Meridional Distribution of Stratospheric Trace Constituents

Abstract

Vertically stratified stratospheric tracers such as methane and nitrous oxide tend to have constant mixing ratio surfaces that slope downward toward the poles in the meridional plane. The equilibrium tracer slope results from the competition between the slope steepening effects of advection by the diabatic circulation and the slope flattening effects of quasi-isentropic eddy transport and photochemical loss. The diabatic circulation itself is, however, driven primarily by eddy transports, which maintain the departure of stratosphere temperatures from radiative equilibrium. If the eddy transports are weak, the diabatic circulation is also weak and the slope is small. Using a simple beta-plane channel model and an eddy diffusion parameterization for the eddy potential vorticity and tracer transports, we show that the slope is a maximum for a value of eddy diffusion such that the dynamical time scale is between the radiative and chemical time scales.