Transient Eddies and the Seasonal Mean Rotational Flow

Brian J. Hoskins Department of Meteorology, University of Reading, Reading, United Kingdom

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Prashant D. Sardeshmukh Department of Meteorology, University of Reading, Reading, United Kingdom

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Abstract

Virtually all investigations of transient-eddy effects on the large-scale mean vorticity start from the premise that only the rotational transient motion need be considered. In this paper, the seasonal mean vorticity balance at 250 mb is examined, with particular emphasis on those transient term that are associated with the horizontally divergent transient motion. The largest transient terms are, in fact, found to be the advection of vorticity by the divergent flow and the stretching term. These am only a factor of 2 smaller than the mean flow terms. However, these transient term have a strong mutual cancellation. Their residual, the convergence of the vorticity flux associated with the divergent motion, although much smaller, is comparable on the planetary scale with the similar term associated with the rotational motion. These properties are interpreted using simple models. It is concluded that a representation of the vorticity flux by the transient divergent flow may be necessary in an accurate parameterization of transient eddies in global-scale climate models, and that any analysis of transient effects must include the divergent motions in a consistent manner.

Abstract

Virtually all investigations of transient-eddy effects on the large-scale mean vorticity start from the premise that only the rotational transient motion need be considered. In this paper, the seasonal mean vorticity balance at 250 mb is examined, with particular emphasis on those transient term that are associated with the horizontally divergent transient motion. The largest transient terms are, in fact, found to be the advection of vorticity by the divergent flow and the stretching term. These am only a factor of 2 smaller than the mean flow terms. However, these transient term have a strong mutual cancellation. Their residual, the convergence of the vorticity flux associated with the divergent motion, although much smaller, is comparable on the planetary scale with the similar term associated with the rotational motion. These properties are interpreted using simple models. It is concluded that a representation of the vorticity flux by the transient divergent flow may be necessary in an accurate parameterization of transient eddies in global-scale climate models, and that any analysis of transient effects must include the divergent motions in a consistent manner.

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