Gräwe, U., Holtermann, P., Klingbeil, K.:

 

The North Sea and the Baltic Sea can be regarded as prototypes for shelf seas. Although both regions are tightly coupled, they do show unique features, which pose challenges to the modelling community.
Whereas the North Sea is dominated by tidal mixing, the mixing in the nearly tideless Baltic Sea is controlled by boundary mixing. However, both show strong temperature stratification during summer, which is limiting the exchange across the thermocline. Additionally, the stratification leads to a shrinking of the baroclinic Rossby radius, which limits the ability of numerical models to resolve this important length scale. Even if they are able to do so, the models might not be capable of representing the energy fluxes between the mesoscale and submesoscale, or the transition from the 2D geostrophic turbulence to the ageostrophic regime. Here, the horizontal mixing parameterisation, which is either not grid sensitive or based on isotropic 3D turbulence, is a limiting factor.  Moreover, giving a high enough spatial resolution, the internal wave field and its impact on the mixing is only partially resolved. This makes is hard to apply internal wave mixing parameterisations, like in global ocean models.
Since shelf sea models are always regional models, they rely on boundary conditions from global ocean models or a nesting hierarchy. Both approaches might miss variations on different time and length scales, limiting their applicability. The same holds for the atmospheric forcing. Although more state of the art forcing datasets are available, the effect of land-sea transition might also limit their usage for local modelling studies.
In summary, we will highlight some achievements, challenges and potential future pathways in our efforts to improve shelf sea modelling.