Hans BurchardLeibniz Institute for Baltic Sea Research Warnemünde, Germany

Cooperation: Thomas Badewien1, Johannes Becherer2, Kaveh Purkiani2

Götz Flöser3, Ulf Gräwe2, Robert Hetland5, Volker Mohrholz2, Rolf Riethmüller3, Henk Schuttelaars4, Joanna Staneva3, Lars

Umlauf2

1. 1. University Oldenburg, Germany2. 2. Leibniz Institute for Baltic Sea Research Warnemünde, Germany3. 3. Helmholtz Center Geesthacht, Institute for Coastal Research, Germany4. 4. TU Delft, The Netherlands5. 5. Texas A&M, College Station, TX, USA

Thermohaline circulation in the Wadden Sea

www.rapid.ac.uk

Wadden Sea … and … thermo-haline circulation?

Warming Precipitation

Weak tidal mixing: vertically stratified

Strong tidal mixing: horizontally stratified

Land Ocean

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 theWadden Sea of theGerman Bight, recordingtemperature 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

75% level

Result:Tidal straining makes about 2/3 of estuarine circulation.

With full-scale 1Dmodel (GOTM):Gravitational circulation and tidal straining profiles

Burchard and Hetland (JPO 2010)

Estuarine circ.Straining

Gravitational

Tidally-averaged currents fortypical Wadden Sea conditions

Enhancement of estuarine circulation in channelised tidal flow

(2D slice modelling with GETM)

Burchard et al. (JPO 2011)

www.getm

.eu

Circulation in transverse estuary

Transverse structure of estuarine circulation

Burchard et al. (JPO 2011)

Tidal straining circulation Gravitational circulation

Advective circulation Barotropic 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:W

ater

col

umn

stab

ility

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 3D model (100 m resolution)

Tidally averaged water column stability

Purkiani et al. (in prep.)

Results of fully baroclinic 3D model (100 m resolution)

Purkiani et al. (in prep.)

(S1)

(S2)

Results of fully baroclinic 3D model (100 m resolution)

Purkiani et al. (in prep.)

Flood Ebb

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 concentrationsare substantially increased in theWadden Sea of the German Bight, withouthaving significantsources at the coast.

Why ?

Total suspended matter from MERIS/ENVISAT on August, 12, 2003.

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 /

km3

Burchard et al. (JPO 2008)

Total water and SPM volume

Without density differences

V /

km3

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?“ (2011-2014)

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 transportin 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 subsequentsediment fluxes on human intervention and natural change?

Derived specific questions

Up to which degree of sea level rise will the intertidal flats of theWadden Sea be prevented from drowning?

Will further deepening of navigational channelsin tidal estuaries turn them into gigantic sediment traps?

Is nutrient supply through particulate matter transport providingincreased carrying capacity in the coastal zone to give room forinvasive species?

Note that for answering any of these questons, the currentprocess 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: