ON ANOMALOUS MERIDIONAL CIRCULATIONS AND ELIASSEN-PALM FLUX DIVERGENCES IN AN IDEALIZED MODEL OF DISSIPATING, NON-BREAKING ROSSBY WAVES

Even in idealized models of steady, dissipating, non-breaking Rossby w aves at small wave amplitude, and even in the absence of barotropic an d baroclinic shear instabilities, there can be an anomalous Eliassen-P alm flux divergence in the sense that the divergence is positive when the background potential-vorticity gradient is also positive, implying upgradient eddy potential-vorticity transport. The phenomenon is illu strated in the simplest possible case of dissipation by Rayleigh frict ion and Newtonian cooling, and is shown by a more general argument not to be restricted to that case. The physical reason is that infrared r adiative damping can act anti-dissipatively on potential-vorticity ano malies whenever the vertical disturbance structure is diffractive or e vanescent, as with most real stratospheric synoptic and sub-synoptic-s cale disturbances forced from below. Associated with this phenomenon a re anomalous (eastward) phase tilts with height, and equatorward trans formed Eulerian-mean (TEM) meridional velocities. It is pointed out th at the latter is a clearcut example of a TEM circulation whose sense i s opposite to that of the generalized Lagrangian-mean circulation (and the effective transport circulation in the sense of Plumb and Mahlman ) induced by the same steady, small-amplitude disturbance. (C) 1997 El sevier Science B.V.