On the formation of ice on deep weakly stratified water

Making use of the simplest possible model, we analyse the formation of ice on top of a weakly stratified ocean. Our interest is concentrated on the consumption of freshwater associated with ice formation and the dynamics of the system when the cooling continues beyond the point where further ice formation would destroy the stability of the system. We find that after an initial stage of ice formation the system will not overturn but go into a stage of development which may be called “freeze melting”. This stage is characterized by increasing mixed layer depth, slowly decreasing ice thickness and small but finite stability. If the freeze melting continues for a sufficiently long time, considerably longer than required for the initial ice formation, the ice cover may be removed altogether, whereupon the stratification overturns and the fresh-water in the top layer gets lost. It is suggested that if this happens one year it will contribute to preconditioning the system for ice-free conditions the following year. An essential condition for the analyses, which may be put in question, is the presence of at least some wind generated turbulence and that competing mixing processes, e.g., associated with cabbeling do not become dominating. Observations from the Wedell sea give support to the conclusion that late winter conditions in this area may be well described in terms of a freeze melting stage of development.