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

The energy cascade in ocean mixing caused by stratified turbulence remains poorly understood due to the wide separation of scales at very high Reynolds numbers Re. We present a new conceptual model for this cascade, grounded in high-resolution multibeam echo-sounding observations from the mouth of the Connecticut River, a shallow salt-wedge estuary with intense interfacial mixing. During flood tide, large-scale topography and hydraulics slope the pycnocline, generating interfacial shear and Kelvin-Helmholtz billows on a vertical scale of ~1-2 m. The multibeam captures instantaneous two-dimensional images that resolve the true slopes and geometry of these instabilities, revealing the structure and evolution of turbulent mixing using acoustic backscatter as a proxy for salinity microstructure dissipation. At Re ~ 10^6, we find that mixing is dominated not by the slowly evolving billow cores, which rarely overturn, but by fast, sustained turbulence within the braids that connect them, energized by baroclinic shear within their slopes. Secondary shear instabilities within the braid are predicted by two-dimensional direct numerical simulation with parameters matching the field values. Braid dissipation and mixing is quantified by scaling arguments derived from laboratory experiments in an inclined channel, and may explain why the primary billows do not overturn. This braid-dominated mixing contrasts with the core-dominated mixing seen in transient simulations at Re ~ 10^3-10^4. We conclude that high-Re mixing hotspots continuously driven by large-scale shear – including in estuaries, wind-driven surface currents, and deep overflows – operate through fundamentally different cascade physics than implied by existing low-Re paradigms.