Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification
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Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex bathymetric shoal/channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing. The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly, this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence stratification, mixing, and transport.
KeywordsFront Frontogenesis Trapping Convergence front Lateral circulation
Thanks to those at the Stanford EFML, APL–UW, and other members of the COHSTREX team who provided help in the field, particularly N. Nidzieko, J. Hench, K. Davis, L. Walter, B. Hayworth, P.J. Rusello, T. Litchendorf, E. Boget, C. Craig, and F. Karig. Special thanks to Nick Nidzieko, Mark Stacey, and Rocky Geyer for helpful discussions and to three anonymous reviewers whose detailed responses greatly improved this manuscript. This research was supported by the Office of Naval Research through grants N00014-05-1-0485 and N00014-10-1-0236. Additional support for SNG was provided by the National Science Foundation, a Stanford Graduate Fellowship, and the Achievement Rewards for College Scientists Foundation.
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