Formation of Polynyas
To create a polynya, one
has to remove sea ice from a region and/or prevent it from forming.
The first of these mechanisms
(removal of sea ice) is the primary way that polynyas form close
to the coastline. Strong surface winds blow the ice off-shore,
opening up the surface waters and allowing the formation of more
sea ice, indeed coastal polynyas have been likened to 'sea ice
factories'.
The second of these mechanisms
(prevention of sea ice formation) is perhaps more interesting.
How can one keep the sea surface temperature above freezing thereby
preventing the formation of sea ice? Remember, we are talking
about the depth of the Antarctic winter: the atmosphere is very
cold and there is little or no sunlight! The answer is the deep
ocean. There are regions where the water at some depth below the
surface is actually warmer than the water at the surface. It is
also saltier and denser and therefore not easily brought to the
surface. However, if the surface waters can be cooled and made
more salty (by losing heat and moisture to the atmosphere) there
is the possibility that the warm deep water can be exchanged with
colder surface waters through a process known as convection. This
process is thought to be the mechanism by which open ocean polynyas
form and are maintained.
The presence of open ocean
polynyas are therefore important as they signal that ocean is
undergoing convective overturning.