The Role of Boundary Wave Activity in Stationary Periods for Northern Hemisphere Rossby Waves

Pseudomomentum and pseudoenergy are both measures of wave activity for disturbances in a fluid, relative to a background state. Together they give information on the propagation, growth and decay of disturbances. Here, both wave activities are calculated using forms that obey conservation laws for large-amplitude disturbances described by the primitive equations on the sphere in isentropic coordinates. This includes new expressions for the contribution to pseudoenergy where isentropic surfaces intersect the ground, obtained by the Casimir technique. The pseudomomentum flux from large-amplitude theory differs from the E-vector in isentropic coordinates (or the Eliassen-Palm flux in the zonal average) only through the nonlinear horizontal advection of wave activity density.

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.