Geoscience Reference
In-Depth Information
each depression in the series. Eventually, the front
trails far to the south and the cold polar air forms
an extensive meridional wedge of high pressure,
terminating the sequence.
Another pattern of development may take
place on the warm front, particularly at the point
of occlusion, as a separate wave forms and runs
ahead of the parent depression. This type of
secondary is more likely with very cold (cA, mA
or cP) air ahead of the warm front, and it tends
to form when mountains bar the eastward
movement of the occlusion. This situation often
occurs when a primary depression is situated in
the Davis Strait and a breakaway wave forms south
of Cape Farewell (the southern tip of Greenland),
moving away eastward. Analogous developments
take place in the Skagerrak-Kattegat area when
the occlusion is held up by the Scandinavian
mountains.
North Atlantic, but it decreases eastward in the
North Pacific, perhaps owing to a less pronounced
gradient of sea surface temperature. Frontal
activity is most common in the central North
Pacific when the subtropical high is split into two
cells with converging airflows between them.
Another section of the Polar Front, often
referred to as the
Mediterranean Front
, is located
over the Mediterranean-Caspian Sea areas
in winter. At intervals, fresh Atlantic mP air,
or cool cP air from southeast Europe, converges
with warmer air masses of North African origin
over the Mediterranean Basin and initiates
frontogenesis. In summer, the Azores subtropical
anticyclone influences the area, and the frontal
zone is absent.
The summer locations of the Polar Front over
the western Atlantic and Pacific are some 10
°
further north than in winter (see
Figure 9.19
),
although the summer frontal zone is rather weak.
There is a frontal zone over Eurasia and a
corresponding one over middle North America.
These reflect the general meridional temperature
gradient and also the large-scale influence of
orography on the general circulation (see G, this
chapter).
In the Southern Hemisphere, the Polar Front
is on average about 45°S in January (summer),
with branches spiralling poleward towards it from
about 32°S off eastern South America and from
30
F ZONES OF WAVE
DEVELOPMENT AND
FRONTOGENESIS
Fronts and associated depressions tend to develop
in well-defined areas. The major zones of frontal
wave development are areas that are most
frequently baroclinic as a result of airstream
confluence (
Figure 9.18
). This is the case off East
Asia and eastern North America, especially in
winter, when there is a sharp temperature gradi-
ent between the snow-covered land and warm
offshore currents. These zones are referred to as
the Pacific Polar and Atlantic Polar Fronts,
respectively (
Figure 9.19
). Their position is quite
variable, but they are displaced equatorward in
winter, when the Atlantic Frontal Zone may
extend into the Gulf of Mexico. Here there is
convergence of air masses of different stability
between adjacent subtropical high pressure
cells. Depressions developing here commonly
move northeastward, sometimes following or
amalgamating with others of the northern part of
the Polar Front proper or of the Canadian Arctic
Front. Frontal frequency remains high across the
W in the South Pacific (
Figure 9.20
). In
July (winter), there are two Polar Frontal Zones
spiralling towards Antarctica from about 20
°
S, 150
°
S;
one starts over South America and the other at
170
°
latitude further
poleward than in summer. It is noteworthy that
the Southern Hemisphere has more cyclonic
activity in summer than does the Northern
Hemisphere in its summer. This appears to be
related to the seasonal strengthening of the
meridional temperature gradient (see p. 180).
The second major frontal zone is the Arctic
Front, associated with the snow and ice margins
of high latitudes (see
Figure 9.19
). In summer,
this zone is developed at the land-sea boundary in
°
W. They terminate some 4-5
°
