Using a multi-level, primitive equation model, the zonal mean and time mean structure of the atmospheric zonal wind is investigated. When baroclinic life cycle calculations are performed with an axisymmetric subtropical jet as an initial flow, a distinct midlatitude jet is established, away from the subtropical jet. This results in the co-existence of two jets. Such behavior results from the fact that the latitude of a subtropical jet and that of maximum baroclinicity differ from each other, and that divergence of the baroclinic wave activity flux does not occur at the core of the subtropical jet.
Equilibrium calculations have also been performed using a forced-dissipative model which includes a full zonal wave spectrum. It is found that in the statistically steady state, the distinction between the subtropical and the midlatitude jet disappears, as the radiative equilibrium meridional temperature gradient increases and the static stability decreases. We suggest a mechanism that can explain such a transition from a double to a single jet flow. This mechanism involves baroclinic waves that grow on the zonal wind minimum between the midlatitude and subtropical jets. For a statistically steady flow where the zonal wind minimum exists, we find a decaying normal mode whose meridional flux of zonal momentum converges toward the zonal wind minimum. We then construct a fictitious two jet state in the parameter regime where statistically steady states exhibit only one jet. This state is obtained by linearly extrapolating statistically steady states that show two jets. For such a fictitious state, the decaying normal mode described above becomes linearly unstable. An initial value calculation shows that the fictitious two jet state rapidly disappears, as waves resembling the unstable normal mode grow in the vicinity of the zonal wind minimum.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics