Fig. 7: Residual meridional overturning streamfunction for different idealized configurations.

Streamfunction computed in density coordinate, and then remapped in pseudo-depth coordinate¹⁹², computed for three variants of the idealized DINO configuration¹⁹³ and run to near-equilibrium with the VEROS ocean model¹⁹⁴. The contour interval is 5 Sv. Sigma-2 contours are superimposed in black dashed lines. a The reference 1° configuration, has an elongated 5000 km-wide basin with a 2500 m-deep re-entrant channel in the South (red box) mimicking the Southern Ocean. A zonal wind is applied at the surface, together with thermal and haline restoring to zonally-averaged climatological values. Eddies are parameterized using a GM-like scheme. A vertical diffusivity profile is prescribed, with 0.3 cm² s⁻¹ above 2000 m transitioning to 1.25 cm² s⁻¹ below 3000 m¹⁹⁵. A typical overturning structure is obtained, with tropical cells in the upper thermocline, a deep AMOC-like cell and a bottom AABW-like cell originating from the southern edge. b The configuration is run with a constant 1.25 cm² s⁻¹ vertical diffusivity up to the surface, making the AMOC-like cell deeper and more intense but also more diabatic in the interior. Similarly, the tropical cell becomes deeper and stronger with increased diffusivity. c When the southern channel is blocked, the AMOC-like cell is nearly shut down while the bottom cell is now intensified due to a combination of wind pumping, southern convection and deep mixing. On the other hand, the tropical cells are nearly unchanged, illustrating their primarily wind-driven nature. The pycnocline depth is about 30% shallower than in the case of an open re-entrant channel.