The Role of Atlantic Basin Geometry in Meridional Overturning Circulation

Abstract

We present idealized simulations to explore how the shape of eastern and western continental boundaries along the Atlantic Ocean influences the Atlantic meridional overturning circulation (AMOC). 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 with a large, Pacific-like basin and a small, Atlantic-like basin. We perform simulations with five coastline geometries along the Atlantic-like basin that range from coastlines that are straight to coastlines that are shaped like the coasts of the American and African continents. Changing the Atlantic basin coastline shape influences AMOC strength in a manner distinct from simply increasing basin width: widening the basin while maintaining straight coastlines leads to a 10-Sv (1 Sv ≡ 106 m3 s−1) increase in AMOC strength, whereas widening the basin with the geometry of the American and African continents leads to a 6-Sv increase in AMOC strength, despite both cases representing the same average basin-width increase relative to a control case. The structure of AMOC changes are different between these two cases as well: a more realistic basin geometry results in a shoaled AMOC while widening the basin with straight boundaries deepens AMOC. We test the influence of the shape of the both boundaries independently and find that AMOC is more sensitive to the American coastline while the African coastline impacts the abyssal circulation. We also find that AMOC strength and depth scales well with basin-scale meridional density difference, even with different Atlantic basin geometries, illuminating a robust physical link between AMOC and the North Atlantic western boundary density gradient.