Eliassen-Palm Diagnostics of Wave-Mean Flow Interaction in the GFDL "SKYHI" General Circulation Model

Abstract

The Eliassen-Palm flux is important in analytical studies of small-amplitude waves where it provides a powerful and elegant tool for the description of wave propagation in mean zonal shear flows, as well as for analysis of the effective mean zonal force induced by the waves. Furthermore, it has recently been used as a diagnostic in a number of studies of atmospheric data and numerical models of specific dynamical phenomena. In this paper, we apply it to the GFDL "SKYHI" global general circulation model of the troposphere-stratosphere-mesosphere, and describe computations of the primitive equations, isobaric coordinate form of the Eliassen-Palm flux and its divergence under conditions of annual-mean insolation. The Eliassen-Palm flux diagnostics show a clear picture of planetary wave propagation from the midlatitude troposphere into the stratosphere and mesosphere. In the tropics, the presence of Kelvin waves confuses the picture somewhat (because their phase speeds are eastward with respect to the mean flow) and necessitates additional analysis which is given elsewhere. We find the Eliassen-Palm diagnostics lead to new insights on the forcing of mean flows by the eddies. The implications of the fact that the model waves are not close to "non-acceleration conditions" and the importance of mean diabatic effects in our 30-day average statistics are considered in Appendix B. An important finding is that zonal westerly flows are strongly decelerated by eddies in the midlatitude upper troposphere and the mesosphere, in sharp contrast to the apparent implication of traditional zonal mean balances. On the other hand, the forcing of mean accelerations by waves in the tropics is essentially in agreement with that found in earlier studies. The above inferences from the Eliassen-Palm diagnostics concerning the effect of eddies on zonal flows have been tested in a companion model experiment in which eddies propagating out of the troposphere are strongly damped. This experiment shows the resultant zonal flow accelerations to be fully consistent with the inferences from the Eliassen-Palm diagnostics.