Ultrasound for estuaries: imaging the hidden life of stratified turbulence
| When |
Sep 11, 2025
from 02:00 to 04:00 |
|---|---|
| Where | Salle des thèses |
| Attendees |
Adrien Lefauve |
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The energy cascade in ocean mixing 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, based on high-resolution multibeam echo-sounding observations from the mouth of the Connecticut River (USA), a shallow salt-wedge estuary featuring striking Kelvin-Helmholtz billows on a vertical scale of ~1-2 m (see image). The multibeam captures instantaneous two-dimensional images that reveal the structure and evolution of turbulent mixing using acoustic backscatter as a proxy for salinity microstructure dissipation, i.e. mixing. At Re ~ 106, 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, energised by baroclinic shear within their slopes. Secondary shear instabilities within the braids 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 numerical simulations at Re ~ 103-104, indicating that high-Re mixing hotspots continuously driven by large-scale shear – in estuaries, wind-driven surface currents, and deep overflows – may operate through fundamentally different cascade dynamics than previously thought.