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Recent high-resolution numerical simulations have shown that the diurnal variability in the atmospheric forcing strongly affects the dynamics, stability, and turbulence of submesoscale structures in the surface boundary layer (SBL). Field observations supporting the real-ocean relevance of these studies are, however, largely lacking at the moment. Here, the impact of large diurnal variations in the surface heat flux on a dense submesoscale upwelling filament in the Benguela upwelling system is investigated, based on a combination of densely spaced turbulence microstructure observations and surface drifter data. Our data show that during nighttime and early morning conditions, when solar radiation is still weak, frontal turbulence is generated by a mix of symmetric and shear instability. In this situation, turbulent diapycnal mixing is approximately balanced by frontal restratification associated with the cross-front secondary circulation. During daytime, when solar radiation is close to its peak value, the SBL quickly restratifies, the conditions for frontal instability are no longer fulfilled, and SBL turbulence collapses except for a thin wind-driven layer near the surface. The drifter data suggest that inertial oscillations periodically modulate the stability characteristics and energetics of the submesoscale fronts bounding the filament. [ABSTRACT FROM AUTHOR]

Based on high-resolution turbulence microstructure and near-surface velocity data, frontal instability and its relation to turbulence are investigated inside a transient upwelling filament in the Benguela upwelling system (southeast Atlantic). The focus of our study is a sharp submesoscale front located at the edge of the filament, characterized by persistent downfront winds, a strong frontal jet, and vigorous turbulence. Our analysis reveals three distinct frontal stability regimes. (i) On the light side of the front, a 30-40-m-deep turbulent surface layer with low potential vorticity (PV) was identified. This low-PV region exhibited a welldefined two-layer structure with a convective (Ekman-forced) upper layer and a stably stratified lower layer, where turbulence was driven by forced symmetric instability (FSI). Dissipation rates in this region scaled with the Ekman buoyancy flux, in excellent quantitative agreement with recent numerical simulations of FSI. (ii) Inside the cyclonic flank of the frontal jet, near the maximum of the cross-front density gradient, the cyclonic vorticity was sufficiently strong to suppress FSI. Turbulence in this region was driven by marginal shear instability. (iii) Inside the anticyclonic flank of the frontal jet, conditions for mixed inertial/symmetric instability were satisfied. Our data provide direct evidence for the relevance of FSI, inertial instability, and marginal shear instability for overall kinetic energy dissipation in submesoscale fronts and filaments. [ABSTRACT FROM AUTHOR]

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. Various mixed-layer frontal instabilities, providing potential routes to energy dissipation, have been identified. However, direct turbulence measurements in submesoscale fronts, required to test the relevance of these new theoretical concepts, are so far largely missing. Here, data from a submesoscale upwelling filament (Benguela upwelling system) are presented that combine densely-spaced cross-front turbulence microstructure measurements with simultaneously conducted high-resolution velocity measurements from a towed research catamaran. These data reveal a narrow (width: 2-3 km) submesoscale front associated with an energetic frontal jet reaching speeds up to 1.2 m/s. Our data show a strongly turbulent surface mixing layer down to 30-40 m depth in the frontal region, driven by at least two distinct processes: (i) atmospheric forcing due to down-front winds and surface buoyancy loss on the warm side of the front; (ii) symmetric and inertial instabilities induced mixing in the cyclonic side of the frontal jet, consistent with the strong negative Ekman buoyancy flux in the frontal region. The two fronts bounding the submesoscale upwelling filament exhibited a marked asymmetry in mixing-layer depth and turbulence levels, likely resulting from the stabilizing or destabilizing Ekman transports on the two sides of the filament. Breaking internal waves associated with the collapse of the front were ubiquitous, suggesting an alternative pathway to energy dissipation in frontal regions. [ABSTRACT FROM AUTHOR]