Influence of the stratospheric potential vorticity distribution on the Brewer-Dobson circulation

Propagation of planetary scale Rossby waves within the stratosphere,

and hence the location of wave-induced zonal mean momentum forcing and

the resulting Brewer-Dobson circulation, is dynamically determined by

the distribution of zonal mean potential vorticity; steep potential

vorticity gradients at the vortex edge, for example, act as a

wave-guide, allowing wave propagation to higher altitudes before wave

breaking occurs. We consider a series of forced-dissipative

experiments to test the effect of the potential vorticity distribution

on the Brewer-Dobson circulation in models of varying horizontal

resolution, the latter acting to limit the extent to which wave

breaking may steepen potential vorticity gradients at the vortex edge.

Under perpetual January radiative conditions and with steady wave

forcing at the tropopause the model system exhibits strong internal

variability, with the polar vortex undergoing a series of

quasi-periodic sudden warmings separated by a period of gradual vortex

recovery under radiative forcing. Although under certain forcing

parameters the horizontal resolution is able to influence

qualitatively the nature of the internal variability, in general, the

time-average vertical upwelling at low latitudes in the lower

stratosphere is relatively insensitive to changes in the horizontal

resolution; changes in the wave breaking are balanced by changes in

diffusion, resulting in similar zonal mean momentum forcing. The

distribution of wave flux convergence, details of the potential

vorticity distribution, and structure of the Brewer-Dobson circulation

are investigated across a range of horizontal resolutions spanning

those currently used in climate modeling.