Reconstructing vertical velocities and heat fluxes in the Southern Ocean from SWOT SSH fields

Accessing vertical velocities and heat fluxes globally represents today a major gap in our understanding of the 3D ocean dynamics. SWOT is showing a great advance, namely a sea level resolution up to ten times better than traditional nadir-looking altimeters, allowing for the observation of fine-scale ocean dynamics. Mesoscale dynamics account for 80% of the total kinetic energy in the ocean and smaller mesoscales and sub-mesoscales contribute 50% of the total vertical velocity variance in the upper ocean. In this work, we explore the potential of using SWOT 2D surface topography data to reconstruct vertical velocities and vertical heat fluxes below the Southern Ocean mixed layer using surface Quasi-Geostrophic (sQG) theory for the vertical projection. The upper ocean 3D circulation is calculated from SWOT’s science phase 2D sea surface height (SSH) observations and validated with multi-sourced in-situ data (high-resolution XBT and CTD sections from the SURVOSTRAL and ACC-SMST SWOT CalVal campaigns in the Southern Ocean south of Tasmania). The performance of the SQG methodology, in terms of spatial and temporal correlation, is first assessed using the COAS coupled ocean-atmosphere simulation,  and we find that the SQG reproduces both the spatial distribution and the overall regional variability of the mesoscale structures below the Southern Ocean mixed layer. We then apply the SQG methodology to a high-resolution SSH regional mapping of 3 to 5 days of SWOT science phase data to obtain reconstructed vertical velocities and vertical heat fluxes. The results are validated over the in-situ XBT and CTD data collected in the same time period. The observable structures, resolved dynamics and errors involved in this fine-scale reconstruction are discussed.