Thermohaline residual circulation of the Wadden Sea
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In this study, we present estuarine circulation driven by horizontal density gradients generated by spatially homogeneous surface buoyancy fluxes over sloping bathymetry as a dynamical feature in the coastal zone being potentially relevant for cross-coastal transports. A combination of downward buoyancy flux (net precipitation, net heating) together with tidal mixing may generate a classical estuarine circulation with landward near-bottom residual currents. The Wadden Sea of the south-eastern North Sea is a prototype for such a coastal regime. It is characterised by extensive inter-tidal flats along the coast separated from the open sea by barrier islands. Here, we present long-term observations from the Wadden Sea covering the years 2006–2011. We investigated the statistics of the density gradients. Typical values for the landward density gradient were ∂ x ρ≈−3⋅10−5 kg m−4 and maximum values were ∂ x ρ≈−6.5⋅10−5 kg m−4. The magnitude of the density gradient resulted from the magnitude of the salinity gradient, with some modifications by the positive (towards the coast, in spring) or negative (towards the sea, in autumn) temperature gradient. To explain the generation of estuarine circulation by the surface buoyancy flux, we construct an analytical model representing the geometry and dynamics of a Wadden Sea Basin. With downward buoyancy flux, a weak classical estuarine circulation due to gravitational forcing results, whereas upward buoyancy flux drives inverse estuarine circulation. Finally, a two-dimensional (vertical-longitudinal) numerical model was set up for the idealised geometry, including tidally asymmetric turbulent mixing. This results in significantly stronger estuarine circulation due to the presence of tidal straining. The model assesses the circulation due to neutral and upward surface buoyancy fluxes. We conclude that these mechanisms may be important in many coastal areas and may substantially contribute to coast-to-sea exchange in these areas.
KeywordsWadden Sea Estuarine circulation Surface buoyancy flux Horizontal density gradients
The present study has been carried out in the framework of the project ECOWS (Role of Estuarine Circulation for Transport of Suspended Particulate Matter in the Wadden Sea) funded by the German Research Foundation (DFG) as project BU1199/11 and by the German Federal Ministry of Research and Education in the framework of the project PACE (The future of the Wadden Sea sediment fluxes: still keeping pace with sea level rise?, FKZ 03F0634A). We gratefully acknowledge the technical assistance of Axel Braun, Gerrit Behrens and Waldemar Siewert. We thank Lars Holinde and Anne-Christin Schulz for their assistance in data preparation and validation. The time series station Spiekeroog was initiated by Rainer Reuter and set up as part of the DFG research group BioGeoChemistry of tidal flats. The station is maintained through funds by the federal State of Lower Saxony. We are furthermore grateful to Ulf Gräwe for providing us with the Wadden Sea map shown in Fig. 2.
- Becherer J, Stacey M, Umlauf L, Burchard H (2014) Asymmetric lateral circulation in a well-mixed inlet: mechanisms and implications. J Phys Oceanogr 44:638–656Google Scholar
- Burchard H, Bolding K (2002) GETM—a general estuarine transport model. Scientific documentation. Tech. Rep. EUR 20253 EN, European CommissionGoogle Scholar
- Chant RJ (2002) Secondary circulation in a region of flow curvature: relationship with tidal forcing and river discharge. J Geophys Res 107:14/1-14/11Google Scholar
- Hansen DV, Rattray M (1965) Gravitational circulation in straits and estuaries. J Mar Res 23:104–122Google Scholar
- IOC SCOR, IAPSO (2010) The international thermodynamic equation of seawater 2010: calculation and use of thermodynamic properties. Intergovernmental Oceanographic Commission, Manuals and Guides No. 56, UNESCO (English),196Google Scholar
- Jay DA, Musiak JD (1994) Particle trapping in estuarine tidal flows. J Geophys Res 99:445–461Google Scholar
- Oppenheim AV, Schafer RW, Buck JR (1999) Discrete-time signal processing. Prentice Hall, Upper Saddle River, NJ., 870pp.Google Scholar
- Pritchard DW (1952) Salinity distribution and circulation in the Chesapeake Bay estuarine system. J Mar Res 11:106–123Google Scholar