A Weakly Nonlinear Primitive Equation Baroclinic Life Cycle

Abstract

Abstract A weakly nonlinear baroclinic life cycle is examined with a spherical, multilevel, primitive equation model. The structure of the initial zonal jet is chosen so that the disturbance grows very slowly, that is, linear growth rate less than 0.1 day−1, and the life cycles of the disturbance are characterized by baroclinic growth and followed by barotropic decay. It is found that if the disturbance grows sufficiently slowly, the decay is baroclinic. As a result, the procedure for determining this weakly nonlinear jet is rather delicate. The evolution of the disturbance is examined with Eliassen-Palm flux diagrams, which illustrate that the disturbance is bounded at all times by its critical surface in the model's middle and upper troposphere. The disturbance undergoes two large baroclinic gtowth/barotropic decay life cycles, after which it decays by horizontal diffusion. At the end of the first cycle, the zonally averaged zonal flow is linearly stable, suggesting that the disturbance growth during th...