Using a primitive equation model on the sphere, the properties of multiple zonal jets in a baroclinically unstable atmosphere are examined. The model is forced by relaxing the temperature toward a radiative-convective equilibrium state. It is found that the number of jets in the meridional direction does not increase monotonically with planetary radius, R, because the meridional scale of the jet also increases with R. The latter behavior can be understood by noting that the meridional length scale of the jet is determined by the Rhines scale, Lr, which in turn depends on R. In addition, the increase in R leads to an increase in Lr, indirectly through an increase in the rms wind speed. An explanation for this behavior is discussed in the context of linear instability.
Given the rather complex behavior of the zonal jets with changes in R, the jet behavior is studied in detail as the equilibrium temperature gradient is varied, fixing the value of R at four times the earth radius. At this value of R, a distinct persistent subtropical jet, and at least two persistent eddy-driven jets are obtained in either hemisphere, provided that the equilibrium temperature gradient is sufficiently small. We show that such a flow supports normal modes whose meridional heat flux is maximized at the zonal wind minima between each jet. We refer to these normal modes as inter-jet modes. The inter-jet mode between the two eddy-driven jets is linearly unstable and the associated momentum flux diverges, reinforcing the local jet minimum. On the other hand, the inter-jet mode on the poleward side of the subtropical jet is linearly stable, and the associated momentum flux converges. Spectral analysis of the momentum flux from the nonlinear model is consistent with the properties of the linear modes described above.
The above experiments suggest that a large planet, such as Jupiter, can support chaotic baroclinic eddies even when the pole to equator temperature gradient is much smaller than that of the earth, and that the inter-jet modes can play an important role in maintaining the multiple jets in the Jovian atmosphere.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics