Atlantic pycnocline theory scrutinized using a coupled climate model

Simulations with changed Southern Ocean wind‐stress, oceanic vertical mixing, surface freshwater forcing and global warming confirm the basic equations of Gnanadesikan's (1999) theory for the Atlantic: one vertical scale, the pycnocline depth D , contributes inversely proportional to low‐latitudinal upwelling and linearly to Southern Ocean eddy transport. The maximum Atlantic overturning is confirmed to be quadratic in D but is also shown to be linear in a meridional density difference Δ ρ . Our simulations strongly suggest that the theory needs to be complemented by a dynamical equation for Δ ρ since changes in both D and Δ ρ are significant and mutually independent. While, under global warming D varies strongly and Δ ρ is practically constant, the situation is reversed for altered surface freshwater fluxes. Similarly, variations in the meridional length scale of Southern Ocean outcropping require a dynamical equation in order to capture the fundamental behavior of the Atlantic meridional overturning circulation.