The Weddell Polynya, a large expanse of water that originated over Maud Rise (a bathymetric protrusion centred near 64°30′S, 3°E) and remained open during winter in the late 1970s, may have manifested a mode of deep ocean convection where despite large heat loss at the surface, sustained heat transport from below prevents lasting ice formation. In a different dominant mode (the present one), sea ice forms early in the winter and subsequently provides a thermal barrier that quickly quells incipient deep convection, thus preventing wholesale destruction of the ice cover. A possible mechanism for overcoming the thermal barrier is thermobaricity, the pressure dependence of the thermal expansion factor for seawater. An idealized, two-layer version of actual temperature and salinity profiles from the Weddell illustrates that thermobaric mixing can persist for extended periods in an ice-covered ocean, provided realistic melt rates (controlled by salt exchange at the ice/ocean interface) are specified. This furnishes a possible explanation for transient winter polynyas sometime observed in the ice-covered Southern Ocean. Thermobaricity may provide a trigger for widespread convection with possible climate impact.

The ocean/sea-ice interaction of the Antarctic open ocean region is described through a one-dimensional model. The model includes processes responsible for maintaining stability in this marginally stable region and reveals the importance of the various processes controlling deep water formation/ventilation and sea-ice thickness and their sensitivity to climate change. This information is used to estimate changes, as they impact water column stability, induced by glacial conditions. Increased stability is conducive to greater ice cover and less deep water formation/ventilation; decreased stability conducive to the opposite.

Sensitivity studies show that the system is destabilised given: (1) shallowing of the pycnocline (induced by increased gyre vigor); (2) decrease in the ratio of heat to salt through the pycnocline (induced by introducing a colder and/or saltier deep water or by increasing the salinity of the surface water); (3) decreased pycnocline strength (induced by a fresher deep water or saltier surface water) and (4) increased atmospheric heat loss. Most of the assumed glacial conditions drive the system toward destabilisation, but the critical effect of changes in NADW characteristics depends strongly on the temperature and salinity of the replacement water. The importance of this deep water influence is evident today—as little as 3Wm−2 in the upper ocean heat balance or an additional 15 cm of ice growth is sufficient to overturn the water column in some regions.