Cédric Chavanne (Université du Québec à Rimouski - Institut des sciences de la mer, Canada); Pascal Matte (Environment and Climate Change Canada - Meteorological Research and Development, Canada)

The Surface Water and Ocean Topography (SWOT) mission will significantly improve and expand near-global mapping of tidal cycles that contribute significantly to sea-level variability by collecting, at an unprecedented resolution, sea-level elevation measurements in nearshore areas that were poorly mapped by previous altimetric missions. Despite SWOT’s limitations in capturing tides due to undersampling, mapping tides is a prerequisite for distinguishing them from other physical processes that occur on comparable scales, such as mesoscale and submesoscale eddies and fronts, which are key targets for SWOT. The mechanisms by which energy moves from barotropic and baroclinic tides to smaller scales have been extensively researched in the global ocean. However, tracking the tidal energy budget in coastal shallower areas remains challenging due to the complex nonlinear deformation of sinusoidal tidal waves caused by bathymetric gradients, water discharge, and ambient stratification. Using numerical modeling and SWOT observations, we seek to decipher the tidal energetics of the St. Lawrence Lower Estuary, a major stratified North American waterway strongly driven by tides, and that also received additional attention during SWOT calibration-validation stage and AirSWOT campaign. Our primary finding is that SWOT can observe the prominent surface elevation signal of internal solitary waves generated by the nonlinear steepening of the low-mode baroclinic tide propagating in the St. Lawrence Estuary, which periodically redistributes the tidal energy toward scales of one or a few kilometers. This reveals the potential of SWOT to capture tidal variability at much finer scales previously missed by its predecessors. Future work will focus on understanding the key mechanisms and physical parameters that control the decay of baroclinic tides into internal solitary waves, which could lead to improved detection of baroclinic tides using altimetry.