Eliassen-Palm flux

The formalism of the transformed Eulerian mean circulation shows that meridional transport in the middle atmosphere is generated primarily by non-local momentum forcing associated with wave dissipation. This forcing, represented by the Eliassen-Palm flux divergence in equation (3.67), acts as an extratropical pump producing strong upward air motions in the tropics and downward return ... 

Downward-control principle An analytical result stating that the steady-state, zonal-mean vertical velocity at a given level is a function of the Eliassen-Palm flux divergence integrated vertically between the level in question and the top of the atmosphere. 

Eliassen-Palm flux A measure of the pseudomomentum associated with vertically propagating atmospheric waves. Its divergence gives the acceleration of the zonal-mean zonal wind by atmospheric waves. 

Non acceleration theorem A statement of the conditions under which the divergence of the Eliassen-Palm flux vanishes. According to the theorem, the EP flux divergence is nonzero only for transient or dissipating waves. 

Figure 3.34 Eliassen-Palm (EP) flux (arrows) and its divergence (dashed lines) represented as a function of altitude and latitude for January. The EP flux propagates in the stratosphere only during winter. The EP flux divergence is associated with Rossby wave transience and dissipation. Courtesy of W. Randel, NCAR (2001).

This result may be thought of as a generalized Eliassen-Palm theorem because it shows the relationship between EP flux divergence and nonsteady, nonconservative perturbations. The term in parentheses on the left of Eq. (119) is a measure of wave activity, so that Eq. (119) clearly shows the relationship between EP fluxes and wave activity. 

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