A Convective Model for the Weddell Polynya

Abstract

Mechanisms are considered which may induct the large (over 105 km2) area of open water, or polynya, which frequently occurs within the Weddell Sea winter sea-ice. We propose that when surface cooling and ice formation decrease the temperature and increase the salinity of the surface water (the latter by salt rejection during ice formation) in a preconditioned area, static instability with intense vertical mixing can occur. The upwelled warm, salty deep water can then supply enough heat to melt the ice, or prohibit its formation, even in the middle of winter. A simple two-level model is derived to test this theory and is found to agree well with observations. The process is found to be irregular due to different times of ice onset from one year to the next, and to a lesser extent from variations in surface heating and cooling. Further, it is shown that unless the freshwater input exactly balances the increased salinity from the overturn each year, the system will either gain or lose salt yearly and eventually stabilize permanently (i.e., attain a steady-state condition). The model is insensitive to short term stochastic variations in surface heat flux or freshwater input rates, but is somewhat sensitive to longer scale variations in the net freshwater input and also to the depth of the pycnocline (i.e., preconditioning). It is suggested that upwelling may raise the pycnocline until convection can occur and the polynya form. The preconditioned area then advects westward with the mean flow. Permanent stability finally is attained but the preconditioned area is advected into the western boundary current of the Weddell subpolar gyre and destroyed. Prior to destruction, topographical features may quantitatively affect both the movement and occurrence of the polynya. Regardless of the preconditioning mechanism, if overturning is responsible for the polynya, this would contribute a minimum of 106 m3 s−1 to the total deep water formation, and constitute the largest area of deep open-ocean convection yet discovered.