Geoscience Reference
In-Depth Information
Winter
Summer
N
T
T
R
Figure 9.19
The major Northern Hemisphere frontal zones in winter and summer.
a less pronounced Arctic Frontal Zone in the
North Atlantic-Norwegian Sea area, extending
along the Siberian coast. A similar weak frontal
zone is found in winter in the Southern
Hemisphere. It is located at 65-70°S near the edge
of the Antarctic pack ice in the Pacific sector (see
Figure 9.20
), although few cyclones form there.
Zones of airstream confluence in the Southern
Hemisphere (cf.
Figures 9.2B
and
9.4B
) are fewer
and more persistent, particularly in coastal
regions, than in the Northern Hemisphere.
The principal tracks of depressions in the
Northern Hemisphere in January are shown in
Figure 9.21
. The major tracks reflect the primary
frontal zones discussed above. In summer, the
Mediterranean route is absent and lows move
across Siberia; the other tracks are similar,
although more zonal and located in higher
latitudes (around 60°N).
Between the two hemispherical belts of
subtropical high pressure there is a further major
convergence zone, the Intertropical Convergence
Zone (ITCZ). Formerly this was designated as the
Intertropical Front (ITF), but air-mass contrasts
are not typical. The ITCZ moves seasonally away
from the equator, as the subtropical high pressure
cell activity alternates in opposite hemispheres.
0°
O
90°W
90°E
T
180°
Figure 9.20
The major Southern Hemisphere frontal zones
in winter (Wi) and summer (Su).
Siberia and North America where there is a strong
temperature gradient between the heated snow-
free land and the largely sea ice-covered, cold
Arctic Ocean. In winter over North America, it
is formed between cA (or cP) air and Pacific
maritime air modified by crossing the Coast
Ranges and the Rocky Mountains). There is also
