The Relationship between Zonal Mean Flow and Quasi-Stationary Waves in the Midtroposphere

Abstract

The degree to which quasi-stationary midtropospheric flow is consistent with linear, potential vorticity conservation is investigated. The linear theory suggests there should be a well defined relationship between the zonal mean component of steady flow and departures from the zonal mean. A set of 45 monthly mean winter states is examined to determine if there is a zonal/eddy relationship. Month-to-month variations in zonal mean winds are represented in terms of an EOF analysis. Correlation and compositing techniques isolate two-dimensional patterns which appear to be associated with each zonal EOF. Solutions to the nondivergent barotropic vorticity equation linearized about a zonal mean state suggest that the two-dimensional flow patterns associated with several of the leading zonal mean EOFs are consistent with the dynamical mechanism Rossby first proposed as an explanation for a zonal/eddy relationship. It is suggested that the zonal/eddy relationship examined here may in fact play a role in long time-scale (i.e., several weeks or longer) midtrophospheric phenomena described in previous observational studies. It may make contributions to midlatitude teleconnection patterns, tropical/extratropical relationships and, of course, the index cycle. However, the fraction of the total variance of long time-scale midtropospheric flow for which a zonal/eddy relationship holds may be small.