Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde, Germany Cooperation: Thomas Badewien 1, Johannes Becherer 2, Kaveh Purkiani 2 Götz Flöser 3, Ulf Gräwe 2, Robert Hetland 5, Volker Mohrholz 2, Rolf Riethmüller 3, Henk Schuttelaars 4, Joanna Staneva 3, Lars Umlauf University Oldenburg, Germany Leibniz Institute for Baltic Sea Research Warnemünde, Germany Helmholtz Center Geesthacht, Institute for Coastal Research, Germany TU Delft, The Netherlands Texas A&M, College Station, TX, USA Thermohaline circulation in the Wadden Sea

Wadden Sea … and … thermo-haline circulation?

Warming Precipitation Weak tidal mixing: vertically stratified Strong tidal mixing: horizontally stratified LandOcean Downward surface buoyancy flux Thermohaline estuarine circulation Sea bed River? The Wadden Sea circulation in a nutshell

Global ocean: Spatially inhomogeneous surface buoyancy fluxes plus internal mixing leads to global overturning circulation. Wadden Sea: Spatially homogenous surface buoyancy fluxes over sloping bathymetry plus tidal mixing should lead to residual overturning circulation. But does it really happen?

Conceptual model Burchard et al. (in prep.)

Width-averaged Wadden Sea model Burchard et al. (in prep.)

Width-averaged Wadden Sea model: Net precipitation Burchard et al. (in prep.)

Width-averaged Wadden Sea model: no buoyancy flux Burchard et al. (in prep.)

Width-averaged Wadden Sea model: Net evaporation Burchard et al. (in prep.)

Locations of five automatic monitoring poles in the Wadden Sea of the German Bight, recording temperature and salinity, (and thus density). How can we approach this with observations ? Burchard et al. (JPO 2008)

Climatology: Salinity difference HW-NW Burchard et al. (JPO 2008)

Climatology: Temperature difference HW-LW Burchard et al. (JPO 2008)

Climatology: Density difference HW-LW Burchard et al. (JPO 2008)

Longitudinal density gradients lead to: 1. Gravitational circulation MacCready and Geyer (2010)

MacCready & Geyer (2010) after Jay & Musiak (1994) Longitudinal density gradients & tidal oscillations lead to: 2. Tidal straining 75% level

Result: Tidal straining makes about 2/3 of estuarine circulation. With full-scale 1D model (GOTM): Gravitational circulation and tidal straining profiles Burchard and Hetland (JPO 2010) Estuarine circ. Straining Gravitational Tidally-averaged currents for typical Wadden Sea conditions

Enhancement of estuarine circulation in channelised tidal flow (2D slice modelling with GETM) Burchard et al. (JPO 2011)

Circulation in transverse estuary

Transverse structure of estuarine circulation Burchard et al. (JPO 2011 ) Tidal straining circulation Gravitational circulation Advective circulationBarotropic circulation

Does this all happen in nature? Ok, let’s go out to the Wadden Sea and measure:

Campaign in Lister Deep (April 2008) Becherer et al. (GRL 2011) shoals

Becherer et al. (GRL 2011)

Puzzling however: Water column stability Tidal phase Near lateral shoals, stratification kicks in already during flood... See following model study. Becherer et al. (GRL 2011)

Tides in the Wadden Sea (as seen in 200 m resolution model)

Results of fully baroclinic 3 D model (100 m resolution) Tidally averaged water column stability Purkiani et al. (in prep.)

Results of fully baroclinic 3 D model (100 m resolution) Purkiani et al. (in prep.) (S1) (S2)

Results of fully baroclinic 3 D model (100 m resolution) Purkiani et al. (in prep.) FloodEbb

Residual sediment fluxes in estuaries (analytical solutions) Burchard et al. (JPO, submitted)

Decomposition of sediment flux fluctuation flux transport flux total flux

Residual sediment fluxes in estuaries (transient numerical solutions) Burchard et al. (JPO, submitted) Fully erodable bed Semi-erodable bed Non-erodable bed

Suspended matter concentrations are substantially increased in the Wadden Sea of the German Bight, without having significant sources at the coast. Why ? Total suspended matter from MERIS/ENVISAT on August, 12, Implications for sediment transport

Model approach: 1. Simulating a closed Wadden Sea basin (Sylt-Rømø bight) with small freshwater-runoff and net precipitation. 2. Spin up model with variable and with constant density until periodic steady state. 3. Then initialise both scenarios with const. SPM concentration. 4. Quantify SPM content for control volume. Burchard et al. (JPO 2008)

Surface salinity at high and low water Burchard et al. (JPO 2008)

Total water and SPM volume With density differences V / km 3 Burchard et al. (JPO 2008)

Total water and SPM volume Without density differences V / km 3 Burchard et al. (JPO 2008)

Model system based on GETM: NA: 5.4 km X 5.4 km (2D) NSBS: 1.8 km X 1.8 km (3D) SNS, WBS: 600 m X 600 m (3D) Wadden Sea: 200 m X 200 m (3D) PACE project (NWO-BMBF): „The future of the Wadden Sea sediment fluxes: Still keeping pace with sea level rise?“ ( ) Wadden Sea model Gräwe et al., in prep.

Tides in the Wadden Sea (as seen in 200 m resolution model)

Overarching research questions What are the hydrodynamic processes driving sediment transport in the coastal zone? How do the sediment fluxes affect the coastal ecosystem? How do the sediment fluxes determine the morphological evolution in the coastal zone? How is the susceptibility of these processes and the subsequent sediment fluxes on human intervention and natural change?

Derived specific questions Up to which degree of sea level rise will the intertidal flats of the Wadden Sea be prevented from drowning? Will further deepening of navigational channels in tidal estuaries turn them into gigantic sediment traps? Is nutrient supply through particulate matter transport providing increased carrying capacity in the coastal zone to give room for invasive species? Note that for answering any of these questons, the current process understanding is not sufficient!

Conclusions Other than in real estuaries, the residual circulation in the Wadden Sea is truly thermohaline with sometimes salinity being the major driver and sometimes temperature and it has a significant impact on sediment transport by supporting landward sediment fluxes. The big questions are: How much does thermohaline circulation support sediment accumulation? Can it help the Wadden Sea to survive sea level rise ? Funded by: