Use of the flow relaxation scheme in a three-dimensional baroclinic ocean model with realistic topography

A three-dimensional numerical ocean model was set up for a domain covering the Nordic Seas to produce dynamically consistent climatological archives of sea level, current, salinity and temperature. The flow relaxation scheme (FRS) was applied as open boundary condition. As a first step the model was run in diagnostic mode until a stationary solution was obtained. The only forcing was provided by gridded climatological fields of salinity and temperature. Compared with available data, this model set up produced a satisfactory circulation in the Nordic Seas. The results were stored in The Diagnostic Archive. In the next step, the obtained stationary solutions were used as boundary values in prognostic simulations to produce The Prognostic Archive. This paper describes a method for handling open boundary conditions in diagnostic simulations. The method requires knowledge of the depth mean current. A rather artificial condition of zero depth mean current or transport was chosen. Because the FRS does not conserve mass or momentum, it is able to redistribute the specified zero transports to nonzero in a realistic way through the relaxation zones. However, if the open boundaries are situated in dynamical active areas, in which contour lines of f/H enter or leave the model domain, the production of nonzero transports in the FRS zones is not sufficient to provide satisfactory boundary conditions. Instead, if already computed sea level and currents from a simulation with a larger model domain are used as external solutions at the open boundaries, the model gives excellent results. To illustrate the performance of the FRS when the temperature and salinity fields also are allowed to evolve, i.e., prognostic simulation, an example is shown from The Prognostic Archive. Here, boundary values were extracted from The Diagnostic Archive.