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Abstract The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.
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This content will become publicly available on June 1, 2026
Interplay Between Wind‐Driven Advection and Mixing of Salt and Dissolved Oxygen in a Microtidal Estuary
Abstract Most work on how estuarine dynamics impact dissolved oxygen (DO) distributions has focused on tides, but in shallow estuaries with large fetch or small tides, wind can be the primary mixing agent and also drives advection. To investigate how these processes affect DO distributions, an observational study was conducted in the shallow, microtidal Neuse Estuary. Salinity, DO, and velocity profiles were measured at multiple positions along and across the estuary over a 6‐month period. A one‐dimensional model (General Ocean Turbulence Model) provided additional insight into the response of salinity and DO to wind. Salinity and oxygen conservation equation terms were calculated from observations and simulations. Cross‐estuary wind drove lateral circulation and tilted the isohalines, reducing stratification; lateral advection and enhanced mixing reduced vertical gradients and increased the bottom DO. Down‐estuary wind tended to increase the exchange flow and stratification, but concurrently the surface wind‐mixed layer deepened over time. The balance of these processes determined if the water column became fully mixed or remained stratified, and the depth of the pycnocline and oxycline. An expression for steady state surface layer thickness was derived by considering the competition between the horizontal and vertical buoyancy flux, and the predictions agreed well with observations and simulations. Up‐estuary wind inhibited the exchange flow and the combination of advection and mixing homogenized the water column. While these patterns generally held for purely across‐ or along‐channel wind, the response was often more complex as the wind vector varied in orientation and with time.
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- PAR ID:
- 10621300
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 130
- Issue:
- 6
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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