Geoscience Reference
In-Depth Information
Box 4.5
Antarctic polynyas: a window into the ocean
We have seen that the temperature gradient between the poles and the tropics
drives the global atmosphere and ocean circulations, but these two parts of the
climate system also interact quite closely. As cold air blows over the ocean the
air will be warmed and moistened while the ocean will be cooled. This cooling can
lead to freezing of the seawater and the development of sea ice. The presence of
sea ice insulates the cold air from the relatively warmer water, thus limiting
the exchange of heat and moisture between the ocean and the atmosphere. While
sea ice formation is favoured by cold air
flowing over the ocean, which is a
common occurrence around the margins of the Antarctic continent, very
strong winds can also push this sea ice away from the Antarctic coast leading to
areas of open water adjacent to the continent. These areas are known as polynyas,
and form in similar locations year after year, where winds tend to be strongest.
One of these locations is Terra Nova Bay in the Ross Sea. Like Cape Denison,
Terra Nova Bay is subject to exceptionally strong katabatic winds. Members of
Scott
s Antarctic expeditions that spent the winter on an island in Terra Nova Bay
named the island Inexpressible Island, because the torment of the strong winds
day after day was so unbearable as to be inexpressible to those that had not
experienced it at
'
first hand. The very cold, strong katabatic winds which blow
across Terra Nova Bay remove signi
cant amounts of heat and moisture from the
ocean. As a result the ocean is cooled, and its salinity increases due to the
evaporation of water. This cold, salty water is denser than the surrounding ocean
water and sinks to form what is known as Antarctic bottom water. This Antarctic
bottom water is part of the global thermohaline circulation, which is responsible
for driving the global ocean circulation. During the late winter of 2009, scientists
from the United States made measurements of the atmosphere and ocean over
the Terra Nova Bay polynya, to understand better the processes acting in this
unique environment. Since
fieldwork are limited to the
more mild Antarctic summer this research was conducted using a remotely
controlled aircraft, known more formally as an unmanned aerial vehicle (UAV).
The UAV carried instruments to measure the atmosphere and to observe the
surface of the polynya. Measurements from the UAV showed white-capped
waves being driven by hurricane force winds and heat and moisture transfer from
the ocean that were as large as the heating that could be expected over a mid-
latitude location in summer. These observations will allow scientists to more
accurately represent the heat and moisture exchange between the ocean and the
atmosphere in climate models, and to better characterise the properties of the
Antarctic bottom water that forms here. British scientists have used a similar
UAV to make measurements over the Weddell Sea during the Antarctic summer
(October to December) of 2007.
flight operations and
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