Asymmetric dynamics, energetics, and mixing of a dense submesoscale filament under cross-front winds

Abstract While the effects of along-front winds aligned with the frontal jet in submesoscale surface-layer fronts have been extensively studied in the past, the impact of cross-front winds is less well explored at the moment. This study investigates the effects of cross-front winds on a dense submesoscale filament in the central Baltic Sea based on high-resolution turbulence and velocity observations, supplemented by idealized numerical simulations. Our focus on a dense filament, by definition laterally bounded by two fronts, allowed us to systematically distinguish between fronts with upgradient winds (directed from lighter to denser waters) and down-gradient winds (pointing into the opposite direction). Our data show that upgradient winds induce a down-front Ekman transport that augments the geostrophically balanced frontal jet. This type of forcing also drives near-surface Ekman currents that reinforce the secondary cross-front circulation, triggering rapid restratification and a shoaling of the turbulent surface layer. In this case, the combination of enhanced turbulence shear production and strong stratification leads to vigorous and efficient diapycnal mixing. Conversely, down-gradient winds suppress both the frontal jet and the secondary cross-front circulation, induce shear-driven convection, and result in a well-mixed surface layer with weak vertical shear. Idealized numerical simulations further indicate that the observed asymmetry in frontal mixing reflects distinct kinetic energy exchange pathways within the filament, mediated by geostrophic shear production on time scales of hours.