Remotely-sensed horizontal structure and evolution of the leading and trailing edges of the salt wedge at the Columbia River mouth
Abstract
Salt wedge dynamics are important in the exchange of material between oceans and estuaries, impacting estuarine and coastal ecology, as well as commercial and recreational activities. The frontal boundaries of the salt wedge are of particular interest because they not only spatially delineate regimes of advection and mixing, but their sharp gradients also have implications for navigation and acoustic transmission. However, these sharp gradients are challenging to model over estuarine domains and sample via in-situ methods. We present new remote sensing observations of salt wedge fronts via shore-based, X-band imaging radar at the Mouth of the Columbia River (MCR). These observations provide useful information about the fronts' horizontal structure as well as their location, temporal evolution, and propagation over a wide range of time scales. Specifically, we track position and speed of both the leading bottom and trailing surface fronts of the MCR salt wedge during its ebb-to-flood landward transit of the estuary mouth. Observations of the bottom frontal boundary are particularly unique, as it is a surface signature of a process occurring at full depth. The bottom front is identifiable due to the presence of surface velocity divergence above the head of the front. Non-hydrostatic 3-D simulations (NHwave) confirm that the displacement of fluid above a saline density current can induce such surface divergence leading to the identified backscatter anomaly (i.e. imaging mechanism). Co-located, landward transit of the front was additionally corroborated by sharp gradients in near-bottom salinity time series and shipborne salinity profile transects. The surface front is noteworthy due to its recurrence and complex horizontal evolution. Whereas the bottom front was only observed after large amplitude ebbs, surface frontogenesis was observed to occur at the MCR during all floods, regardless of tidal amplitude or river discharge. Though regular in occurrence, the salt wedge surface fronts exhibit rich variety in both horizontal structure and evolution, highlighting the remarkable complexity of internal dynamics at the MCR. These findings indicate that a radar remote sensing approach, in conjunction with directed subsurface measurements, can facilitate a 4-D perspective of frontal dynamics in the field.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMOS31E..06H
- Keywords:
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- 4544 Internal and inertial waves;
- OCEANOGRAPHY: PHYSICALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICALDE: 4558 Sediment transport;
- OCEANOGRAPHY: PHYSICALDE: 4560 Surface waves and tides;
- OCEANOGRAPHY: PHYSICAL