A nested‐grid ocean model: With application to the simulation of meanders and eddies in the Norwegian Coastal Current

Oceanic flow phenomena vary in such large ranges of time and spatial scales that even with the fastest and largest computer to‐date, one cannot, at reasonable costs, compute large‐scale circulation and at the same time resolve mesoscale features like fronts and/or eddies; yet these features are dynamically important, and their inclusion may determine how well we can model the mean flow, including the deep‐ocean circulation. A two‐way interactive, nested‐grid, primitive‐equation model is developed here for coastal ocean applications. Notable features of the model are (1) nesting can be specified on any subregion of a coarse‐grid, large domain, and there can be more than one nest if required; (2) the nested region can be “hot‐started” from earlier calculation results of the coarse‐grid region, that is, the code automatically (by a change of an input flag) generates topography, wind forcing, climatology, currents, density and other prognostic variables in the nest and steps forward in time; (3) a time‐splitting integration, with small timesteps in the nest and large timesteps in the coarse‐grid domain, is used; and (4) nested variables are driven by coarse‐grid solutions around the nest's boundaries, where a flow relaxation scheme may be used, and at the same time drive the coarse‐grid evolution through its averaged action in the overlapped region. To demonstrate its robustness, the model is applied in a February/March 1988 real‐time simulation of meanders and eddies in the Norwegian Coastal Current, initialized from a 585 days' quasi‐equilibrium calculation. The simulation includes meteorological forcings, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and wintertime hydrography for depths > 500 m. From March 20 to 31, the development of a meander between the Froya and the Halten Banks is simulated. The timing and location of the meander agree well with observed hydrography.