The Brewer-Dobson circulation is a mechanically driven global-scale circulation that transports mass and chemical tracers (such as ozone and water vapor) in the stratosphere from the cold tropical tropopause to warmer extratropical regions. Its mean meridional mass transport part is usually quantified in terms of the residual stratospheric circulation in the transformed Eulerian mean (TEM) sense.
We study transport within this stratospheric circulation by means of (hypothetical) trajectories along the residual circulation, that is by means of trajectories that are driven by the residual mean meridional and vertical velocities. Diagnostics based on these residual circulation trajectories support the notion of two separated stratospheric circulation branches: a deep branch with upwelling over the inner tropics and downwelling over the polar regions, and a shallow branch with the main upwelling significantly displaced off the equator and downwelling over the mid-latitudes. Residual circulation transit times of air traveling from the tropical tropopause to the extratropical lowermost stratosphere are evaluated and their seasonal cycle is discussed in light of observed mean age of air estimates. Recent climate trends in the above described structural characteristics of the stratospheric circulation will be discussed.