Ocean eddies influence regional and global climate through mixing and transport of heat and properties. One of the most recognizable and ubiquitous feature of oceanic eddies are coherent vortices with spatial scales of tens to hundreds of kilometers, frequently referred as “mesoscale eddies”. Coherent mesoscale eddies are known to transport properties across the ocean and to locally affect near-surface wind, cloud properties, and rainfall patterns. Although coherent eddies are ubiquitous, their climatology, seasonality, and long-term temporal evolution remains poorly understood. Here, we examine the kinetic energy contained by coherent eddies and present the seasonal, interannual and long-term variability using satellite observations between 1993 to 2019. A total of $\sim$37 million coherent eddies are detected in this analysis. Around 50% of the kinetic energy contained by ocean eddies corresponds to coherent eddies. Additionally, a strong seasonal cycle is observed, with a 3-6 months lag between the wind forcing and the response of the coherent eddy field. The seasonality of the number of coherent eddies and their amplitude reveals that the number of coherent eddies responds faster to the forcing ($\sim$3 months), than the coherent eddy amplitude (which lags by $\sim$6 months). This seasonal cycle is spatially variable, so we also analyze their climatology in key oceanic regions. Our analysis highlights the relative importance of the coherent eddy field in the ocean kinetic energy budget, implies a strong response of the eddy number and eddy amplitude to forcing at different time-scales, and showcases the seasonality, and multidecadal trends of coherent eddy properties.

Yvan Dossmann

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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.