Identifying internal waves in complex flow fields is a long-standing problem in fluid dynamics, oceanography and atmospheric science, owing to the overlap of internal waves temporal and spatial scales with other flow regimes. Lagrangian filtering — that is, temporal filtering in a frame of reference moving with the flow — is one proposed methodology for performing this separation. Here we (i) describe a new implementation of the Lagrangian filtering methodology and (ii) introduce a freely available, parallelised Python package that applies the method. We show that the package can be used to directly filter output from a variety of common ocean models including MITgcm, ROMS and MOM5 for both regional and global domains at high resolution. The Lagrangian filtering is shown to be a useful tool to both identify (and thereby quantify) internal waves, and to remove internal waves to isolate the ‘balanced’ flow field.

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 internal waves as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream internal wave beams due to a Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a super-buoyancy (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] 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.