The background state used must also be a solution to the primitive equations for the conservation laws to apply. Here, it is defined by the modified Lagrangian mean state obtained by re-arranging the observed 3-D atmospheric state to be zonally symmetric while conserving both mass and circulation enclosed by potential vorticity contours in isentropic layers. This state would be steady if the full flow were adiabatic and frictionless.
The diagnostics have been applied to ERA-Interim data and features of the slowly varying background state and the wave activity on it are highlighted. Events of poleward movement of wave activity at the tropopause are associated with the meridional extension of ridges towards the pole and a baroclinic growth rate rather than group velocity. Periods of westward, stationary (blocked) or eastward propagation ("zonal regime") are shown to be related to the speed obtained from the ratio of hemispheric pseudoenergy to pseudomomentum. Perhaps most surprisingly, variations in this speed are dominated by the lower boundary contribution to pseudoenergy with the westward phases being associated with stronger boundary potential temperature perturbations.