Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 140

temperature over the continent in summer is around the 250 hPa level
at 50 8C; in winter the atmosphere is much less stratified, and the coldest
temperatures with values between 80 and 90 8C are found at the 30 hPa
level. The zonally averaged eastward wind component is, for all altitudes and
seasons, weak over Antarctica. The surface winds are, to a great extent,
decoupled from the wind aloft due to the strong surface inversion. Dome C
(now called Place Concordia), located on the top of a dome at an altitude of
3280m, measured a mean annual wind speed of less than 3m s 1 ; such a low
value is found nowhere else on Earth for a freely exposed station at that
altitude.
Sea ice
Sea ice surrounding Antarctica displays a large annual variation in coverage.
The maximum is normally found in late winter/early spring with 19 10 6 km 2 ,
while the minimum occurs in late summer with only 3.5 10 6 km 2 . The
presence or absence of sea ice strongly influences the energy transfer between
the ocean and atmosphere and is of great importance for understanding the
climate. Systematic observations of sea ice go back some 30 years, and became
possible only by remote sensing from satellite. Passive microwave sensors,
not affected by darkness or clouds, are especially useful. As the Antarctic
continent is surrounded by oceans, the ice drifts steadily north where it melts.
The ice pack consists mostly of first-year ice which is thin ( < 1m), whereas in
the Arctic a large amount of multi-year ice is present. These differences have
been known for a long time owing to whaling activities. For example, inMarch
1840, Charles Wilkes wrote in a letter to Sir James Clark Ross: ''The ice of the
Antarctic is of totally different character from that of the Arctic.'' Most long-
term trend studies have found a decrease in ice extent (Ackerley et al. 2003 ).
However, new studies using satellite data and in part new algorithms to
determine the ice amount came to the opposite conclusion, that is that the
amount of sea ice is increasing (Parkinson 2002 ). This increase has been
verified for the last two decades and cannot be doubted (Zwally et al. 2002 ).
The very strong katabatic winds can drive the ice away from the coastline,
and coastal polynyas (open water areas) can occur, even in midwinter. The
energy exchange over these polynyas can be two orders of magnitude higher
than the heat conduction through sea ice, because cold air, originating from
the interior of Antarctica, is moved over this ''warm'' water with a high
velocity. These are the areas where large amounts of new ice are formed.
During this process, brine is released, which increases the density of the
water. This water sinks, forming the Antarctic bottom water. In fact, 75% of
the bottom water in all oceans originates from Antarctica. The production
areas along the Antarctic coast are limited to confluence zones, such as
Ad´lie Land (Mertz polynya) and Terra Nova Bay (polynya with the same