HOW SLIPPERY ARE PIECEWISE-CONSTANT COASTLINES IN NUMERICAL OCEAN MODELS

Coastlines in numerical ocean models are oriented al various finite an gles to the model grid. The true coastline is usually replaced by a pi ecewise-constant approximation in which the model coastline is everywh ere aligned with the model grid. Here we study the consequences of the piecewise-constant approximation in an idealised shallow-water ocean model. By rotating the numerical grid at various finite angles to the physical coastlines, we are able to isolate the impact of piecewise-li near boundaries on the model circulation. We demonstrate that piecewis e-constant coastlines exert a spurious form stress on model boundary c urrents, dependent on both the implementation of the slip boundary con dition and the form of the viscous stress tenser. In particular, when free-slip boundary conditions are applied, the character of the circul ation can be reduced to no-slip in the presence of a piecewise-constan t boundary. The spurious form stress can be avoided in a free-slip lim it if the viscous stress tensor is written in terms of vorticity and d ivergence.