Modulation of Internal Tides Properties off the Vitória–Trindade Ridge during Contrasted Seasons from Altimetry and a Regional Ocean Model

Abstract. The incoherent fraction of internal tides, generated through interactions with mesoscale eddies and other transient oceanic features, remains poorly understood and challenging to predict. This knowledge gap limits our ability to accurately represent energy transfers and mixing processes in the ocean induced by these waves. The Vitória–Trindade Ridge, located off the Brazilian shelf, provides a particularly relevant natural laboratory to investigate these processes, as it constitutes a hotspot for internal tides generation embedded in a region characterized by strong mesoscale activity and vigorous eddy fluxes. To assess how seasonal stratification and mesoscale variability modulate internal tide properties, we combined two complementary approaches: internal tide signals were extracted from a 27-year satellite altimetry record and compared with a high-resolution (1/36°) regional simulation performed with the ocean model NEMO v4.0.2. This joint analysis allowed for a consistent characterization of the generation, propagation, and dissipation of internal tides under two contrasted oceanic regimes. Austral winter (defined here from May to October) is marked by a deep pycnocline, whereas austral summer (defined here from November to April) is associated with a shallower, sharper seasonal pycnocline. Both the model and observations depict six intense, in-phase reflection beams propagating southward from the ridge. The first two beams of reflection are associated with a wavelength of 100 km approximately corresponding to the mode 1 of propagation, while the beams further from the ridge are separated by about 50 km only, which likely corresponds to the second mode of propagation. Quantification from the model show that internal tide generation rates are 5 to 15 % higher in summer than in winter. Dissipation occurs predominantly in the vicinity of the ridge (45 %) but also extends offshore (40 %), reaching beyond 2 to 3 mode-1 wavelengths. In the open ocean, dissipation rates are up to 40 % higher during winter than in summer. In the model, these seasonal differences result in stronger baroclinic fluxes that propagate farther south during summer, whereas winter fluxes are more rapidly dissipated. Altimetric observations further confirm pronounced seasonal variations in both wavelength and amplitude, in particular for mode-2 internal tides. In addition, a representative case of interaction between internal tides and a mesoscale eddy is documented under summer conditions, showing deviation and diffraction of the baroclinic flux as it encounters the eddy. This study demonstrates that mesoscale variability and seasonal stratification act jointly to modulate the coherence and energy pathways of internal tides. These findings are essential for improving predictions of the incoherent tide and for guiding the interpretation of recent high-resolution altimetric observations.