Linear and Nonlinear Baroclinic Instability of the Northern Hemisphere Winter Zonal Flow

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  • 1 U.K. Universities Atmospheric Modelling Group, University of Reading, England
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Abstract

This study was carried out to investigate the extent to which the results of linear β- or f-plane solutions for baroclinic instability of simple flows can be applied to the Northern Hemisphere winter, zonal mean zonal flow. In particular, the idea of baroclinic adjustment postulated by Stone (1978) in response to a parameterization of poleward heat flux presented by Held (1978) and based on Charney’s β-plane model is considered. This parameterization is very dependent on a simple formulation for the depth scale of baroclinic waves and it is shown here that this is not an appropriate scaling for many of the modes under consideration. The baroclinic adjustment hypothesis is examined by considering first the linear stability of a variety of atmospheres, then the normal modes are integrated using a spectral primitive equation model and their nonlinear development is found to depend on the process of barotropic decay which is not a feature of the simple theoretical models. The time-averaged wave structure, as measured by Eliassen-Palm flux divergence patterns (Edmon et al., 1980), shows a good agreement with climatological patterns.

Abstract

This study was carried out to investigate the extent to which the results of linear β- or f-plane solutions for baroclinic instability of simple flows can be applied to the Northern Hemisphere winter, zonal mean zonal flow. In particular, the idea of baroclinic adjustment postulated by Stone (1978) in response to a parameterization of poleward heat flux presented by Held (1978) and based on Charney’s β-plane model is considered. This parameterization is very dependent on a simple formulation for the depth scale of baroclinic waves and it is shown here that this is not an appropriate scaling for many of the modes under consideration. The baroclinic adjustment hypothesis is examined by considering first the linear stability of a variety of atmospheres, then the normal modes are integrated using a spectral primitive equation model and their nonlinear development is found to depend on the process of barotropic decay which is not a feature of the simple theoretical models. The time-averaged wave structure, as measured by Eliassen-Palm flux divergence patterns (Edmon et al., 1980), shows a good agreement with climatological patterns.

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