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 limits our ability to accurately represent energy transfers and mixing induced by these waves. The Vitória–Trindade Ridge off the Brazilian shelf is a relevant natural laboratory to investigate these processes, as a hotspot for internal tides generation embedded in a region of intense mesoscale activity. To assess how seasonal stratification and mesoscale variability modulate internal tides, we compared a 27-year satellite altimetry record with a high-resolution (1/36°) regional simulation using NEMO v4.0.2. This joint analysis allows us to characterize the generation, propagation, and dissipation of internal tides under two contrasted regimes: austral winter (defined here from May to October) marked by a deep pycnocline, and austral summer (defined here from November to April) with a shallower and sharper seasonal pycnocline. Both model and observations depict six intense, in-phase beams of the baroclinic flux propagating southward from the ridge. The first two have a wavelength of 100 km approximately corresponding to the mode-1 of propagation, while more distant beams are spaced by about 50 km only, which likely corresponds to the mode-2 of propagation. Quantification from the model shows that generation rates are 5 %–15 % higher in summer than in winter. Dissipation occurs predominantly near the ridge (45 %) but also extends offshore (40 %), reaching beyond 2–3 mode-1 wavelengths. In the open ocean, dissipation is up to 40 % stronger in winter, leading to a weaker baroclinic flux propagating on shorter distances compared to summer. Altimetry confirms seasonal variations in both wavelength and amplitude, especially for mode-2 internal tides. Finally, 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. This study demonstrates that mesoscale variability and seasonal stratification jointly modulate the coherence and energy pathways of internal tides. These findings are essential for improving predictions of the incoherent tide and for interpreting high-resolution altimetric observations.