Weather and climate in middle and high latitudes - Atmosphere, Weather and Climate - page 285

Broadly, there are three prominent cyclone
tracks across the continent in winter (see Figure
9.21 ). One group, known as 'Alberta clippers',
moves from the west along a more or less zonal
path about 45-50

95°W, from 60° to 40°N, the dew-point temper-
atures reported in the four air masses were -8 ° C,

1°C, 4°C and 13°C, respectively.
In summer, east coast depressions are less
frequent and the tracks across the continent are
displaced northward, with the main tracks moving
over Hudson Bay and Labrador-Ungava, or along
the line of the St Lawrence. These are associated
mainly with a poorly defined maritime frontal
zone. The Arctic Front is usually located along the
north coast of Alaska, where there is a strong
temperature gradient between the bare land and
the cold Arctic Ocean and pack ice. East from
here, the front is very variable in location from day
to day and year to year. It occurs most often in the
vicinity of northern Keewatin and Hudson Strait.
One study of air-mass temperatures and airstream
confluence regions suggests that an arctic frontal
zone occurs further south over Keewatin in July
and that its mean position ( Figure 10.15 ) is closely
related to the boreal forest-tundra boundary.
This relationship reflects the importance of arctic
air-mass dominance for summer temperatures
and consequently for tree growth, yet energy
budget differences due to land cover type appear
insufficient to determine the frontal location.
Several circulation singularities have been
recognized in North America, as in Europe (see
A.4, this chapter). Three that have received atten-
tion in view of their prominence are: (1) the
advent of spring in late March; (2) the mid-
summer high-pressure jump at the end of June;
and (3) the Indian summer in late September (and
late October).
The arrival of spring is marked by different
climatic responses in different parts of the
continent. For example, there is a sharp decrease
in March to April precipitation in California,
due to the extension of the Pacific high. In the
Midwest, precipitation intensity increases as a
result of more frequent cyclogenesis in Alberta
and Colorado, and northward extension of mari-
time tropical air from the Gulf of Mexico. These
changes are part of a hemispheric readjustment
of the circulation; in early April, the Aleutian

N, whereas a second loops
southward over the central United States and then
turns northeastward towards New England and
the Gulf of St Lawrence. Some of these depressions
originate over the Pacific, cross the western ranges
as an upper trough and redevelop in the lee of the
mountains. Alberta is a noted area for this process
and also for primary cyclogenesis, since the arctic
frontal zone is over northwest Canada in winter.
This frontal zone involves much-modified mA air
from the Gulf of Alaska and cold, dry cA (or cP)
air. Cyclones of the third group form along
the main polar frontal zone, which in winter is
off the east coast of the United States, and move
northeastward towards Newfoundland. Some-
times, this frontal zone is present over the
continent at about 35°N with mT air from the
Gulf and cP air from the north or modified mP air
from the Pacific. Polar front depressions forming
over Colorado move northeastward towards the
Great Lakes; others developing over Texas follow
a roughly parallel path, further to the south and
east, towards New England. Anomalies in winter
climate over North America are strongly influ-
enced by the position of the jet streams and the
movement of associated storm systems. Figure
10.13 illustrates their role in locating areas of
heavy rain, flooding and positive/negative temper-
ature departures in the winters of 1994-1995 and
1995-1996.
Between the Arctic and Polar Fronts, Canadian
meteorologists distinguish a third frontal zone.
This maritime (arctic) frontal zone is present
when mA and mP air masses interact along their
common boundary. The three-front ( i.e., four air-
mass) model allows a detailed analysis to be made
of the baroclinic structure of depressions over the
North American continent using synoptic weather
maps and cross-sections of the atmosphere. Figure
10.14 illustrates the three frontal zones and
associated depressions on 29 May 1963. Along

°