SUMMARY

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. The upper-level subtropical anticyclones and polar vortex in both hemispheres illustrate this 'thickness' relationship. 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 northwest 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 poleward, 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 poleward by ocean current systems. Surface currents are mostly wind driven, but the slow deep ocean circulation (global conveyor belt) is due to thermohaline forcing.

The circulation in the northern hemisphere mid-latitudes is subject to variations in the strength of the zonal westerlies lasting three to eight weeks (the index cycle) and interannual differences in the north-south pressure gradient in the North Atlantic (the NAO) that lead to a west-east 'seesaw' in temperature and other anomalies. This has major effects on the climate of Europe and eastern North America and west Greenland.

The ocean's vertical structure varies latitudinally and regionally. In general, the thermocline is deepest in mid-latitudes, thus permitting greater turbulent mixing and atmospheric heat exchanges. The oceans are important regulators of both atmospheric temperatures and CO2 concentrations. Ocean dynamics and circulation features are analogous to those in the atmosphere on both the meso- and macroscale. The wind-driven Ekman layer extends to 100 to 200 m. Ekman transport and coastal upwelling maintain normally cold sea surfaces off western South America and southwest Africa in particular.

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