Mesoscale patchiness in SWOT data

Principle Investigator: J. Tom Farrar (Woods Hole Oceanographic Institution)

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The SWOT satellite will provide a new window into oceanic variability at spatial scales of 10-100 km. This range of spatial scales includes several distinct kinds of physical flows, including mesoscale eddies, internal waves, fronts, filaments, current jets, Rossby waves, and topographic waves. Because of the limited temporal sampling of the SWOT satellite and measurement noise in the 10-100 km range, these distinct phenomena will be difficult to distinguish and clearly identify. Most of the oceanographic signals in the 10-100 km wavelength band have relatively short persistence time scales (hours to days), but some of them will persist across many SWOT orbit repeat cycles.

We expect that time-dependent ocean currents and topography should produce strong variability at scales smaller than the large mesoscale eddies that are well observed by current altimeters. A close look at data from conventional nadir altimetry and ocean model fields reveals that eddy kinetic energy is patchy—that is, there are small-scale variations in the amplitude of oceanic sea surface height and velocity variability. Prior work and our preliminary results suggest that hot spots of ocean eddy activity are co-located with topographic features in much of the world ocean. Gridded versions of conventional altimetry smooth and attenuate flows smaller than about 200 km, so the amplitude of small mesoscale patchiness is likely to be drastically underestimated. SWOT will provide an exciting new window into these underappreciated small mesoscale eddies. We expect these eddies to be detectable even if SWOT noise is greater than anticipated, because they evolve on a much longer timescale than internal waves and other potential sources of SWOT noise, so they should persist after averaging to reduce the noise. A detailed characterization of topographically induced small mesoscale eddies from SWOT data will enable us to build an understanding of their dynamics and to assess their impact on ocean mixing and climate. If correct, our hypothesis that mesoscale eddy kinetic energy exhibits strong variations and patchiness on short spatial scales could have significant implications for parameterizations in climate models.