Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 145

pressure in the CPT (Bromwich and Parish 1998 ). These centers, reaching
around 980 hPa, exist between 0 and 408 E longitude (south of Southern
Africa), 80 to 1008 E longitude (Southern Indian Ocean), and 160-1708W
longitude (southeast of New Zealand). The physical barriers created by the
continent to the south, the strength of the pressure gradients, plus the open
ocean to the north and east, encourage the strong zonal flow that defines the
mid-latitude westerlies in the Southern Hemisphere.
The climatic picture shown in Figure 5.6 obscures the strong pressure varia-
tions that occur within the CPT (Simmonds 1998 ). Standard deviations in
pressure can vary by more than 10 hPa over periodicities that exceed 10 days.
Over shorter time periods, pressure may change by up to 25 hPa. One major
result is the creation of a large number of intense synoptic-scale low pressure
systems (see section 5.5 ). The seasonal variations in pressure in the CPT are
controlled by the Antarctic Semi-Annual Oscillation (AAO, see Section 2.7 )in
pressure. Spatial variations in the CPT are also linked to sea ice extent. The
coastal locations closest to the CPT, such as Dumont d'Urville, thus have the
greatest pressure variations associated with the AAO (K¨nig-Langlo et al.
1998 ). A pressure minimum occurs in October when the CPT is also the furthest
south in location, and sea ice extent is at maximum. A secondary minimum in
pressure, and a secondary maximum in latitude, occurs in March and April.
North of 608 latitude, the influence of AAO on pressure variations diminishes
considerably, and the phase reverses. Table 5.1 lists some reasons for the
development and structure of the AAO, and its major influences.
Over the continent, seasonal surface pressure variations are greatest near 908 S
(Bromwich and Parish 1998 ). Here, in August and September, average pressure
is 15-18 hPa lower than in January. Seasonal variations become much less close
to the coast. The terrain influences the spatial distribution of surface pressure

Table 5.1 Reasons for the development of the AAO around Antarctica, and its major influences (after
Simmonds 1998 ; Turner 2004 )

Reasons for development of AAO Major influences on climate by the AAO

Differing annual temperature variations between
land and water

Semi-annual cycle of pressure
Variations in the circumpolar trough

Differences in thermal inertia between land and
water

Mean wind strength and direction
Mean distribution of sea ice

Oceans cool in autumn faster than warm
in spring

Expansion and contraction of circumpolar vortex
Meridional shifts of the Polar Front jet stream

Antarctic coast warms faster in spring than cools
in autumn
Phase of annual temperature cycle differs
between land and water