Global Ocean spectral slope rupture evidenced by SWOT 21-day orbit observations

Embarking both the revolutionary KaRIn 2D imaging SAR interferometer and a Jason-class nadir altimeter, the SWOT mission provides sub-50 km wavelength ocean sea surface height observations. With its unprecedented spatial resolution, coupled with an extremely low noise, the SWOT KaRIn observations will allow us to characterize ocean dynamics for wavelengths well below the capabilities of conventional nadir altimetry.  

Along-track Sea Surface Height (SSH) observations (1 Hz) have been recently used to tease out the spectral characteristics of oceanic variability in the meso- to sub-mesoscale wavelength range (wavelength < 100 km). Conventional altimetry reveals a regime shift in many tropical and subtropical regions, where a quasi-geostrophic energy cascade changes to a flatter, non-geostrophic dynamical regime at smaller wavelengths. Yet at higher latitudes, with smaller Rossby radius, this regime shift was not apparent, hidden by the higher conventional altimetry noise at smaller wavelengths. In the present study, we take advantage of the multi-instrumental characteristics of the SWOT mission, embarking a conventional nadir altimeter in addition to KarIn with its improved signal-to-noise ratio, to characterize the spectral slope rupture at global scale. We use SWOT data available during the first months of the mission’s 21-day repeat orbit and compare these results against latest model and in situ estimates.  

This wavelength scale, where the geostrophic energy cascade becomes dominated by non-geostrophic dynamics (e.g. internal gravity waves including internal tides) is important for using and interpreting SSH-derived geostrophic current data, and to quantify the regions and seasons where the geostrophic assumption breaks down. Our results could therefore be used as an indicator for this scenario, as more high-resolution SWOT data becomes available and is integrated into multi-mission global SSH products and their derived geostrophic currents.