Southern Ocean 3D Eddy Diagnostics Derived From SWOT

Abstract

Observations of sea surface height (SSH) from the Surface Water and Ocean Topography (SWOT) satellite have demonstrated remarkable potential for resolving mesoscale and submesoscale ocean features, which are crucial for deriving vertical velocities, a key variable for understanding the transport of heat, carbon, and nutrients between the ocean surface and interior. In the mesoscale energetic region south of Tasmania, we evaluate the accuracy of SWOT small-scale observations using high-resolution ($\sim$8 km) in situ observations of surface tracer fields and vertical temperature. Surface features derived from SWOT align with subsurface horizontal temperature gradients derived from the in situ measurements. These fine scales (100 km) reveal dynamic phenomena, such as a sudden front deflection near the Macquarie Ridge, obscured in conventional low-resolution products. We then quantify the contributions of these small scales to the currents' energy and shape, compared to the larger mesoscales ($>$100 km) observable with traditional nadir-looking altimetry. Although larger scales predominantly influence the geostrophic velocity, smaller scales contribute significantly to current stretching and straining, showing up to threefold stronger strain and tenfold stronger vorticity than larger scales in a few hotspots. Finally, we investigate the contribution of these small scales to the vertical velocities $\left(w\right)$ down to 1,000 m that are reconstructed using effective surface Quasi-Geostrophic theory. SWOT-derived $w$ exhibit twice the amplitude compared to nadir altimetry, underscoring SWOT's capacity to resolve energetic meso- and submesoscale ocean dynamics. These results highlight the need to fully harness SWOT's high-resolution data in gridded products, as current smoothing limits the retention of valuable small-scale information.