SWOT Observations of the Mode-1 M2 Internal Tide in the Southern Ocean

The Surface Water and Ocean Topography (SWOT) satellite mission, launched in late 2022, provides two-dimensional sea surface height anomaly observations resolving ocean variability down to 15–30 km scales, offering new opportunities to study internal tides. This study investigates mode-1 M$_2$ internal tides in the Southern Ocean using data collected during the Calibration/Validation (Cal/Val) phase from 29 March to 10 July 2023, during which near-daily sampling shortens the tidal aliasing period and reduces mesoscale interference. Surface amplitudes, phases, and propagation directions are derived using a two-dimensional plane-wave fitting approach that resolves multiple wave components, with amplitude and phase uncertainties quantified. Despite the short (102-day) record, internal tides are reliably resolved, with consistent estimates at crossover regions within the expected range of mesoscale variability. SWOT reveals spatially variable mode-1 M$_2$ internal tides across the Southern Ocean, in good agreement with the High-Resolution Empirical Tide (HRET) model, the reference product for internal tide corrections in SWOT altimetry observations. SWOT Cal/Val observations also provide new measurements in regions previously less detectable by conventional nadir altimetry, benefiting from the mission’s swath coverage and fast sampling, particularly in eddying regions such as Drake Passage and the Southwest Indian Ridge. SWOT further reveals interactions between mode-1 M$_2$ internal tides and mesoscale variability: coherent beams weaken after crossing fronts, while phase curvature appears within regions of strong, spatially coherent mesoscale flow, consistent with refraction. Depth-integrated energy and energy flux of mode-1 M$_2$ internal tides are estimated through modal structure functions derived from local climatological stratification.