Rapid Energy Exchange Between Balanced Eddies and Near-Inertial Waves at Fronts.

Inertia-gravity waves, mesoscale eddies, and density fronts are ubiquitous in the ocean. Classical theory predicts that the interaction between the fast, unbalanced waves and the slow, balanced eddies should be weak. A new theory will be described that 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 near-inertial waves 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 theory predicts that the kinetic energy transferred from eddies is ultimately lost to the unbalanced ageostrophic circulation and hence the near-inertial waves play a catalytic role in loss-of-balance. The theory is tested with numerical simulations configured with an array of barotropic eddies that strain a density front. Simulations run with and without a field of near-inertial waves are compared to isolate the wave-mean flow interactions. The modifications of the polarization relations and consequent energy exchange with the mean flow predicted by the theory are realized in the simulations. However, unlike the theory, inhomogeneities in the wave field are allowed in the simulations and are analyzed to quantify the wave-driven accelerations of the mean flow.