| Abstract: An expression is derived for the surface salt input
needed to induce complete convective overturning of a polar water column
consisting of 1) a layer of sea ice, 2) a freezing temperature mixed layer,
3) a pycnocline with linearly varying temperature and salinity, and 4)
deep water with fixed temperature and salinity. This quantity has been
termed the bulk stability by Martinson. The bulk stability is found
to consist of three components. The first two make up Martinson's salt
deficit and are the salt input needed to increase the density of the
mixed layer and the pycnocline layer to that of the deep water (the mixed
layer stability and pycnocline layer stability, respectively).
The third component is Martinson's thermal barrier: the potential
for pycnocline heat to melt ice, reducing the surface salinity. It is found
that when the pycnocline density gradient due to temperature offsets more
than one half of that due to salinity, the pycnocline layer stability is
negative. Consequently, it is possible for a stably stratified water column
to have zero or negative bulk stability. |