When do marginal seas and topographic sills modify the ocean density structure?

We ask what effect marginal seas at high latitudes have on the abyssal densities and stratification of the oceans. Although marginal seas are not necessary for the formation of dense abyssal waters, topographic sills tend to restrict exchange flows and increase density differences. Laboratory experiments with a steady state large‐scale overturning circulation, forced by a gradient in surface temperatures or heat fluxes, show that a marginal sea and topographic sill influence the abyssal density when the sill depth is less than twice the oceanic thermocline depth. The overflow over shallow sills interacts directly with the surrounding thermocline stratification, decreasing the abyssal density relative to an ocean with no marginal sea and increasing the density difference between the sea and the ocean. Experiments show that the effect of the sill depends on the surface boundary conditions and indicate a larger response for conditions approaching a prescribed distribution of temperature rather than of heat flux. Application of the results to the North Atlantic circulation suggests that the Greenland‐Scotland Ridge is shallow enough to lead to a significant reduction of the density of North Atlantic Deep Water, and this is consistent with the conclusions from an analysis of water mass properties. The Greenland‐Scotland Ridge is therefore likely to have a significant effect on the density structure and heat transport in the global oceans.