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 amplitude 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, temperatures 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 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, quasipermanent 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 northeastern 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 southwards 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,

Distribution System Pressure Zones

Figure 10.13 Jet streams, pressure distribution and climate for North America during the winters of 1995 to 1996 and 1994 to 1995.

Source: US Department of Commerce, Climate Prediction Center. Courtesy of US Department of Commerce.

Figure 10.13 Jet streams, pressure distribution and climate for North America during the winters of 1995 to 1996 and 1994 to 1995.

Source: US Department of Commerce, Climate Prediction Center. Courtesy of US Department of Commerce.

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. Sometimes, 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 influenced strongly 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 temperature departures in the winters of 1994 to 1995 and 1995 to 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 airmasses interact along their common boundary. The three-front (i.e. four airmass) 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 95°W, from 60° to 40°N, the dew-point temperatures reported in the four airmasses 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 ones 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

Pressure Systems Map Asia
Figure 10.14 A synoptic example of depressions associated with three frontal zones on 29 May 1963 over North America. Source: Based on charts of the Edmonton Analysis Office and the Daily Weather Report.

of northern Keewatin and Hudson Strait. One study of airmass 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 related closely to the boreal forest-tundra boundary. This relationship reflects the importance of Arctic airmass 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 attention in view of their prominence are (1) the advent of spring in late March; (2) the midsummer 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 maritime tropical air from the Gulf of Mexico. These changes are part of a hemispheric readjustment of the circulation; in early April, the Aleutian low-pressure cell, which from September to March is located about

Aleutian Low Pressure System

Figure 10.15 Regions in North America east of the Rocky Mountains dominated by the various airmass types in July for more than 50 per cent and 75 per cent of the time. The 50 per cent frequency lines correspond to mean frontal positions.

Source: After Bryson (1966).

Figure 10.15 Regions in North America east of the Rocky Mountains dominated by the various airmass types in July for more than 50 per cent and 75 per cent of the time. The 50 per cent frequency lines correspond to mean frontal positions.

Source: After Bryson (1966).

55°N, 165°W, splits into two, with one centre in the Gulf of Alaska and the other over northern Manchuria.

In late June, there is a rapid northward displacement of the Bermuda and North Pacific subtropical high-pressure cells. In North America, this also pushes the depression tracks northward with the result that precipitation decreases from June to July over the northern Great Plains, part of Idaho and eastern Oregon. Conversely, the southwesterly anticyclonic flow that affects Arizona in June is replaced by air from the Gulf of California, and this causes the onset of the summer rains (see B.3, this chapter). Bryson and Lahey suggest that these circulation changes at the end of June may be connected with the disappearance of snow cover from the Arctic tundra. This leads to a sudden decrease of surface albedo from about 75 to 15 per cent, with consequent changes in the heat budget components and hence in the atmospheric circulation.

Frontal wave activity makes the first half of September a rainy period in the northern Midwest states of Iowa, Minnesota and Wisconsin, but after about the 20 September, anticyclonic conditions return with warm airflow from the dry southwest, giving fine weather - the so-called Indian summer. Significantly, the hemispheric zonal index value rises in late September. This anticyclonic weather type has a second phase in the latter half of October, but at this time there are polar outbreaks. The weather is generally cold and dry, although if precipitation does occur there is a high probability of snowfall.

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Responses

  • immacolata
    Is the great basin dominated by low pressure during the summer?
    6 years ago
  • kayleigh
    How is global warming affecting pressure systems#?
    1 year ago

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