What controls the overturning circulation of the stratosphere?

Tracer analysis is blended with residual mean circulation and downward control calculations to explore the stratospheric overturning circulation, or Brewer-Dobson Circulation, in idealized general circulation models. In particular, we focus on the role(s) of the troposphere and stratosphere in controlling the strength and depth of the circulation. While the troposphere appears to largely set the input of planetary and synoptic waves into the stratosphere, subsequent propagation and dissipation of the wave activity is substantially affected by the state of the stratosphere. Thus the overturning circulation depends critically on coupling between the two layers of the atmosphere.

The age of air spectrum complements dynamical circulation metrics, and highlights the importance of isentropic mixing in the transport of mass through the stratosphere. It also reveals the potentially adverse effects of numerical diffusion on chemical transport in models. We find that that overturning circulation can be quite sensitive to model numerics (pseudo-spectral versus finite volume) and resolution, highlighting the importance of small scale motions in the stratosphere, even though its energy spectrum is dominated by large scale planetary waves. Systematic resolution sweep experiments enable us to set a benchmark for the net overturning circulation, and implications of these numerical sensitivities for more comprehensive models are explored.