  In numerical ocean models coast lines change the direction from one grid cell
to its neighbor and the value for viscosity is set to be as small as possible.
Therefore, model simulations are not converged with resolution and boundary
separation points differ in essential properties from flow separation in
continuous flow fields. In this paper, we investigate the quality of the
representation of boundary separation points in global ocean models. To this
end, we apply well established criteria for boundary separation within an
idealized ocean model setup. We investigate an eddy-resolving as well as a
steady test case with idealized and unstructured coast lines in a shallow water
model that is based on a finite element discretization method.
  The results show that well established criteria for separation fail to detect
boundary separation points due to an insufficient representation of ocean flows
along free-slip boundaries. Along no-slip boundaries, most separation criteria
provide adequate results. However, a very sophisticated criterion based on
dynamical system theory reveals that the representation of boundary separation
is limited for these flows as well. We conclude that the representation of
boundary separation points in numerical ocean models is not satisfying. This
will have an impact on the separation of boundary currents in global ocean
models.
