The structure and lifecycle of stratified mixing by shear instabilities in continuously forced flows

The turbulent energy cascade of ocean mixing remains poorly understood because of the wide separation of scales at high Reynolds numbers (Re). Here we propose a new conceptual model for the structure of small-scale mixing grounded in high-resolution multibeam echosounding observations from the mouth of the Connecticut River, a shallow salt-wedge estuary. Tidal forcing and bottom topography slope the pycnocline, sustaining interfacial shear and Kelvin–Helmholtz instabilities well below the marginal instability threshold on a vertical scale of order 1 m. Acoustic backscatter, used here as a proxy for salinity microstructure dissipation, provides time-resolved, two-dimensional imagery of the true structure and evolution of turbulent mixing. At Re~10^6, we find that mixing is dominated not by the collapse of slowly evolving billow cores as at Re~10^3-10^4, but instead by fast turbulence within the ~10 cm thin, shallow-sloping braids that connect them, energized by baroclinic shear. We explain these observations using two-dimensional direct numerical simulation at field-matched parameters, which predicts the emergence of secondary Kelvin–Helmholtz instabilities and turbulence within the braids, pre-empting further steepening and primary overturn. Laboratory experiments in an inclined duct, used as an analogue of the turbulent braid regime, provide estimates and visualizations of mixing down to the dissipative scales, where it remains organized in myriad thin, sheared filaments rather than overturns. We conclude that high-Re mixing hotspots sustained by baroclinic shear forcing follow fundamentally different dynamics than previously thought.