Asymmetric internal tide generation in the presence of a steady flow

Abstract

The generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi-steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of IWs as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream IW beams due to a quasi-Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a superbuoyancy (𝜔 > N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream-bias energy flux-and consequently net horizontal momentum-described in Shakespeare (2020, https://doi.org/10.1175/JPO-D-19-0179.1) is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW-induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.Plain Language Summary When tides and currents interact with abyssal topographies, such as ridges and hills, they generate internal waves that propagate in the ocean interior. The energy transported by these waves sustains the largest scale oceanic motions. To improve our understanding of how and where energy is transferred to oceanic currents, an important step is to describe the fate of internal waves, from their generation to their breaking. Previous studies have independently described the dynamics of internal waves generated by tides, or by a steady current. Here we combine the two types of currents-a situation that is met at many oceanic sites-using laboratory experiments and a linear model. The combined currents cause an asymmetry in the internal wave structure. Internal waves are more energetic on the upstream side of the ridge, a phenomenon that is amplified when decreasing the ridge width. The measured energy matches the model prediction in all but one experiment. This gap is likely related to the formation of a vertical swirl close to the ridge that limits energy transfers to internal waves. These results contributes to improve our understanding of the internal wave dynamics and to better represent their effects in oceanic models.

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

“…Readers can access the supporting data and linear model via an online repository available at https://doi. org/10.5281/zenodo.3889612 (Dossmann et al, 2020).…”