Internal wave generation and its impact at the island Shelf
This doctoral dissertation provides an integrated analysis of island-shelf circulation and the generation of nonlinear internal waves (NIWs) in the Madeira Archipelago, a region characterized by steep bathymetry, a narrow insular shelf, and strong atmosphere–ocean interactions. Although Madeira has been widely studied in terms of wake dynamics, mesoscale variability, and wind–island interactions, no systematic assessment of internal wave generation had been conducted in this area of the northeastern Atlantic. This thesis addresses this major gap by combining in-situ observations, satellite imagery, and high-resolution numerical modeling. The first component examines island-shelf circulation under different forcing regimes, including tides, local winds, and large-scale background flow. A high-resolution (1 km) COAWST configuration is used together with ADCP measurements, meteorological stations, Sentinel imagery, and surface drifters. Results show that Madeira’s coastal circulation is governed by the superposition of these forcings, producing alternating patterns of retention, recirculation, and coastal transport. The spatial heterogeneity of these patterns is strongly controlled by the steep and irregular island topography. The second component presents the first direct observational evidence of NIW generation along the ridge connecting Madeira and the Desertas Islands. Using long-term mooring data, a Wirewalker profiler, a thermistor chain, and hydrographic observations, the analysis documents isopycnal displacements, internal bores, and phase-dependent variations of the Froude number associated with the barotropic tidal cycle. Both Sentinel-1 SAR and Sentinel-2 optical imagery reveal surface signatures consistent with internal wave fronts aligned with the ridge. Based on these observations, a new conceptual model is proposed linking tidal-phase transitions, subcritical–supercritical flow regimes, and the periodic release of nonlinear internal waves. The third component evaluates the ability of a hydrostatic numerical framework (ROMS/COAWST) to reproduce these processes. Three vertical mixing schemes—GLS, MY25, and LMD—are tested. The MY25 configuration shows the highest skill in reproducing observed density structure, isopycnal displacement, Richardson-number variability, and the timing of NIW generation and propagation. Using this optimal configuration, the analysis is extended along the entire Madeira–Desertas Ridge, revealing pronounced spatial heterogeneity in the intensity, location, and extent of NIW activity. Overall, this dissertation establishes a comprehensive dynamical framework for understanding stratified flow–topography interactions in Madeira. It provides the first observational characterization of NIWs in the region, demonstrates their tidal-phase dependence, and identifies the physical mechanisms underpinning their release. It further shows that, when properly configured, high-resolution hydrostatic models can be effective tools for investigating nonlinear internal waves in steep and complex insular environments. The findings offer new insights applicable to other deep-ocean island systems and contribute to improving the representation of internal wave processes in coastal and regional ocean models.
circulación insular, ondas internas no lineales, COAWST, Madeira–Desertas Ridge, estratificación, mezcla vertical, nonlinear internal waves, island-shelf circulation, stratification, vertical mixing
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