Abstract
A new formulation of an approximate conservation relation of wave-activity pseudomomentum is derived, which is applicable for either stationary or migratory quasigeostrophic (QG) eddies on a zonally varying basic flow. The authors utilize a combination of a quantity A that is proportional to wave enstrophy and another quantity
A dynamical interpretation is presented for each term of such a phase-independent flux of the authors or Plumb. Terms that consist of eddy heat and momentum fluxes are shown to represent systematic upstream transport of the mean-flow westerly momentum by a propagating wave packet, whereas other terms proportional to eddy streamfunction anomalies are shown to represent an ageostrophic flux of geopotential in the direction of the local group velocity. In such a flux, these two dynamical processes acting most strongly on the node lines and ridge/trough lines of the eddy streamfunction field, respectively, are appropriately combined to eliminate its phase dependency. The authors also derive generalized three-dimensional transformed Eulerian-mean equations with the residual circulation and eddy forcing both expressed in phase-independent forms.
The flux may not be particularly suited for evaluating the exact local budget of M, because of several assumptions imposed in the derivation. Nevertheless, these assumptions seem qualitatively valid in the assessment based on observed and simulated data. The wave-activity flux is a useful diagnostic tool for illustrating a“snapshot” of a propagating packet of stationary or migratory QG wave disturbances and thereby for inferring where the packet is emitted and absorbed, as verified in several applications to the data. It may also be useful for routine climate diagnoses in an operational center.
Corresponding author’s address: Koutarou Takaya, Dept. of Earth and Planetary Sciences, Graduate School of Science, University of Tokyo, Sci. Bldg. 1, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.