Shortwaves

The co-existing short Rossby waves have wavelengths of a few thousand kilometres at most and are transient, developing and decaying within a week or two. Their short length means rapid twisting, i.e. considerable vorticity (Note12.K ). They travel eastward at a varying pace, typically 1,000 km/day, slightly slower than the winds travelling through them.

Shortwaves are associated with disturbances at the polar front, when a tongue of cold air extends towards the equator, for instance. This causes a lowering of isobaric surfaces in the upper troposphere, i.e. a trough of low pressure (Figure 12.15), and therefore a clockwise turning in the southern hemisphere. Shortwaves in the upper westerlies are linked with frontal disturbances beneath, which cause weather. This is the reason for weather forecasters focusing attention on upper-level charts of winds and vorticity.

The short Rossby waves and the associated frontal disturbances can play an important role in the redistribution of heat across the globe (Section 5.2). That is because large amounts of sensible and latent heat are advected poleward ahead of a frontal disturbance (Chapter 13), and the disturbance is followed by an outbreak of polar air towards the equator. Either way, there is a lessening of the temperature difference between equator and pole.

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