Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 165

Summer sources and transport patterns are more diffuse, reflecting the attrac-
tion of the weak polar low. Summer cyclones are weaker (averaging 1.5 hPa
higher in central pressure than in winter), but have longer lifetimes (Zhang et al.
2004 ). Speed of movement is highest in the winter half of the year (especially
March), and lowest in August and September (Przybylak 2003 ).
Figure 5.12c and d shows the spatial distribution of anticyclone frequency
over the Arctic Basin. Anticyclones are weak and slow moving with sluggish
circulation. They are not particularly well defined. Stronger in winter, they are
most frequent over the Beaufort Sea and East Siberian Arctic, and inland Canada
and Siberia. The contours over Greenland in Figure 5.12 may reflect altitude
more than the existence of high pressure. In summer, Arctic anticyclonic
frequency is similar to winter, with an additional center over the Novaya
Zemlaya area.
As in Antarctica, meso-scale lows can develop and decay quite rapidly
(Przybylak 2003 ). Often independent of the larger-scale circulation, these can
cause significant local changes to weather, and bring considerable precipitation.
5.12 Polar night jet and stratospheric ozone depletion
The late winter and early spring general circulation over the Arctic has some
broad similarities to Antarctica. The strength and variability of the PNJ, as the
leading edge of the circumpolar vortex, are strongly linked to the semi-annual
oscillations (AO, AAO) in both hemispheres (Kuroda and Kodera 2001 ). The
Arctic stratospheric circulation (zonal-mean zonal winds) is poleward, and
subsiding, bringing colder air toward the surface, in a similar manner to
Antarctica (Kuroda and Kodera 2004 ). Changes in Rossby wave number 1
mainly determine variability in the PNJ/circumpolar circulation.
However, major differences occur between the hemispheres, which allow
stratospheric warming to occur over the Arctic, but not the Antarctic, at this
time of year. There is no high-altitude continental ''anchor'' in the Arctic, and
the circulation meanders over wide areas. The circulation can break down on an
irregular basis, allowing major incursions of warm air to offset the winter
cooling associated with stratospheric subsidence. The overall wind speeds in
the vortex are lower than over Antarctica, and more variable.
This type of circulation pattern does not allow polar-wide stratospheric ozone
depletion in the Arctic. While cold, protracted winters can occur, the overall
decrease in Arctic stratospheric ozone in six worst-case years in the 1990s
averaged less than half the decrease over Antarctica (Bobylev et al. 2003 ).
Overall average ozone depletion is less than 15%. There are periods during the
season when temperatures in parts of the Arctic drop low enough to form polar
stratospheric clouds, and therefore significant ozone depletion can occur, but
these periods are not very consistent (Turco 2002 ). Over the Arctic, stratospheric
aerosols can be important as a substitute for PSCs, activating heterogeneous