Abstract

Our environment is shaped by stably stratified air and water masses and the turbulent mixing of heat and chemicals within them. This talk addresses the challenge of understanding and predicting the density-stratified turbulent mixing driven by shear instabilities at high Reynolds numbers (Re > 10^5). These shear instabilities are a key process in the turbulent energy cascade in the ocean, spanning a great range of scales, from coherent instabilities at kilometre scales to the smallest eddies at micrometre scales. We present observational data taken by collaborators from the Woods Hole Oceanographic Institution at the mouth of the Connecticut River, a shallow salt-wedge estuary. Multi-beam echo-sounding imagery provides access to the spatial structure and temporal evolution of the intense interfacial mixing with unprecedented detail. The data demonstrate that mixing occurs primarily by turbulence in the ‘braids’ connecting the ‘cores’ of Kelvin-Helmholtz billows, rather than within the cores themselves. This secondary braid turbulence appears to be continuously forced by the baroclinic generation of shear in the tilted braid. This finding challenges the prevailing paradigm built upon direct numerical simulations (DNS) at lower Reynolds and Prandtl numbers, where mixing tends to occur primarily by overturning in the billow cores. This distinction may represent a shift in our understanding of mixing in highly dissipative hotspots driven by large-scale shear — not only in estuaries but also in wind-driven surface currents and deep oceanic overflows.