PHD defence of Zoi Kourkouraidou: Uncovering the effects of agulhas eddies on low and high internal tide modes and their energetics in high-resolution icon

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Abstract

The interaction between internal tides and mesoscale eddies is one of the processes which are thought to significantly contribute to energy dissipation and diapycnal mixing in the open ocean. Yet the details of such interactions still remain poorly understood. Observational, theoretical as well as numerical studies in idealized conditions have tried to shed light to these interactions. However, these can only capture aspects of the effects of mesoscale eddies on internal tides that are very localized, confined in time or oversimplified.

To date, no study has examined eddy - internal tide interactions using a comprehensive ocean model that resolves both features in a realistic setting. In this thesis I investigate these interactions in the southeast Atlantic, near the Walvis Ridge, where both features are strongly present, using the ocean-only ICON model with very high horizontal resolution.

The study proceeds in three parts: First, focusing on the modal decomposition of the internal tidal (IT) waves resolved and diagnosed by the model, I highlight the limitations of the standard method based on linear wave theory and present an alternative based on data-driven empirical orthogonal function (EOF) analysis, which is more suitable for our model output since it inherently accounts for the influence of the bathymetry and stratification, but also of the mean currents resolved by our model.

Second, focussing on the effects of Agulhas rings on the low and high modes of the internal tide, I use both decomposition methods to quantify the eddy effects during two seasons. An Agulhas ring that crosses a beam of M2 mode-1 IT is causing horizontal refraction, seen as a southward shift of the beam, without significantly altering the vertical component. This indicates that the Agulhas ring does not scatter the mode-1 IT into higher modes. Instead, higher IT modes are found to be trapped within the Agulhas rings before the interaction with mode-1 beams, implying that they do not necessarily arise from this interaction.

Third, focussing on the energetics of the internal tide - eddy interactions we aim to answer whether and to what extent the overall energetics in the study area are altered due to the presence of eddies and whether or not the energy flux of the low-mode IT is more strongly damped in the presence of eddies.

Together, my results offer new insights into IT-eddy interactions and their energetics in a realistic setting and introduce a framework for analyzing internal tides beyond linear wave assumptions, which is essential as computational power and model resolution continue to advance, making the resulting data increasingly complex.