Manabe and Stouffer (2000) reported coupled ocean-atmosphere experiments in which a large freshwater discharge into the North Atlantic drove the thermohaline circulation from its active mode toward an 'inactive reverse mode' with no North Atlantic deep-water formation, and companion work showed that whether such a collapsed state persists as a stable equilibrium depends on the model's vertical diapycnal diffusivity, with a critical value above which the two equilibria do not coexist. The question they left open - whether the collapsed state is a physically realizable, long-lived equilibrium of the real ocean or a model-dependent artifact, and hence whether meltwater pulses or anthropogenic forcing can trigger a quasi-irreversible collapse - is the modern problem of Atlantic meridional overturning circulation (AMOC) bistability and tipping. Twenty-five years of intercomparisons, eddy-permitting simulations, and observational early-warning studies have transformed but not settled it: coupled models still disagree on whether the present ocean is monostable or bistable, diagnosed tipping thresholds depend on model bias and even on the salinity-compensation convention of the hosing protocol, and recent claims of imminent tipping remain contested against the IPCC AR6 assessment of no abrupt collapse before 2100 (medium confidence).

Background

The Atlantic meridional overturning circulation (AMOC), historically called the thermohaline circulation (THC), is the global-scale overturning in which warm, salty upper-ocean water flows northward in the Atlantic, cools and densifies in the Labrador and Nordic Seas, sinks to form North Atlantic Deep Water (NADW), and returns southward at depth; its strength is measured in Sverdrups (1 Sv = one million cubic metres per second), and it carries roughly a petawatt of heat northward, warming the North Atlantic region relative to comparable Pacific latitudes. Antarctic Bottom Water (AABW) is the dense water mass formed around Antarctica that fills the deepest Atlantic; in a collapsed overturning state, AABW penetrates far northward while NADW formation ceases.

Because seawater density depends on both temperature and salinity, the overturning can in principle possess more than one stable equilibrium. Stommel (1961) showed in an idealized two-box model that the transport of freshwater by the circulation itself can act as a positive feedback (the salt-advection feedback): a weaker overturning recirculates less salt, weakening it further. When this feedback is strong enough, two stable regimes coexist - an active 'on' state and a collapsed 'off' state - separated by a threshold (a bifurcation point), so that slowly varying freshwater forcing produces hysteresis: less forcing is needed to drive the circulation into collapse than to restore it.

Manabe and Stouffer (1995, 1997, 2000), using the Geophysical Fluid Dynamics Laboratory (GFDL) coupled ocean-atmosphere model with flux adjustment (empirical corrections added to the surface heat and freshwater fluxes so that the simulated control climate stays realistic), discharged 0.1 Sv of freshwater for 500 years either over the high-latitude North Atlantic (50-70 degrees N) or over the subtropical Gulf of Mexico region that glacial meltwater routed through North America would have reached. High-latitude freshening capped the surface with a low-salinity layer (a halocline) that suppressed deep convection, weakened and shoaled the overturning, intensified AABW penetration, and cooled the northern North Atlantic, Greenland, Scandinavia and parts of western Europe - a spatial pattern resembling that of the Younger Dryas (YD), the last major abrupt cold event (about 12,900-11,700 years ago), one of a family of millennial-scale abrupt swings (Dansgaard-Oeschger events) recorded in Greenland ice during the last glacial period. In the standard model the circulation recovered after the freshwater stopped. However, the model also possessed a second equilibrium, an 'inactive reverse mode' with Southern Ocean sinking, and in the companion study (Manabe and Stouffer 1999) its persistence depended on the coefficient of vertical diapycnal diffusivity (mixing across density surfaces, comprising both real small-scale turbulence and spurious numerical diffusion introduced by the finite-difference discretization): above a critical diffusivity, the reverse mode transformed back to the active mode and only one equilibrium survived.

This raised the central question: is the off state a real, long-lived state of the ocean - such that past meltwater events and future anthropogenic freshening could push the Atlantic across a tipping threshold with quasi-irreversible consequences - or an artifact of model mixing physics? The source experiments also used the simulated modern climate as control rather than glacial or deglacial boundary conditions (larger ice sheets, expanded sea ice, different winds and greenhouse-trace-gas concentrations), so it was unknown how the same perturbations project onto the colder deglacial ocean, whether meltwater routing (subtropical versus high-latitude entry into the North Atlantic) controls the trigger, and whether the modeled hemispheric fingerprints - including the Southern Hemisphere response and the latitudinal reach of cooling - match proxy reconstructions. No monitoring base adequate to detect and attribute future change existed at the time.

The modern toolkit for this problem comprises: hosing experiments (imposed freshwater-flux experiments used to probe overturning stability, now run as multi-model intercomparisons, whose protocols differ in whether and where the added freshwater is compensated so that the basin-mean salinity budget is preserved - a convention choice that shifts the diagnosed thresholds); the net freshwater transport by the AMOC across its southern boundary near 34 degrees S (often written Fov), whose sign has been proposed as an indicator of whether the circulation is in a multiple-equilibria regime; statistical early-warning signals (rising variance and autocorrelation, 'critical slowing down', expected as a bifurcation is approached); the RAPID array of moorings that has monitored the AMOC at 26 degrees N continuously since 2004; and synchronously coupled transient simulations of the last deglaciation (such as TraCE-21ka with the NCAR Community Climate System Model, CCSM3; Liu et al. 2009) that impose reconstructed ice-sheet, meltwater and greenhouse-gas changes.

Problem Statement

Manabe and Stouffer's experiments left open one coherent determination: whether the Atlantic overturning is a bistable system in the real ocean - whether its collapsed 'off' (inactive reverse) state is a physically realizable, stable equilibrium that realistic forcing can reach and in which the ocean can remain for centuries to millennia - and, if so, where the tipping threshold lies. This determination comprises four components: (a) the stability regime of the present and deglacial ocean - whether the current climate lies in the monostable or the bistable regime - established from the sign and magnitude of the salt-advection feedback, for which the net freshwater transport by the overturning across its southern boundary is a relevant indicator, with quantified uncertainty from model freshwater biases, diapycnal mixing, resolution, and hosing-protocol conventions; (b) the threshold: the critical freshwater input and/or warming level, and its dependence on forcing rate and path, beyond which North Atlantic deep-water formation collapses and fails to recover after the forcing ceases (hysteresis width and off-state persistence time); (c) paleo consistency: whether meltwater forcings compatible with proxy constraints on routing, timing and volume, applied under glacial or deglacial boundary conditions, reproduce the magnitude, timing and spatial fingerprints (including the Southern Hemisphere response and the latitudinal reach of cooling) of Younger Dryas- and Dansgaard-Oeschger-type events; and (d) the observable consequences: what the existence and location of the threshold imply for detecting and attributing anthropogenic AMOC change in the observational record. The common objective uniting these components is to decide whether freshwater-triggered, quasi-irreversible overturning collapse is a real mechanism of past and future climate change, as opposed to a model-dependent artifact of imperfectly represented vertical mixing and mean climate. Of the four, (a) and (b) jointly constitute the central determination, while (c) and (d) are supporting determinations.

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