weddell basin
and form Antarctic Bottom Water.