Surface Quasi Geostrophic Reconstruction of Vertical Velocities and Vertical Heat Fluxes in the Southern Ocean: Perspectives for SWOT.

Mesoscale currents account for 80% of the ocean's kinetic energy, whereas submesoscale currents capture 50% of the vertical velocity variance. SWOT's first sea surface height (SSH) observations have a spatial resolution an order of magnitude greater than traditional nadir‐looking altimeters and capture mesoscale and submesoscale features. This enables the derivation of submesoscale vertical velocities, crucial for the vertical transport of heat, carbon and nutrients between the ocean interior and the surface. This work focuses on a mesoscale energetic region south of Tasmania using a coupled ocean‐atmosphere simulation at km‐scale resolution and preliminary SWOT SSH observations. Vertical velocities (w), temperature anomalies and vertical heat fluxes (VHF) from the surface down to 1,000 m are reconstructed using effective surface Quasi‐Geostrophic (sQG) theory. An independent method for reconstructing temperature anomalies, mimicking an operational gridded product, is also developed. Results show that sQG reconstructs 90% of the modeled w and VHF rms at scales down to 30 km just below the mixed layer and 50%–70% of the rms for scales larger than 70 km at greater depth, with a spatial correlation of ∼ ${\sim} $ 0.6. The reconstruction is spectrally coherent (>0.65) $(> 0.65)$ for scales larger than 30–40 km at the surface, slightly degrading (∼ ${\sim} $ 0.55) at depth. Two temperature anomaly data sets yield similar results, indicating the dominance of w on VHF. The RMS of sQG w $w$ and VHF derived from SWOT are twice as large as those derived from conventional altimetry, highlighting the potential of SWOT for reconstructing energetic meso and submesoscale dynamics in the ocean interior. Plain Language Summary: This work focuses on vertical ocean dynamics induced by small‐scale ocean surface currents, especially those at the mesoscale and submesoscale (20–100 km in diameter). These ocean vertical velocities pump heat and carbon from the surface to depth and carry nutrients important for biomass development. Our motivation is the first data from the new satellite SWOT (Surface Water and Ocean Topography), which provides observations of surface geostrophic currents at much higher resolution compared to traditional satellites. We study a Southern Ocean area near Tasmania using a realistic coupled ocean‐atmosphere simulation at a km‐scale resolution and initial SWOT data and reconstruct vertical velocities, temperature, and heat fluxes using a vertical projection theory. Results show that the vertical reconstruction coherently represents ocean vertical velocities and heat fluxes below the surface mixed layer larger than 30–40 km and that smaller scales are not well reconstructed. In the computation of vertical heat fluxes, we compare two different ways of measuring temperature anomalies, finding that, overall, the difference is not significant because vertical velocities dominate the heat fluxes. This study highlights the potential of SWOT for studying small‐scale ocean dynamics at the surface and reconstructed at depth. Key Points: Vertical velocities can be coherently reconstructed in the Southern Ocean at scales down to 30–40 kmVertical heat fluxes can be retrieved using effective surface quasi‐geostrophic reconstruction and are dominated by vertical velocitiesVertical velocities reconstructed from SWOT observations have 50% more variance than those derived from traditional altimetry [ABSTRACT FROM AUTHOR]