Antarctic Sea Ice and Polynyas
Kent Moore , Department of Physics ,University
of Toronto,
Keith Alverson , PAGES, International Project Office,
and
Ian Renfrew , British Antarctic Survey
July 22 1999
What's new:
jump to high resolution infra-red satellite
images of the 1999 Cosmonaut Sea Polynya
jump to time series of embayment/polynya area
during July 1999
The waxing and waning of
sea ice cover off Antarctica is
one of the most striking examples of the dramatic transitions
in our physical environment that is brought about by the earth's
seasonal cycle. At its minimum in February,
the areal extent of the ice cover is approximately 3.5 million
square kilometers. Many sections of the Antarctic coastline are
ice free. At its maximum in September,
the areal extent is approximately 19 million square kilometers
and the entire continent is surrounded by a thick ring of sea
ice. The seasonal change in sea cover in Antarctica is much larger
that than that in the Arctic where the presence of land surrounding
the Arctic Ocean limits the growth of ice.
In the fall and winter within
the ice pack, large areas of open water known as polynyas are
sometimes observed. Polynyas are of great biological and physical
interest. These are areas where sea mammals have convenient access
to the atmosphere and where birds have easy access to the ocean.
These are also regions where the atmosphere and ocean can interact
without the usual insulating intermediary -sea ice. In polynyas,
the sea surface temperature is, by definition, above freezing
(approximately -1.8 deg C for sea water). In the depths of the
Antarctic winter, surface temperatures over the ice pack are perhaps
20 or 30 degrees colder. In such a situation, one would expect
a vigorous transfer of heat and moisture from the relatively warm
ocean to the very cold and dry atmosphere. This results in a warming
and moistening of the atmosphere and a cooling and salinization
of the ocean. This often results in the formation of interesting
clouds
over the polynya .
There are two basic types
of polynyas and two different formation
mechanisms. Those that form close to the coastline are called
coastal polynyas and those that develop deep in the ice pack far
from shore are called open ocean polynyas.
Our ability to monitor ice
conditions deep in the Antarctic ice pack has only existed since
1972 when NASA's Nimbus 5 was launched with the first spaceborne
passive microwave radiometer- an instrument able to distinguish
between open water and sea ice. The successors to this remarkable
instrument are used to provide daily global retreivals of sea ice concentration and are used on this
site to monitor real time changes in the sea ice cover off Antarctica.
The most dramatic example
of an open ocean polynya occurred during the mid-1970's when a
large region of the Weddell Sea
remained ice free during each of three winters.
We are currently monitoring
anomalous sea ice conditions in the Cosmonaut
Sea and have observed the formation an open ocean polynya.