The performance of implicit ocean models on B- and C-grids

Fully-implicit primitive equation ocean models are useful to study the sensitivity of steady ocean flows to parameters, to determine bifurcations of these flows associated with instabilities and to use relatively large time steps in transient flow computations. This paper addresses a problem related to the origin of wiggles occurring in fully-implicit C-grid models. The situation considered is the computation of three-dimensional thermally-driven steady flows in a midlatitude spherical sector. We determine the reason why in a coarse resolution C-grid implicit model, the values of the lateral friction coefficients are restricted to far higher values than for the same B-grid model. The analysis also reveals why the B-grid discretization is superior for the computation of this type of flows.