An oceanic general circulation model framed in hybrid isopycnic-Cartesian coordinates

A newly developed hybrid-coordinate ocean circulation model is documented and tested. Coordinate surfaces in this model adhere to isopycnals wherever this does not violate minimum layer thickness requirements; elsewhere, coordinate surfaces are geometrically constrained. The intent of this approach, some of whose features are reminiscent of the Arbitrary Lagrangian–Eulerian (ALE) technique, is to combine the best features of isopycnic-coordinate and fixed-grid circulation models within a single framework. The hybrid model is an offshoot of the Miami Isopycnic Coordinate Ocean Model whose solutions, obtained under identical geographic and forcing conditions, serve as reference. Century-scale simulations on a coarse-mesh near-global domain show considerable similarities in the modeled thermohaline-forced circulation. Certain architectural details, such as the choice of prognostic thermodynamic variables (ρ,S versus T,S) and the algorithm for moving coordinate surfaces toward their reference isopycnals, are found to only have a minor impact on the solution. Emphasis in this article is on the numerical resiliency of the hybrid coordinate approach. Exploitation of the model's flexible coordinate layout in areas of ocean physics where pure isopycnic coordinate models only have limited options, such as mixed-layer turbulence parameterization, will be the subject of forthcoming articles.