Rapid energy exchange between inertia-gravity waves and mesoscale eddies at fronts

Inertia-gravity waves, mesoscale eddies, and density fronts are ubiquitous in the ocean and atmosphere. Classical theory predicts that the interaction between the fast, unbalanced waves and the slow, balanced eddies should be weak. A new theory demonstrates, however, that this interaction can be strong in regions of frontogenesis, where mesoscale strain intensifies lateral density gradients. Such frontogenetic strain leads to an exponentially fast increase in the vertical shear of the along-front geostrophic flow and a concomitant cross-front ageostrophic circulation that is vertically-sheared as well. These changes in geostrophic flow modify the polarization relation of inertia-gravity waves that are present, making their horizontal velocity rectilinear and resulting in a Reynolds stress that draws energy from the eddies. The process is most effective for waves of low frequency and for a geostrophic flow with low Richardson number. Nonetheless, even in a background flow that is initially strongly stratified, frontogenesis leads to an exponentially fast reduction in the Richardson number, facilitating a rapid energy extraction by the waves. The kinetic energy transferred from eddies is ultimately lost to the unbalanced ageostrophic circulation through shear production and hence the inertia-gravity waves play a catalytic role in loss-of-balance. In the oceans, a large fraction of the kinetic energy is contained in low-frequency inertia-gravity waves and fronts are widespread. Therefore, this mechanism could play a significant role in the removal of kinetic energy from both the internal wave and mesoscale eddy fields.