Regional simulations of the Faroe Bank Channel overflow in a level model

The work presented in this paper is part of an effort to understand and improve the representation of overflows in large scale, coarse resolution ocean climate models. To this end we developed a regional model of the Faroe Bank Channel overflow using the MITgcm (Massachusetts Institute of Technology General Circulation Model), a typical global ocean model using discrete levels as the vertical co-ordinate. In order to isolate the numerical diffusion resulting from the advection of tracers, the model is run without any turbulence closure schemes, without convective adjustment or any other physically based parameterization of mixing. Comparison between the model results and recent observations of the Faroe Bank Channel plume allows assessment of the model performance, including its ability to correctly represent the mixing and the downslope transport in the plume. It is found that at the highest resolution used in this paper (2.5 km - horizontal and 25 m - vertical) the structure of the modeled plume and the magnitude of the entrainment is comparable to the observed plume. The dependence of the mixing on various model parameters, such as vertical and horizontal resolution, vertical viscosity, drag coefficient and inflow forcing, is tested extensively. The numerical mixing in the model is found to be most sensitive to changes in the horizontal resolution, and to a lesser extent on vertical resolution and vertical viscosity. The inflow forcing and drag coefficient show only a very minor effect on the mixing. The results presented in the paper identify the shortcomings of the model at coarser resolutions which need to be addressed when attempting to represent such overflows realistically in large scale climate and ocean models.