Disturbances and Eddy Fluxes in Northern Hemisphere Flows: Instability of Three-Dimensional January and July Flows

Abstract

A study is made of the instability properties of three-dimensional Northern Hemisphere flows for the months of January and July 1978. The growth rates and phase frequencies of growing baroclinic disturbances and the perturbation streamfunctions and eddy heat and momentum fluxes are obtained using a five-level spherical quasi-geostrophic spectral model. It is found that the presence of the planetary waves in the basic states cause a considerable increase in the growth rates of the fastest growing modes, compared with the case for zonally averaged January and July flows. The results of three-dimensional instability theory am compared with the observed regions of most actively developing baroclinic disturbances and eddy fluxes. For both months, there is reasonable agreement between linear theory and observations as far as the geographical locations of the largest disturbance amplitudes and eddy fluxes are concerned. However, for the January three-dimensional basic state, the usual vertical structure-problem of instability theory occurs with the disturbance quantities being too large at the surface compared with those at the tropopause. For the July three-dimensional basic state, where local horizontal sheer or barotropic wave instability plays an important role, the disturbance streamfunctions and eddy fluxes are too large in the middle troposphere, compared with those at the surface. For both months, the linear solutions are very sensitive to local potential vorticity gradients.