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- Title
Using Tracer Variance Decay to Quantify Variability of Salinity Mixing in the Hudson River Estuary.
- Authors
Warner, John C.; Geyer, W. Rockwell; Ralston, David K.; Kalra, Tarandeep
- 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. Plain Language Summary: The Hudson River Estuary is a location where salty ocean water mixes with fresh river water. The mixing of these different water sources occurs along the entire length of the estuary, with more mixing at certain locations at different times. In this study we study where and when the mixing occurs. It was found that there are certain locations that create more mixing due to changes in the channel width and the shape of the channel cross‐section. More mixing occurred on the ebb tides than on the flood. This is important to understand how salt is mixed out of the estuary because these processes also control transport of other tracers in the estuary, as well as help us understand how changes to the estuary (like channel dredging or sea level rise) will affect salt intrusion. Key Points: Greater mixing during ebb (average 60% of total) than flood (40% of total)Mixing occurs at fronts where strong stratification and active turbulence overlapTracer variance approach directly quantifies spatial and temporal distribution of mixing
- Subjects
HUDSON River (N.Y. &; N.J.); SEAWATER salinity; ESTUARY hydrodynamics; OCEANIC mixing; GROUNDWATER tracers; TIDES; OCEAN circulation
- Publication
Journal of Geophysical Research. Oceans, 2020, Vol 125, Issue 12, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016096