On the Role of Potential Vorticity Perturbations in the Spontaneous Generation of Gravity Waves

Abstract

Mechanisms of internal gravity wave generation have been intensively studied because of the role of gravity waves in the large-scale atmospheric circulation. One source of gravity waves that is poorly understood is spontaneous wave generation by the low frequency mesoscale and large scale motions in the atmosphere. In this work, we study the interaction among potential vorticity perturbations and gravity waves in a stably stratified, horizontally sheared zonal flow, with the goal of clarifying the role of these interactions in the process of spontaneous wave generation. We find that vorticity perturbations amplify in energy due to downgradient Reynolds stress when their phase lines tilt towards the west with increasing latitude for a northward increasing shear flow. For a Froude number of order one, there is strong coupling between the vorticity perturbations and the gravity wave manifold. As a result, the large growth attained by vorticity perturbations is transferred to propagating gravity waves. When the flow geometry permits wave propagation, significant gravity wave emission occurs.