Polar water column stability

An expression is derived for the surface salt input needed to induce comple te connective overturning of a polar water column consisting of 1) a layer of sea ice, 2) a freezing temperature mixed layer, 3) a pycnocline with lin early varying temperature and salinity, and 4) deep water with fixed temper ature and salinity. This quantity has been termed the bulk stability by Mar tinson. The bulk stability is found to consist of three components. The fir st two make up Martinson's salt deficit and are the salt input needed to in crease the density of the mixed layer and the pycnocline layer to that of t he deep water (the mired layer stability and pycnocline layer stability, re spectively). The third component is Martinson's thermal barrier: the potent ial fur pycnocline heat to melt ice, reducing the surface salinity. It is f ound that when the pycnocline density gradient due to temperature offsets m ore than one half of that due to salinity, the pycnocline lover stability i s negative, Consequently. it is possible for a stably stratified water colu mn to have zero or negative bulk stability.