The TRR 181 seminar is held by Jen-Ping Peng(IOW, PhD in T2) on December 9, tba, during the "Warnemünde Turbulence Days" on the island Vilm.
"Frontal instability and energy dissipation in a submesoscale upwelling filament"
Theory and numerical simulations suggest that submesoscale fronts and filaments are subject to various types of instabilities, providing a potentially important pathway for the downscale transport and dissipation of mesoscale energy in the ocean. Here, we discuss the real-ocean relevance of these recent concepts based on high-resolution turbulence microstructure and near-surface velocity data from a transient upwelling filament in the Benguela upwelling system (South-East Atlantic). The focus of the study is a sharp submesoscale front at the edge of the filament, characterized by downfront winds, a strong frontal jet, and vigorous turbulence. On the light side of the front, we identified a 30-40 m thick turbulent surface layer with low potential vorticity (PV) and a distinct two-layer structure: turbulence in the upper well-mixedpart of this low-PV layer was driven by convective mixing due to the destabilizing cross-front Ekman transport. The lower part was characterized by stable stratification and negative PV, indicating forced symmetric instability (FSI), similar to previous idealized simulations. Dissipation rates in this region scaled with the Ekman buoyancy flux, and quantitatively agreed with theoretical predictions for FSI. In the frontal outcropping region, however, the cyclonic shear associated with the frontal jet was sufficiently strong to suppress FSI. There, turbulence was driven by marginal shear instability. Our data show that both FSI and shear instability provide relevant routes to kinetic energy dissipation in dense upwelling filaments.