Self‐adapting open boundaries for a sigma coordinate model of the eastern North Atlantic

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Abstract
A regional non‐eddy‐resolving configuration of the SPEM5.1 primitive equation model is used to study the dynamics of the eastern North Atlantic. Along most of the domain perimeter, extensive open boundaries (OBs) evacuate outgoing perturbations, while the outer ocean is simulated by lateral relaxation of some of the model variables. Unlike in existing regional models, this lateral constraint only concerns the tracers and the baroclinic velocities, for which good climatological estimates exist. Along the OBs, the barotropic stream function is entirely determined by the model response to the inner dynamics, without relaxation to any questionable a priori estimate. The adjustment processes and their interaction with the open boundaries are described. The final solution is remarkably stable and quite realistic. The net transports crossing the boundaries are prescribed; we therefore investigated the impact of these parameters. We first show that the prescribed mass exchange between the basin and the rest of the Atlantic has almost no influence on the interior circulation since the western and southern lateral conditions naturally adjust to the interior dynamics. The net circulation constrained around Iceland, however, has large‐scale consequences: Through the intensity of the overflow above the Iceland‐Scotland Ridge (ISR), this parameter affects the stratification north and south of the Rockall Plateau, the subpolar dynamics, and finally the North Atlantic Current path up to the Charlie Gibbs Fracture Zone. This slowly propagating dynamical effect, already noticed in previous studies, is shown to be forced baroclinically near the ISR.