The structure of stratified mixing by shear instability in baroclinically forced shear flows

We present observational data from the mouth of the Connecticut River, a shallow salt-wedge estuary characterised by intense interfacial mixing. The motivation is to better understand, and ultimately predict, density-stratified turbulent mixing driven by shear instabilities at high Reynolds numbers (Re > 10^5). Such processes span an immense turbulent energy cascade across eight orders of magnitude, from coherent instabilities at kilometre scales to the smallest mixing eddies at micrometre scales. Using multi-beam echo-sounding imagery, we reveal the spatial structure and temporal evolution of turbulent mixing with unprecedented detail. During the flood tide, large-scale topography and hydraulics cause the pycnocline to slope, which triggers, through baroclinic forcing, primary shear instabilities in the form of long trains of Kelvin-Helmholtz billows. Our data demonstrate that at Re ~ 5x10^5, mixing occurs primarily by turbulence in the braids connecting the cores of the billows rather than within the cores themselves. This secondary 'braid turbulence' is continuously forced by the secondary baroclinic generation of shear within the sloping braids. This finding challenges the prevailing paradigm built upon direct numerical simulations (DNS) at lower Reynolds numbers (Re ~ 10^3-10^4), where mixing is thought to occur primarily by overturning in the billow cores. This distinction is a significant shift in understanding the high-Re turbulent cascade in mixing hotspots, with potential implications for mixing parameterisations in the coastal ocean.