Ageostrophic Effects on the Stratospheric Residual Circulation and Tracer Distributions

Abstract

We have examined an idealized, zonally averaged, nonlinear, ageostrophic circulation forced by differential heating and parameterized eddy mixing for a range of mixing values and boundary conditions. Using a simple ƒ-plane channel model, we show that geostrophic and ageostrophic flows can have fundamentally different behaviors which may have important implications for the circulation and trace gas distributions in the stratosphere. Our main conclusions are: 1) As eddy forcing vanishes, an ageostrophic system is not constrained to approach radiative equilibrium (unlike a geostrophic system) and may tend to the limit of an inviscid diabatic circulation under a range of boundary conditions. 2) For stratospheric applications, we show that reduced eddy mixing in an ageostrophic model leads to a pronounced meridional contraction in the residual circulation (thereby limiting its poleward transport); as the eddy mixing vanishes, the circulation tends to an inviscid limit with a well-defined meridional width. This is characteristically different from the behavior of a geostrophic circulation, which vanishes in approximate proportions to the amount of eddy mixing. 3) Reduced eddy mixing in an ageostrophic model can ultimately lead to the steepening of the meridional slope of a long-lived tracer in the region between maximum rising and sinking motions. An implication of this study is that the comparatively weak wave driving in the Southern Hemisphere could produce a contracted residual circulation during the Southern winter, which may partially account for the asymmetry in the observed zonal-mean ozone column abundances between the Northern and Southern springs.