Frontal Instability and Energy Dissipation in a Submesoscale Upwelling Filament

In frontal regions of the ocean, submesoscale motions are frequently observed features in the surface layer. Due to their important role for surface-layer restratification, lateral dispersion, and the turbulent energy cascade, this type of motion has been extensively studied over the past decade. However, direct high-resolution turbulence observations inside submesoscale features (e.g., fronts and filaments), required to test the relevance of new theoretical concepts, are so far virtually lacking. Unlike previous studies focusing on persistent fronts (e.g., Gulf Stream and Kuroshio), here we present high-resolution turbulence microstructure and velocity data from a transient submesoscale upwelling filament, entrained between two mesoscale eddies 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 vigorous turbulence, down-front winds, and a pronounced low-PV layer near the surface. Two distinct stability regimes are identified: (i) the main frontal region, where the cyclonic cross-front shear is strong enough to suppress symmetric instability, despite strong baroclinicity. Turbulence in this region is locally driven by Kelvin-Helmholtz instability; (ii) a neighboring region with a 30-35 m deep low-PV layer and strong horizontal density gradients, where down-front winds induce a vertical two-layer structure: the upper part of the low-PV layer is characterized by convective mixing due to the destabilizing cross-front Ekman transport, while in the lower part turbulence is driven by forced symmetric instability. These are the first direct field observations supporting the relevance of forced symmetric instability that has so far only been identified in theoretical and numerical investigations.