Planetary Waves

Rossby waves are of two kinds, short and long. The latter are known as planetary waves. They are almost stationary and last from a few weeks to a season. They usually have a wave number of two in the northern hemisphere (because of the two land masses, Eurasia and North

Figure 12. 14 The effect of warm land on atmospheric pressures. The land heats the air above, which therefore expands lifting the higher atmosphere, so that the level at a pressure of 200 hPa, for instance, is raised (a). In other words, pressures over the land are greater than at the same height over the sea, especially at higher levels. For example, the pressure on top of the shaded block of warm air in (a) exceeds the 200 hPa of adjacent air at that level. The pressure difference leads to an outwards flow at the upper levels. This reduces the weight of air above the land, lowering the surface pressure there (b). That causes an inwards flow near the ground. The contrary flows at different levels create a convective circulation (c).

Figure 12. 14 The effect of warm land on atmospheric pressures. The land heats the air above, which therefore expands lifting the higher atmosphere, so that the level at a pressure of 200 hPa, for instance, is raised (a). In other words, pressures over the land are greater than at the same height over the sea, especially at higher levels. For example, the pressure on top of the shaded block of warm air in (a) exceeds the 200 hPa of adjacent air at that level. The pressure difference leads to an outwards flow at the upper levels. This reduces the weight of air above the land, lowering the surface pressure there (b). That causes an inwards flow near the ground. The contrary flows at different levels create a convective circulation (c).

jet stream jet stream

Figure 12.15 The upper-troposphere winds in the vicinity of a shortwave trough or low.

America), and one in the southern hemisphere because of the asymmetry of Antarctica about the South Pole; Antarctica bulges towards the Indian Ocean. (This asymmetry is one reason why Perth at 32°S in west Australia receives 865 mm/a of rain, whereas Santiago at 33.5°S in Chile receives only 318 mm/a.) There is also a simultaneous weaker undulation of wavenumber three in the southern hemisphere, due to South America (where much of the Andes is above 3,000 m), Africa (where most of the land in the south is above 1,000 m), and the 'island continent' consisting mainly of Australia and its Dividing Range, as well as mountainous New Zealand.

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