Interaction of Internal Gravity Waves with Meso-Scale Eddies

Abstract

We investigate the interaction of internal gravity waves (IGW) with mesoscale eddies using the novel numerical Internal Wave Energy Model (IWEM). With IWEM, we integrate the radiative transfer equation to investigate the propagation and refraction of IGWs, and the energy exchange between IGWs and mean (eddying) flow. We evaluate the evolution of a typical IGW spectrum with energy density in physical and wavenumber space along a single column and over an eddy cross-section. We compare the simulations with the observations of a coherent mesoscale eddy in the Canary Current System. Results show that the changes in IGW energy are dominated by wave propagation effects, wave-mean flow interaction and wave breaking at critical layers, while wave capture effects are two orders of magnitude smaller. The wave propagation terms transport IGW energy from the eddy center to the rim. Energy gain by wave-mean flow interaction is dominated by low-frequency waves in the eddy center, while high-frequency waves are trapped in a cyclo-stationary up-/downward propagation cancelling out their gain or loss of energy. Energy loss by wave-mean flow interaction or wave breaking is largest at the eddy rim, where IGWs undergo a downscale energy transfer to small vertical scales and to the inertial frequency. Mooring observations agree with our model results on higher IGW energy values at the eddy center compared to the rim.  Following the Osborn-Cox relation, wave-breaking induced vertical diffusivities are found to be maximal at the eddy rim and range between κ≅10-7-10-5m2s-1, partly in range with the observed values in the ocean. The interaction of IGWs and mesoscale eddies is therefore a plausible process for explaining the near-surface enhanced mixing.