Open boundary conditions for long-term integration of regional oceanic models

Abstract

Regional oceanic models can be developed and used efficiently for the investigation of regional and coastal domains, provided a satisfactory prescription for the open boundary conditions (OBCs) is found. We propose in this paper an adaptive algorithm where inward and outward information fluxes are treated separately. Because of the essentially hyperbolic nature of the incompressible, hydrostatic Primitive Equations, external data are required only for inward boundary fluxes. The outward fluxes are treated with a new algorithm for two-dimensional radiation. Special attention is given to the estimation of the radiation phase speed, essential for detecting the direction of boundary fluxes. The boundary conditions are applied and assessed on a US West Coast (USWC) configuration of the Regional Oceanic Modeling System (ROMS). Our guiding principles are that the numerical solution be stable over multiple years, reach a meaningful statistical equilibrium, and be realistic with respect to the available observational data. A sensitivity analysis suggests that the oblique radiation is robust and sufficiently accurate to detect the direction of information fluxes. The adaptive nudging adequately incorporates the external information minimizing over- and under-specification problems. In addition, a volume constraint based on global correction of normal barotropic velocities improves the overall performances of the open boundary conditions.