Alejandra Sanchez-Rios (University of Hawaii, USA); Leo Middleton (University of Gothenburg, Sweden); Harper Simmons (University of Washington, USA); Jennifer MacKinnon (Scripps Institution of Oceanography, USA); Ilker Fer (University of Bergen, Norway); Ailin Brakstad (University of Bergen, Norway)

Satellite altimetrically derived velocity fields have provided valuable insights into horizontal stirring and tracer transport at the ocean surface, as well as mixing along isopycnals in the ocean interior. Here, we present novel results demonstrating the powerful capability of SWOT to resolve the connectivity between surface-layer stirring and subsurface isopycnal mixing in the Nordic Seas, a region of critical climatic importance, where warm, saline North Atlantic waters mix with cold, fresh polar waters to form North Atlantic Deep Water that feeds the lower limb of the Atlantic Meridional Overturning Circulation. By combining SWOT-derived velocity fields with in-situ observations of a subducted warm water mass in the Jan Mayen channel, we trace the evolution of these flows across the Arctic front and along isopycnal surfaces from the surface to the interior.
Understanding the role of mesoscale and submesoscale eddies in water mass transformation in the Nordic Seas has been limited by the coarse resolution of geostrophic velocities derived from conventional nadir altimetry at high latitudes. The finer-scale velocity fields provided by SWOT, in combination with high resolution in-situ measurements, reveal the critical role of small-scale eddies in stirring tracers in these seas. The in-situ data resolve the structure of warm, saline Atlantic water that was subducted beneath colder polar waters along the boundary of the Arctic front and was being transported away from the front by eddying flows. Dynamic height inferred from density profiles agrees with SWOT observations, and the patchy structure of the subducted water suggests stirring by eddies with horizontal scales of ~20 km that are consistent with those resolved by SWOT.
Synthetic sea surface temperature (SST) fields advected using SWOT velocities reproduce stirring patterns seen in the observations. However, these patterns align not with satellite-observed SST, but rather with the along-isopycnal temperature structure of the subducted water mass. Together, these results support a scenario in which surface waters are stirred, subducted in regions of strong lateral gradients, and continued to be advected below the surface by SWOT-resolved eddying flows. Estimates of along isopycnal diffusivity from in-situ observations (~300 m^2/s) closely match those derived from the advected tracer fields and lie within the range of previous drifter-based estimates (O(100–500 m^2/s)). These findings offer new insight into the role of small-scale processes and three-dimensional transport in Nordic Seas water mass transformation, and highlight a new frontier in the use of SWOT to understand stirring by small-scale upper-ocean macroturbulence.