Geoscience Reference
In-Depth Information
the Arctic or the Gulf of Mexico to sweep across the
interior lowlands, causing wide extremes of weather
and climate. Maritime influences in eastern North
America are greatly limited by the fact that the
prevailing winds are westerly, so that the temperature
regime is continental. Nevertheless, the Gulf of Mexico
is a major source of moisture supply for precipitation
over the eastern half of the United States and, as a
result, the precipitation regimes differ from those in East
Asia.
We look first at the characteristics of the atmospheric
circulation over the continent.
tinent, causing a reversal of the usual weather pattern,
since upper northwesterly airflow can bring cold, dry
weather to the west while in the east there are very mild
conditions associated with upper southwesterly flow.
Precipitation amounts also depend on the depression
tracks. If the upper trough is far to the west, depressions
form ahead of it (see Chapter 9G) over the south central
United States and move northeastward towards the
lower St Lawrence, giving more precipitation than usual
in these areas and less along the Atlantic coast.
The major features of the surface pressure map
in January (see Figure 7.9) are the extension of the
subtropical high over the southwestern United States
(called the Great Basin high) and the separate polar
anticyclone of the Mackenzie district of Canada. Mean
pressure is low off both the east and west coasts of
higher mid-latitudes, where oceanic heat sources
indirectly give rise to the (mean) Icelandic and Aleutian
lows. It is interesting to note that, on average, in
December, of any region in the northern hemisphere for
any month of the year, the Great Basin region has the
most frequent occurrence of highs, whereas the Gulf of
Alaska has the maximum frequency of lows. The Pacific
coast as a whole has its most frequent cyclonic activity
in winter, as does the Great Lakes area, whereas over
the Great Plains the maximum is in spring and early
summer. Remarkably, the Great Basin in June has the
most frequent cyclogenesis of any part of the northern
hemisphere in any month of the year. Heating over
this area in summer helps to maintain a shallow, quasi-
permanent low-pressure cell, in marked contrast with
the almost continuous subtropical high-pressure belt
in the middle troposphere (see Figure 7.4). Continental
heating also indirectly assists in the splitting of the
Icelandic low to create a secondary centre over north-
eastern Canada. The west coast summer circulation
is dominated by the Pacific anticyclone, while the
southeastern United States is affected by the Atlantic
subtropical anticyclone cell (see Figure 7.9B).
Broadly, there are three prominent cyclone tracks
across the continent in winter (see Figure 9.21). One
group moves from the west along a more or less zonal
path about 45 to 50°N, whereas a second loops south-
wards 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,
1 Pressure systems
The mean pressure pattern for the middle troposphere
displays a prominent trough over eastern North America
in both summer and winter (see Figures 7.3A and 7.4A).
In part, this is a lee trough caused by the effect of the
western mountain ranges on the upper westerlies, but at
least in winter the strong baroclinic zone along the East
Coast of the continent is a major contributory factor.
As a result of this mean wave pattern, cyclones tend
to move southeastward over the Midwest, carrying
continental polar air southward, while the cyclones
travel northeastward along the Atlantic coast. The
planetary wave structure over the eastern North Pacific
and North America is referred to as the Pacific-North
America (PNA) pattern. It refers to the relative ampli-
tude of the troughs over the central North Pacific and
eastern North America, on the one hand, and the ridge
over western North America on the other. In the positive
(negative) mode of the PNA, there is a well-developed
storm track from East Asia into the central Pacific and
then into the Gulf of Alaska (cylones over East Asia
move northeastward to the Bering Sea, with another
area of lows off the west coast of Canada). The positive
(negative) phases of PNA tend to be associated with El
Niño (La Niña ) events in the equatorial Pacific.
The PNA mode has important consequences for the
weather in different parts of the continent. In fact, this
relationship provides the basis for the monthly forecasts
of the US National Weather Service. For example, if the
eastern trough is more pronounced than usual, temper-
atures are below average in the central, southern and
eastern United States, whereas if the trough is weak
the westerly flow is stronger with correspondingly less
opportunity for cold outbreaks of polar air. Sometimes,
the trough is displaced to the western half of the con-
