A numerical study of the three-dimensional structure of the Brewer-Dobson circulation

A 3-dimensional framework for studying the middle atmosphere in relation to upper-tropospheric structure is developed. The numerical model is formulated from the primitive equations in isentropic coordinates, which directly characterize diabatic processes forcing the Brewer-Dobson circulation. It is anchored in observed tropospheric variability, so integrations provide middle atmospheric behavior that tracks observed variations in the upper troposphere.;The numerical framework achieves enhanced performance by incorporating eigen-functions of the primitive equations to represent structure spectrally in all three coordinates. Scale-selective dissipation can then be applied entirely at 6th order, which leaves all but the shortest vertical scales undamped. This feature allows vertical diffusion to be made small enough to represent stratospheric transport as advective (rather than diffusive) for most of the scales carried in the integration. Transport across the model's lower boundary, which is positioned near the tropopause, is calculated prognostically from diabatic processes in the middle atmosphere, in concert with tropospheric influences imposed at the bottom.;Integrations forced by observed tropospheric behavior are validated against climatological structure, as well as tracer behavior deduced from satellite measurements. The isentropic formulation, together with 6th order vertical dissipation, enable potential vorticity to be conserved quite accurately. The results throw light on the 3-dimensional structure of the Brewer-Dobson circulation and how it follows from diabatic processes operating in the middle atmosphere and tropospheric processes.;Lagrangian calculations of diabatic motion, supported by its 3D structure, indicate that the Brewer-Dobson circulation receives a major component of its forcing from thermal dissipation of planetary waves, especially at high latitudes. Associated with irreversible heat transfer, that nonconservative behavior is introduced whenever the circulation is disturbed from polar symmetry. Air orbiting about the displaced vortex then experiences a large cyclic departure from radiative equilibrium. This renders its heat transfer irreversible, introducing a hysteresis into the (otherwise cyclic) potential temperature of individual air parcels. Successive orbits about the displaced vortex then yield a systematic drift of air to lower theta, which is accompanied by compression and adiabatic warming. Were it not for this contribution to diabatic motion, wintertime temperatures over the Arctic would be several tens of Kelvin colder.