Numerical simulations of the North Atlantic subtropical gyre: sensitivity to boundary conditions

Abstract

Abstract A series of simulations have been carried out with the Miami Isopycnic Coordinate Ocean Model (MICOM) in the context of the Data Assimilation and Modeling Evaluation Experiment-North Atlantic Basin (DAMEE-NAB), in order to explore the model's ability to reproduce the wind- and thermohaline-driven circulations in the North Atlantic subtropical gyre. This paper addresses the constraints imposed on the simulations by the domain size and by the position of the ocean lateral boundaries. Comparisons between simulations performed in a large CME-like domain (28°S to 65°N) and in a smaller DAMEE-NAB domain (6°N to 50°N) are discussed in detail. In the large configuration, the flow into the subtropics is related to the model circulation in the subpolar and equatorial regions, while in the smaller configuration, that flow depends solely on the interaction between the circulation in the subtropics and the restoring forcing at the boundaries. Despite the fact that the boundaries in the small domain are located adjacent to the region of interest, and that these boundaries intersect strong meridional surface flows in the western basin (in contrast to the situation in the large domain), the restoring force is able to generate appropriate inflow and outflow conditions without significantly disrupting the circulation in the subtropical gyre. The impact of grid spacing is also assessed by doubling the horizontal resolution. This increase in resolution implies an increase in the number of grid points inside the buffer zones, which modifies the vertical mass transfer within the boundaries and consequently the efficiency of the buffer zones. This is shown to be particularly important in the sponge layer located in the Gulf of Cadiz to simulate the outflow of salty Mediterranean waters into the North Atlantic, which under certain circumstances, can significantly modify the interior water mass properties. Furthermore, changes in the flow conditions in the Gulf of Cadiz, associated with this increase in resolution, also have an important impact upon the model's ability to simulate the Azores Current.