Page 194 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

158 part I The energy–atmosphere System
midlatitude migratory pressure systems and topographic barriers that can change wind direction.
Subpolar Lows: Cool and Moist In January, two low­ pressure cyclonic cells exist over the oceans around 60° N latitude, near their namesake islands: the North Pa­ cific Aleutian Low and the North Atlantic Icelandic Low (see Figure 6.10a). Both cells are dominant in winter and weaken or disappear in summer with the strengthening of high­pressure systems in the subtropics. The area of con­ trast between cold air from higher latitudes and warm air from lower latitudes forms the polar front, where masses of air with different characteristics meet (air masses and weather are the subjects of Chapter 8). This front encircles Earth, focused in these low­pressure areas.
Figure GIA 6 illustrates the polar front, where warm, moist air from the westerlies meets cold, dry air from the polar and Arctic regions. Warm air is displaced upward above the cool air at this front, leading to condensation and precipitation (frontal precipitation is discussed in Chapter 8). Low­pressure cyclonic storms migrate out of the Aleutian and Icelandic frontal areas and may produce precipitation in North America and Europe, respectively. Northwestern sections of North America and Europe generally are cool and moist as a result of the passage of these cyclonic systems onshore—consider the weather in British Columbia, Washington, Oregon, Ireland, and the United Kingdom. In the Southern Hemisphere, a discon­ tinuous belt of subpolar low­pressure systems surrounds Antarctica.
Polar Highs: Frigid and Dry Polar high­pressure cells are weak. The polar atmospheric mass is small, receiving little energy from the Sun to put it into motion. Variable winds, cold and dry, move away from the polar region in an anticyclonic direction. They descend and diverge clockwise in the Northern Hemisphere (counterclockwise in the Southern Hemisphere) and form weak, variable winds of the polar easterlies (shown in Figure GIA 6).
Of the two polar regions, the Antarctic has the stronger and more persistent high­pressure system, the Antarctic High, forming over the Antarctic landmass. Less pronounced is a polar high­pressure cell over the Arctic Ocean. When it does form, it tends to locate over the colder northern continental areas in winter (Canadian and Siberian Highs) rather than directly over the rela­ tively warmer Arctic Ocean.
upper Atmospheric Circulation
Circulation in the middle and upper troposphere is an important component of the atmosphere’s general circula­ tion. For surface­pressure maps, we plot air pressure using the fixed elevation of sea level as a reference datum—a constant height surface. For upper­atmosphere pressure maps, we use a fixed pressure value of 500 mb as a refer­ ence datum and plot its elevation above sea level through­ out the map to produce a constant isobaric surface.
Figure 6.12a and b illustrate the undulating surface elevations of a 500­mb constant isobaric surface for an April day. Similar to surface­pressure maps, closer spac­ ing of the height contours indicates faster winds; wider spacing indicates slower winds. On this map, altitude variations in the isobaric surface are ridges for high pres­ sure (with height contours on the map bending poleward) and troughs for low pressure (with height contours on the map bending equatorward).
The pattern of ridges and troughs in the upper­air wind flow is important in sustaining surface cyclonic (low­pressure) and anticyclonic (high­pressure) circula­ tion. Along ridges, winds slow and converge (pile up); along troughs, winds accelerate and diverge (spread out). Note the wind­speed indicators and labels in Figure 6.12a near the ridge (over Alberta, Saskatchewan, Montana, and Wyoming), and compare them with the wind­speed indi­ cators around the trough (over Kentucky, West Virginia, the New England states, and the Maritimes). Also, note the wind relationships off the Pacific Coast.
Figure 6.12c shows convergence and divergence in the upper­air flow. Divergence aloft is important to cyclonic circulation at the surface because it creates an outflow of air aloft that stimulates an inflow of air into the low­ pressure cyclone (like what happens when you open an upstairs window to create an upward draft). Similarly, convergence aloft is important to anticyclonic circulation at the surface, driving descending airflows and causing airflow to diverge from high­pressure anticyclones.
Rossby Waves Within the westerly flow of geostrophic winds are great waving undulations, the Rossby waves, named for meteorologist Carl G. Rossby, who first de­ scribed them mathematically in 1938. Rossby waves occur along the polar front, where colder air meets warmer air, and bring tongues of cold air southward, with warmer tropical air moving northward. The development of Rossby waves begins with undulations that then in­ crease in amplitude to form waves (Figure 6.13, page 160). As these disturbances mature, circulation patterns form in which warmer air and colder air mix along distinct fronts. These wave­and­eddy formations and upper­air di­ vergences support cyclonic storm systems at the surface. Rossby waves develop along the flow axis of a jet stream.
Jet Streams The most prominent movement in the upper­ level westerly geostrophic wind flows are the jet streams, irregular, concentrated bands of wind occurring at sev­ eral different locations that influence surface weather systems (Figure GIA 6.1 shows the location of four jet streams). The jet streams normally are 160–480 km wide by 900–2150 m thick, with core speeds that can ex­ ceed 300 km·h−1. Jet streams in each hemisphere tend to weaken during the hemisphere’s summer and strengthen during its winter as the streams shift closer to the equa­ tor. The pattern of high­pressure ridges and low­pressure troughs in the meandering jet streams causes variation in jet­stream speeds.