The gradient genesis of stratospheric trace species in the subtropics and around the polar vortex

We investigated mechanisms that control the formation and decay of meridional gradients in stratospheric trace species in the subtropics and around the polar vortex. We developed a gradient genesis equation that uses mass-weighted isentropic zonal means. The trace species included in the analysis was global $N_2O$ data output from a global chemical transport model; the analysis showed that shearing deformation in the mean vertical transport increased the meridional gradient of the trace species from the subtropics to mid-latitudes. Shearing deformation is related to overturning in the Brewer-Dobson circulation. Mean meridional transport advects the subtropical edge of the trace species toward mid-latitudes, and the stairstep effect in the eddy transport (steepening at the edge of the well-mixed region because of a meridional gradient in the diffusion coefficient) nearly compensates for the reduction in the gradient that arises because of mean meridional transport around the subtropics. The gradient at the edge of the trace species in the middle stratosphere in the subtropics reaches a maximum in early spring and autumn. Mechanisms controlling the evolution of these gradients in the subtropics differ between the two seasons. Development of the autumnal subtropical tracer gradient maximum is mostly controlled by shearing deformation in the mean-meridional flows. In spring, the stairstep effect also influences the generation of the gradient at the edge. A significant difference also exists between gradient genesis mechanisms in the subtropics and around the Antarctic polar vortex.