Assessing Submesoscale Sea Surface Height Signals From the SWOT Mission

The sea surface height (SSH) field measured by Surface Water and Ocean Topography mission's wide-swath altimeter is analyzed with a focus on submesoscale features. Along-track wave number spectra of SSH variance are estimated for the global ocean using the 1-day repeat period from March 26 to 10 July 2023. In regions with an energetic mesoscale eddy field, the spectra have a mesoscale plateau, a steep drop-off due to balanced submesoscale turbulence, and a much flatter power-law tail at small scales. These spectra are characterized by fitting a spectral model. For the balanced signal, this fit yields a power-law exponent between -4 and -6 for most regions, broadly consistent with expectations and previous observations. The amplitude of the distinct small-scale signal, which typically dominates at wavelengths less than 30 to 50 km, is strongly correlated in time and space with the height of surface gravity waves, suggesting aliased wave signals as the most likely source. A simple method is proposed to isolate the balanced signal in regions with negligible internal tides. Maps of the balanced signal in the Antarctic Circumpolar Current show compact cyclones with geostrophic relative vorticities frequently in excess of the local planetary vorticity, challenging the quasi-geostrophic framework commonly used to interpret altimetric data.

This work was supported by NASA Grants 80NSSC20K1140 and 80NSSC24K1652. The authors gratefully acknowledge stimulating discussions with Albion Lawrence and Jack Skinner as well as feedback on the manuscript from Dudley Chelton, Carl Wunsch, and two anonymous reviewers.

This work was supported by NASA Grants 80NSSC20K1140 and 80NSSC24K1652.