In addition to turbulence and waves, the giant planet atmospheres exhibit one more, very distinctive planetary-scale motion peculiar to them: long-lived vortices, or oval circulations such as the GRS on Jupiter, and the dark spots on Neptune. Such systems typically appear at latitudes of large horizontal wind shear and all vortices seen in the giant planet atmospheres are seen to have a relative vorticity of the same sign as the jets within which they lie. In one respect their presence is expected from two-dimensional turbulence since any small vortices generated in such shear regions between the belts and zones are expected to merge with other vortices and grow.
However, what is not clear is by what instability mechanism these vortices are initiated, and how they are maintained once they are established.
A fundamental clue to the nature of the large ovals is to examine the distribution of cyclones and anticyclones across the different planets. The geostrophic equations derived in Section 5.2.1 are completely symmetric with respect to the sign of the vorticity and thus if the atmospheres of the giant planets were purely geostrophic, we might expect that cyclonic ovals were equally numerous as anticyclonic ones. However, almost all observed ovals on the giant planets are anticyclonic (e.g., on Jupiter 90% of all ovals are anticyclones). Where might such an asymmetry arise? One way of achieving such an asymmetry is to include centrifugal forces. Consider the acceleration of air moving with speed V in a circle of radius R. Starting with the horizontal momentum equations, and ignoring friction, we find that (e.g., Chamberlain and Hunten, 1987; Holton, 1992; Houghton, 1986)
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