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Ocean Science An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/os-2017-88
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
02 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).
Shelf sea tidal currents and mixing fronts determined from ocean glider observations
Peter M. F. Sheehan1, Sarah L. Hughes2, Barbara Berx2, Alejandro Gallego2, Rob A. Hall1, Karen J. Heywood1, and Bastien Y. Queste1 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, United Kingdom
2Marine Scotland Science, 375 Victoria Road, Aberdeen, AB11 9DB, United Kingdom
Abstract. Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. We use dive-average currents from a two-month (12th October–2nd December 2013) glider deployment along a zonal hydrographic section in the northern North Sea to determine M2 and S2 tidal velocities, which agree well with tidal velocities measured by current meters and extracted from a tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We use the glider-derived tidal velocities to investigate tidal controls on the location of a tidal mixing front. During the deployment, the front moves offshore at a rate of 0.51 km day−1. During the first period of the deployment (i.e. until mid November), the front's position is explained by the local balance between tidal mixing and surface heat fluxes: as heat is lost to the atmosphere, full-depth tidal mixing is able to occur in progressively deeper water. In the latter half of the deployment, the output of a simple one-dimensional model suggests that the front should have decayed. By comparing this model output to hydrographic observations from the glider, we attribute the persistence of the front beyond this period to the advection of cold, saline Atlantic-origin water across the deeper portion of the section. The glider captures the transition of the front from being one controlled by the balance between tidal mixing and surface heating, to being one controlled by advection of buoyancy. Fronts in shelf regions with oceanic influence may be geographically fixed and persist during periods of little to no thermal stratification, with implications for the thermohaline circulation of shelf seas.

Citation: Sheehan, P. M. F., Hughes, S. L., Berx, B., Gallego, A., Hall, R. A., Heywood, K. J., and Queste, B. Y.: Shelf sea tidal currents and mixing fronts determined from ocean glider observations, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-88, in review, 2017.
Peter M. F. Sheehan et al.
Peter M. F. Sheehan et al.
Peter M. F. Sheehan et al.

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Short summary
We calculate tidal velocities using observations of ocean currents collected by an underwater glider. We use these velocities to investigate the location of sharp boundaries between water masses in shallow seas. Narrow currents along these boundaries are important transport pathways around shallow seas for e.g. pollutants and organisms. Tides are an important control on boundary location in summer, but seawater's salt concentration can also influence boundary location, especially in winter.
We calculate tidal velocities using observations of ocean currents collected by an underwater...
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