Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 146

changes, with diabatic cooling rates creating the strongest pressure differences
on steeper coastal slopes.
5.5 Cyclogenesis and cyclonicity
The CPT around Antarctica is one of the most active cyclonic regimes on Earth
(Bromwich and Parish 1998 ;KingandTurner 1997 ;Simmonds 1998 ). Not only
does the CPT spawn storm systems through cyclogenesis, there are also a large
number of low pressure systems circulating within the CPT. More than half of the
extra-tropical cyclonic systems in the Southern Hemisphere form in the CPT. Strong
baroclinic influences associated with wind and temperature are a major factor.
Due to the barrier created by the land mass, cyclogenesis is restricted to the
offshore area. There are significant spatial variations, but a similar seasonal
pattern. Cyclogenesis occurs mainly in the West Antarctic section of the region,
on either side of the Antarctic Penisula. The East Antarctic is a relatively quiet
zone. Three nodes dominate the spatial distribution. In both seasons, cyclogen-
esis is strong in the Ross Sea area, where the combination of cyclonic turning
of katabatic flows, the instability associated with the ice-ocean temperature
contrast, and confinement from the surrounding coastal elevations minimize
horizontal movement. A second area is the lee of the Antarctic Peninsula, around
658 S, where the higher elevations of the Peninsula interrupt the westerly air-
flow, and create a cyclonic circulation area. The third area is in the Weddell Sea.
A considerably greater number of cyclones form in winter, when the described
contrasts are the greatest. Particularly in the Ross and Weddell Sea areas, the
density of cyclogenesis doubles in winter compared to summer.
The spatial differences in cyclone system density reflect their development
and mobility within the CPT circulation, and thus differ from the major centers
of cyclogenesis (Simmonds 1998 ). Figure 5.7a and b compare summer and
winter cyclone system density, and again indicate three areas of major activity:
off the East Antarctic coast south of Southern Africa; off the East Antarctic coast
south of Australia; and off the West Antarctic coast west of the Antarctic
Peninsula. These locations relate strongly to the AAO, and although the frontal
systems may not be as well developed as in the mid-latitudes, they are the main
source for the poleward flux of energy and moisture.
The offshore flow of katabatic winds, the influence of Coriolis force, and the
channeling effect of the Antarctic coast itself encourage further easterly flow and
cyclonic circulation (Bromwich and Parish 1998 ). Depending on location, coastal
locations show variations in wind (and temperature) according to katabatic or
cyclonic dominance (K¨nig-Langlo et al. 1998 ). At Dumont d'Urville, katabatic
flows dominate, bringing cold dry air from the continent (wind direction
125-1858). Winds occasionally exceed 50m s 1 . On the opposite (NW) side of
the continent, winds at both Halley Bay and Neumayer are dominated by direc-
tions from 80 to 908. Katabatic influences and cyclonic disturbances control the