Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 142

RADARSAT was turned around to look to the south so that it could obtain
accurate altitudes of all of Antarctica. In 2000, the mission was repeated, this
time without turning the satellite around, hence not obtaining data for the
center of Antarctica. A new satellite with laser altimetry has recently been
launched. Highly accurate data are expected to be obtained, and when
repeated, they will give elevation changes from which the mass balance of
Antarctica can be accurately calculated.
5.3 Upper air circulation and wind
The interaction between the high-altitude ice mass that makes up Antarctica, the
surrounding ice shelf and ocean, and the Southern Hemisphere general circula-
tion creates a complex climatology of airflows in the region. Detailed and
accurate information about the upper atmosphere and its processes over the
Antarctic region is limited due to lack of long-term data, lack of radiosonde
data, and difficulties in interpreting information from satellites that are normally
oriented toward the mid-latitudes (Simmonds 1998 ).
Figure 5.5a illustrates a simple model of the overall circulation structure over
the continent (King and Turner 1997 ). Inflows from the mid-latitudes occur in
the upper troposphere and lower stratosphere, creating confluence and conver-
gence, and bringing warmer air to the polar region. This air subsides very slowly
over the land mass. The subsidence is centered over the East Antarctic Plateau,
the region of highest altitude. Cyclonic vorticity associated with the subsiding
air creates a westerly vortex in the atmosphere above 500 hPa (Figure 5.5b ). The
climatological center of the vortex tilts poleward with height, being located over
the Ross Sea in winter at 500 hPa and over the South Pole at 300 hPa. On a day-
to-day basis, cyclonic weather systems can disrupt the circulation pattern,
allowing irregular meridional transport of moisture and energy from the north
(K¨nig-Langlo et al. 1998 ). This upper air circulation structure acts as a com-
pensation for, and a controller for, surface airflow, especially the katabatic
winds. At the surface, below about 2 km, the vortex circulation is easterly
(Figure 5.5b ).
In late winter, when the Sun is in the Northern Hemisphere, and the polar-
equatorial and land-ocean temperature gradients the strongest, upper air circula-
tion around Antarctica is the strongest for the year. The polar night jet (PNJ)
( > 75 m s 1 at 10 hPa) dominates the circulation above the tropopause between
50 and 608 S (see Section 5.7 ). The long polar night, and the low temperature
associated with the ''coreless winter'' strengthen both the upper air and near-
surface circulation (Bromwich and Parish 1998 ).
In summer, the westerly circulation is weaker, especially above 300 hPa, and
becomes stratospheric easterlies above 50 hPa. In this season, the tropopause
drops in height from about 100 hPa at 508 S to 300 hPa over the continent. The