Hans Burchard Baltic Sea Research Institute Warnemünde, Germany Collaboration: Lars Arneborg, Thomas Badewien, Karsten Bolding, Jorn Bruggeman, Volker Fiekas, Götz Flöser, Hans Ulrich Lass, Volker Mohrholz, Rolf Riethmüller, Piet Ruardij, Joanna Staneva, Lars Umlauf Applications of the General Estuarine Transport Model (GETM) for coastal ocean process studies

Program today Tools: General Ocean Turbulence Model, General Estuarine Transport Model, Applications: Western Baltic Sea – dense bottom currents Wadden Sea – suspended matter accumulation

GOTM is a water column model with modules for state-of-the-art turbulence closure models biogeochemical models of various complexities

GETM is a 3D numerical model for estuarine, coastal and shelf sea hydrodynamics with applications to the Tidal Elbe Wadden Sea Limfjord Lake of Geneva, Western Baltic Sea, North Sea – Baltic Sea system …

Present GETM characteristics... physics... Solves three-dimensional primitive equations with hydrostatic and Boussinesq approximations. Based on general vertical coordinates. Options for Cartesian, spherical and curvilinear coordinates. Fully baroclinic with tracer equations for salinity, temperature, suspended matter and ecosystem (from GOTM bio module). Two-equation turbulence closure models with algebraic second-moment closures (from GOTM turbulence module). Wetting and drying of intertidal flats is supported also in baroclinic mode.

Present GETM characteristics... numerics... Consistent explicit mode splitting into barotropic and baroclinic mode. High-order positive-definite TVD advection schemes with directional split. Choice of different schemes for internal pressure gradient calculation. Consistent treatment of zero-velocity bottom boundary condition for momentum. Positive-definite conservative schemes for ecosystem processes (in GOTM-Bio module).

GETM-GOTM-Bio coupling example: ERSEM simulation for North Sea (Simulation and animation by Piet Ruardij, NIOZ, The Netherlands)

Western Baltic Sea Study

Kriegers Flak Motivation: wind farms in the Western Baltic Sea

Western Baltic Sea monitoring stations Darss Sill: 19 m + Drogden Sill: 8 m + MARNET (IOW/BSH) Farvandsvæsenet

baroclinicbarotropic Inflows over Drogden Sill surface bottom Source: Farvandsvæsenet

Where does the Sound plume go ? ?

5 days 15 days 31 days Sound lock-exchange experiment with GETM Main plume goes via north of Kriegers Flak: Is this real ? Bottom salinity: 8 – 25 psu

Plume passing Kriegers Flak (Feb 2004)

GETM Western Baltic Sea hindcast

Model validation: Darss Sill

Model derived annual mean mixing in Western Baltic Sea

Nov 2005: Velocity structure of dense bottom current Ship A: TL-ADCP Ship B: Microstructure View 1 km Flow East comp. North comp. Can we explain the flow structure ?

GETM 2DV Slice Model: Transverse gravity current structure

Western Baltic Sea conclusions: Density currents in Western Baltic Sea are highly variable, show a complex transverse structure and induce substantial natural transports and mixing. For evaluating additional anthropogenic mixing due to offshore structures on these currents, proper parameterisations need to be found and inserted into 3D model (QuantAS-Off). Multiple bridges and wind farms may result in cumulative mixing effects.

Suspended matter concentrations are substantially increased in the Wadden Sea of the German Bight. Total suspended matter from MERIS/ENVISAT on August, 12, Wadden Sea study

The areal view shows locations of five automatic monitoring poles in the Wadden Sea of the German Bight, operated by GKSS and the University of Oldenburg. They record several parameters in the water column, such as temperature and salinity.

Salinity difference HW-LW

Temperature difference HW-LW

Density difference HW-LW

Hypothesis: This must have a dynamic impact on tidal flow and SPM transport, see the theory of Jay and Musiak (1994) below.

Testing with GOTM supports hypothesis: Residual onshore near-bed current Along-tide salinity gradient prescribed Bottom-surface salinity

3D simulations with GETM for the Sylt-Rømø bight Approach: Simulating a closed Wadden Sea basin (Sylt-Rømø bight) with small freshwater-runoff and net precipitation. Spin up model with variable and with constant density until periodic steady state. Then initialise both scenarios with const. SPM concentration. Quantify SPM content of fixed budget boxes.

The Sylt-Rømø bight

Bottom salinity at high and low water during periodically steady state.

Vertically averaged current velocity during full flood and full ebb.

Cross-sectional dynamics

Total water volume and SPM unit mass in budget boxes Case with density differences, tidal periods # 46-55

Total excess SPM mass in budget boxes Case with density differences, tidal periods # 46-55

Total water volume and SPM unit mass in budget boxes Case with no density differences, tidal periods # 46-55

Total excess SPM mass in budget boxes Case with no density differences, tidal periods # 46-55

Wadden Sea conclusions: The hypothesis is strongly supported. Other mechanisms than density differences which are also reproduced by the model system (such as settling lag and barotropic tidal asymmetries) do not play a major role in this scenario. Now, targeted field studied are needed for further confirming the hypothesis.

RV Gauss leaving Warnemünde for its last research cruise General conclusion: Sufficient knowledge about coastal processes is a prerequisite for assessing regional consequences of climate and anthropogenically induced change.