Baroclinic Instability on the Sphere: Solutions with a More Realistic Tropopause

A. J. Simmons U.K. Universities' Atmospheric Modelling Group, Department of Geophysics, University of Reading, England

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B. J. Hoskins U.K. Universities' Atmospheric Modelling Group, Department of Geophysics, University of Reading, England

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Abstract

The authors' previous study of baroclinic instability using a primitive equation model with spherical geometry is extended to include more realistic initial distributions of zonal wind and temperature in the upper troposphere and lower stratosphere. Results show little difference in the low-level structure of normal modes, but generally larger upper-level amplitudes for wavelengths close to or longer than that giving maximum linear growth rate. Near the tropopause these disturbances may extend significantly toward the equator, a result shown to be consistent with the forced barotropic response of tropical regions. Their eddy momentum fluxes exhibit some of the variability noted previously, but transfer tends to be predominantly poleward at upper levels. The upper-level heat flux is stronger relative to the momentum flux than is found in general circulation statistics.

Abstract

The authors' previous study of baroclinic instability using a primitive equation model with spherical geometry is extended to include more realistic initial distributions of zonal wind and temperature in the upper troposphere and lower stratosphere. Results show little difference in the low-level structure of normal modes, but generally larger upper-level amplitudes for wavelengths close to or longer than that giving maximum linear growth rate. Near the tropopause these disturbances may extend significantly toward the equator, a result shown to be consistent with the forced barotropic response of tropical regions. Their eddy momentum fluxes exhibit some of the variability noted previously, but transfer tends to be predominantly poleward at upper levels. The upper-level heat flux is stronger relative to the momentum flux than is found in general circulation statistics.

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