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

Mechanisms that control the formation and decay of meridional gradients in stratospheric trace species in the subtropics and around the polar vortex are investigated using a gradient genesis equation that uses mass-weighted isentropic zonal means. Application of this method to global nitrous oxide (N₂O) data output from a global chemical transport model shows that mean vertical transport increases the meridional tracer gradient from the subtropics to midlatitudes through the shearing deformation, particularly related to overturning of the Brewer-Dobson circulation. Mean meridional transport advects the subtropical tracer gradient toward midlatitudes, while the eddy stairstep effect, steepening at the edge of the well-mixed region because of a meridional gradient in the diffusion coefficient, increases the tracer gradient in the subtropics and around the polar vortex. Mechanisms controlling the evolution of the tracer gradients in the subtropics differ between spring and autumn. The autumnal subtropical tracer gradient maximum is generated mainly from shearing deformation of the mean vertical transport, but less from mean and eddy meridional fluxes. In spring, the eddy stairstep effect also contributes to the generation of the subtropical tracer gradient maximum. Strong divergence forces stretching deformation that causes the springtime subtropical tracer gradient to decay. The gradient genesis mechanism around the Antarctic polar vortex is significantly different from that in the subtropics. Development of the tracer gradient around the Antarctic polar vortex is mostly controlled by mean meridional stretching motion in the middle stratosphere. Vertical advection and eddy smoothing effects flatten the tracer gradient as the polar vortex decays.