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The Atlantic meridional overturning circulation (AMOC) carries large amountsof heat into the North Atlantic influencing climate regionally as well asglobally. Palaeo-records and simulations with comprehensive climate modelssuggest that the positive salt-advection feedback may yield a thresholdbehaviour of the system. That is to say that beyond a certain amount offreshwater flux into the North Atlantic, no meridional overturningcirculation can be sustained. Concepts of monitoring the AMOC and identifyingits vicinity to the threshold rely on the fact that the volume flux definingthe AMOC will be reduced when approaching the threshold. Here we advanceconceptual models that have been used in a paradigmatic way to understand theAMOC, by introducing a density-dependent parameterization for the SouthernOcean eddies. This additional degree of freedom uncovers a mechanism by whichthe AMOC can increase with additional freshwater flux into the NorthAtlantic, before it reaches the threshold and collapses: an AMOC that ismainly wind-driven will have a constant upwelling as long as the SouthernOcean winds do not change significantly. The downward transport of tracersoccurs either in the northern sinking regions or through Southern Oceaneddies. If freshwater is transported, either atmospherically or viahorizontal gyres, from the low to high latitudes, this would reduce the eddytransport and by continuity increase the northern sinking which defines theAMOC until a threshold is reached at which the AMOC cannot be sustained. Ifdominant in the real ocean this mechanism would have significant consequencesfor monitoring the AMOC

Mechanism for potential strengthening of Atlantic overturning prior to collapse