Learning by OSMOSIS: The seasonal cycle of submesoscale motions

The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradi- ents, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 square km region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (~20 km) hydrographic sections. The importance of mixed layer instabilities on the re-stratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter and emphasize that accurate parameteriza- tions of submesoscale processes are needed throughout the ocean.

Speaker: Andy Thompson, CalTech