Лекция: Circulations in the vertical and horizontal planes

The vertical change of pressure with height depends on the temperature structure. High-(low-) pressure systems intensify with altitude in a warm (cold) air column; thus warm lows and cold highs are shallow features. This 'thickness' relationship is illustrated by the upper-level subtropical anticyclones and polar vortex in both hemispheres. The intermediate mid-latitude west­erly winds thus have a large 'thermal wind' component. They become concentrated into upper tropospheric jet streams above sharp thermal gradients, such as fronts. The upper flow displays a large-scale long­-wave pattern, especially in the northern hemi­sphere, related to the influence of mountain barriers and land-sea differences. The surface pressure field is dominated by semi-permanent subtropical highs, subpolar lows and, in winter, shallow cold continental highs in Siberia and north-western Canada. The equatorial zone is predominantly low pressure. The associated global wind belts are the easterly trade winds and the mid-latitude westerlies. There are more variable polar easterlies and over land areas in summer a band of equatorial westerlies repre­senting the monsoon systems. This mean zonal (west-east) circulation is intermittently inter­rupted by 'blocking' highs; an idealized sequence is known as the index cycle. The atmospheric general circulation, which transfers heat and momentum polewards, is predominantly in a vertical meridional plane in low latitudes (the Hadley cell), but there are also important east-west circulations (Walker cells) between the major regions of subsidence and convective activity. Heat and momentum exchanges in middle and high latitudes are accomplished by horizontal waves and eddies (cyclones/anticyclones). Substantial energy is also carried polewards by ocean current systems. Surface currents are mostly wind driven, but the slow deep ocean circulation (global conveyor belt) is due to thermohaline forcing.

Упражнение 10.

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Упражнение 11.

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