Numerical simulations of nonlinear baroclinic instability with a spherical wave-mean flow model Public Deposited

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  • A global, multi-level, wave-mean flow model based on an approximate version of the primitive equations is developed to investigate the development of a baroclinic wave field initially confined to a single zonal wavenumber. The effects of physical processes (surface drag and thermal damping) and internal diffusion on the evolution have been examined. The nature of the mean flow adjustment by the nonlinear baroclinic waves is also studied. For a simulation with a relatively strong internal diffusion it is found that a single life cycle characterized by baroclinic growth and barotropic decay is obtained (as in Simmons and Hoskins, 1978), whereas with weaker diffusion the wave undergoes secondary life cycles before a nearly wave-free state is reached (as in Barnes and Young, 1991). In an experiment with weak 4th order diffusion secondary life cycles occur with little net decay. Relatively strong barotropic growth follows the initial life cycle. In experiments with surface drag (Rayleigh friction) and thermal damping (Newtonian cooling), repeated life cycles of baroclinic growth and barotropic decay can be obtained. It is found that in the complete absence of surface drag, the flow evolves to a nearly wave-free state after one secondary cycle. This demonstrates that surface drag plays an important role in nonlinear baroclinic instability. With relatively strong surface drag multiple life cycle behavior is found for sufficiently strong thermal damping. Such a behavior strengthens for very strong thermal damping. A steady wave state in which the wave amplitude equilibrates at an essentially constant level has only been obtained with very strong "potential vorticity damping". Both the "barotropic governor" process (James and Gray, 1986) and the baroclinic adjusment process are responsible for major parts of the stabilization of the mean flow in simulations with and without surface drag and thermal damping. However, the "barotropic governor" process dominates the flow evolution in the model simulations without surface drag and thermal damping. The "barotropic governor" modifies the meridional gradient of zonal mean potential vorticity, which influences the baroclinic adjustment.
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