Evolution of the southern ocean winter mixed layer and sea ice: Open ocean deepwater formation and ventilation

Abstract

An analytical model has been developed to describe the nature of the winter ocean/sea ice interaction in the southern ocean, to determine the principal processes responsible for maintaining stability in this marginally stable region, and to predict the response to changes in the external forcing and initial conditions. The model accurately describes the sparse winter observations and the general temporal evolution of the ice, mixed layer depth, and salinity for the period following the elimination of the seasonal pycnocline by rapid fall ice growth. A scale analysis and sensitivity study reveal the system parameter dependence. This indicates that the magnitude of entrainment is proportional to Fst/(h0∇S), where Fs is the external salt flux; h0, the depth of the winter mixed layer; and ∇S, the salinity gradient through the pycnocline. Thermodynamic ice growth reflects predominantly the competition between the oceanic diffusive heat gain across the base of the mixed layer and the loss to the atmosphere through the ice and leads. A strong thermal gradient across the pycnocline leads to a significant diffusive flux which balances most of the heat lost to the atmosphere. This limits the amount of ice growth possible and plays the principal role in maintaining the system stability, since the ice growth is required for destabilization through haline rejection during the growth process. The system is also sensitive to the large ratio of heat to salt in the pycnocline. This introduces a negative feedback where increasing ice growth induces entrainment by the associated salt flux, which in turn drives an entrainment heat flux which compensates for heat lost to the atmosphere. This moderates the ability to grow ice and drive further entrainment. The absolute strength of the salinity gradient, on the other hand, dictates the effectiveness which a same magnitude external salt flux will have in driving entrainment. The effectiveness with which changes in these parameters (as well as several other external parameters) can influence the southern ocean stability is explicitly evaluated through the sensitivity study and analysis.