The Impact of Atlantic Basin Geometry on Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) plays a fundamental role in setting regional and global climate, but the role of basin geometry in shaping its magnitude has not been fully explored. Previous studies used aquaplanet GCMs with thin meridional continental barriers to examine the influence of basin width and length on AMOC. Here, we use a state-of-the art ocean—sea-ice model (MOM6 and SIS2) with idealized, zonally-symmetric surface forcing and a range of idealized continental configurations to explore how the shape of the Atlantic basin coastlines affects MOC. We consider the ocean circulation in a large Pacific-like basin and a small Atlantic-like basin and explore five different coastline geometries along the small basin: 1) straight meridional boundaries used as a control case, 2) straight meridional boundaries with a widened small basin and unchanged large basin, 3) a western boundary shaped like a simplified version of the east coast of the Americas to target the impact on the western boundary currents, 4) an eastern boundary shaped like the African west coast, and 5) realistic boundaries on both sides of the basin to estimate any nonlinear effects on overturning and heat transport caused by the shape of both coasts. We find that changing basin shape influences AMOC strength in a manner inconsistent with simply changing basin width while keeping straight meridional coastlines: the wide small basin case shows a ~30% increase in MOC strength over the control case, as compared to a ~15% increase for the shaped coast case, even though both have the same average basin width. AMOC strength is most sensitive to changes in coastline shape along the North Atlantic subpolar gyre region. We also find that AMOC strength and depth scales well with meridional density difference, even with different Atlantic basin geometries, illuminating a robust physical link between AMOC and North Atlantic Western Boundary density gradient.