Back in the 1970s, NASA investigators divided the circumpolar
Southern Ocean sea ice pie into 5 pieces and began to monitor their
sea ice extents and concentrations. The overall ice extent has
increased since then, mainly due to changes in the Ross sector, in
contrast to the widely publicized declines in the Arctic summer ice
minimum. However, the Amundsen and Bellingshausen sector has bucked
the Antarctic trend, with the length of its sea ice season declining
by more than two months. This is comparable to Arctic changes over
the same period and, along the west side of the Antarctic Peninsula,
has been associated with one of the largest rates of regional surface
warming on the planet.
The Amundsen Sea differs from its Ross and Bellingshausen neighbors
by harboring broad, quasi-permanent areas of fast ice, held in place
by myriads of large, grounded icebergs. It experiences prevailing E/
SE winds, less conducive to ice export than the Ross Sea southerlies.
Since the melting point of ice in seawater decreases as pressure
increases, upwelling beneath its deep-rooted ice shelves will be
stronger than in the Bellingshausen Sea, where the ice shelves are
thinner. But like the Bellingshausen, its precipitation is relatively
high, leading to 'snow ice' formation. This occurs when seawater
floods ice floes with negative freeboards, sunk by their snow loads,
with the ocean to atmosphere heat flux essentailly making an end run
around the tattered sea ice blanket.
Is the Amundsen sea ice thinner now in response to increased
upwelling of deep water that has not lost all its heat to melting
glacial ice? Is its snow cover thicker due to increased precipitation
in an enhanced hydrological cycle? Are there more icebergs to ground
the fast ice and also drive cooling and upwelling, now that the
glaciers are moving/calving faster? Is sea ice production and melting
higher or lower because wind strength and directions have changed due
to the deepening of atmospheric lows along the continental margin? We
don't yet know the answers to these and similar questions, in part
because few sea ice measurements have been made prior to NBP09-01
over the large but remote Amundsen continental shelf.
In this context, the sea ice component of O-274 has occupied 20 ice
stations and bagged 90+ m of ice core along N-S and E-W transects,
noting that the amount retrieved for structural analyses is a record
compared to the 3.6 m length of the sea ice team. Their work has been
reliably supported by the RPS MTs and others, and extensively
photographed and videotaped, including provocative pictures of the
bottoms of sea ice floes. Along with visual and camera logging of
surface conditions underway, the sea ice measuring, sampling and
subsequent modeling will be compared with satellite records of the
sea ice cover, and with data from three deployed ice mass balance
buoys currently reporting to collaborators on the home front. When
all is said and done, we should have a much better understanding of
the role of sea ice in the Amundsen Sea's deep water heat sink.
Macrophotographs of snowflakes were taken during 8 of the 12 short
snow showers that punctuated the otherwise sunny summer weather
during the first 6 weeks of the cruise. These photographs are being
analyzed to determine the size distribution and crystal habits of
falling snow, to complement records of the timing and intensity of
snowfall events measured with photoelectric particle counters mounted
on the ship's ice tower. The timing and relative intensity of
snowfall measured on the ship during cruises NBP07-02 and NBP07-09
are well correlated with the timing and relative intensity of
precipitation forecast by the ECMWF and some other weather models. To
correlate the absolute magnitude of documented precipitation events
with forecasts and reanalyses, one must know the size distribution
and water equivalent mass contained in the falling crystals. Most
snow observed during the warm summer snowstorms consisted of rimed
dendrites, with more plates, sector plates and rare hollow columns
closer to the continent, and one snowfall event composed entirely of
ice needles up to 3.5 mm in length.
Work on the continental shelf concluded near the end of this week,
and for mile after mile on and north of the shelf we weaved our way
through what may be the largest iceberg graveyard in the Southern
Ocean. Their common presence in this deep region suggests a large
area of weak or waffling currents. The varied features on overturned
bergs provided a crash course on what the bottom surfaces of melting
ice shelves probably look like.
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