WHOI PO

Title:
Global Dynamics of Mixed Layer Eddy Restratification from SWOT

Abstract:
Submesoscale features play an important role in mixed-layer dynamics. As the mixed layer deepens in winter, it energizes submesoscale motions that subsequently contribute to springtime restratification by advecting denser water beneath lighter water. This restratification occurs through buoyancy production, linking mixed-layer energetics to the vertical transport of heat, carbon, and other tracers. Enhanced vertical velocities at submesoscales allow these motions to drive vigorous vertical transport, but their small spatial scales make them difficult to observe and model. Here, we show that measurements of sea-surface height from the Surface Water and Ocean Topography (SWOT) satellite can be used to infer submesoscale restratification and diagnose mixed-layer variability globally.

We estimate submesoscale restratification using the mixed-layer eddy parameterization. We argue that gradients of the small-scale sea-surface height anomaly observed by SWOT reflect mixed-layer steric height anomalies which allows us to obtain an entirely SWOT-derived estimate of submesoscale restratification. These restratification rates exhibit pronounced seasonality, with enhanced rates during the spring restratification period. We find that these estimates are most reliable in highly energetic western boundary currents and the Antarctic Circumpolar Current. Combining our estimates with Argo mixed-layer depth observations and reanalysis surface buoyancy fluxes, we show that submesoscale restratification is sufficient to explain the early shoaling of the mixed layer relative to the timing expected from surface buoyancy fluxes alone in western boundary currents and the North Atlantic.