As freshwater from melting ice sheets enters the North Atlantic, the AMOC weakens. As it weakens, it imports less salty subtropical water northward, further reducing surface salinity and sinking — which further weakens the AMOC. This dynamic is an example of:
ANegative feedback that will eventually stabilize the AMOC at a new, weaker equilibrium
BPositive feedback that can drive runaway weakening toward a collapsed state
CThermohaline equilibration, where thermal effects compensate for salinity changes
DIsostatic adjustment of ocean basins in response to reduced ice mass
This is the salt-advection positive feedback: weaker AMOC → less salt imported → lower surface density → less sinking → weaker AMOC. Positive feedback amplifies perturbations rather than restoring the original state. Once freshwater forcing crosses the tipping threshold, this loop accelerates rather than decelerating, which is why the transition to a collapsed state can be rapid and why recovery requires conditions substantially more favorable than those that triggered the collapse. Negative feedback would act in the opposite direction — for example, cooling from reduced heat transport might slightly increase density, partially counteracting the freshwater dilution.
Question 2 Multiple Choice
What does 'bistability' mean in the context of AMOC dynamics?
AThe AMOC oscillates periodically between a strong and weak state on a predictable schedule
BThe AMOC can exist in two distinct self-reinforcing stable states: a vigorous 'on' circulation and a collapsed 'off' state
CThe AMOC's stability is governed by exactly two variables: North Atlantic temperature and salinity
DThe AMOC has two circulation cells — a surface cell and a deep cell — that operate independently
Bistability means the system has two stable equilibria separated by an unstable threshold. In the 'on' state, the salt-advection feedback sustains the overturning: salty water brought north keeps surface density high, which drives sinking, which keeps the circulation strong. In the 'off' state, no salty water is imported, the North Atlantic stays fresh, density is too low for sinking, and the circulation stays collapsed. Both states are self-reinforcing. The system is like a light switch: it can remain stably in either position, but once pushed past the threshold, it switches states and stays there.
Question 3 True / False
Paleoclimate records from Greenland ice cores show that the AMOC has mainly weakened gradually over geological time and has rarely undergone rapid collapse.
TTrue
FFalse
Answer: False
Greenland ice cores record the opposite: Heinrich events and Dansgaard-Oeschger oscillations during the last glacial period show North Atlantic temperatures changing by 5–10°C within decades — far too fast for gradual forcing. These events are best explained by rapid AMOC shutdowns triggered by massive freshwater pulses from collapsing ice sheets, followed by restarts. The paleoclimate record is one of the strongest lines of evidence that AMOC bistability and rapid transitions are real physical phenomena, not model artifacts. This historical evidence is why current observations of AMOC weakening and Greenland melt are taken seriously as potential precursors.
Question 4 True / False
Once the AMOC collapses into its 'off' state, removing the freshwater forcing that triggered the collapse is sufficient to immediately restart circulation.
TTrue
FFalse
Answer: False
Bistability implies hysteresis: the conditions required to restart the AMOC are more demanding than simply reversing the conditions that caused it to collapse. In the collapsed state, the North Atlantic freshens (no salty subtropical water being imported) and the positive feedback maintains the off state. Restarting requires either a sufficiently large perturbation of salty water into the sinking regions or reduction of freshwater input far below the threshold that triggered collapse — not just returning to the pre-collapse forcing level. This irreversibility is what makes AMOC collapse a potential climate tipping point rather than a reversible weakening.
Question 5 Short Answer
Explain how the salt-advection feedback creates bistability in the AMOC, and why the collapsed 'off' state is self-reinforcing.
Think about your answer, then reveal below.
Model answer: In the 'on' state, the AMOC transports warm, salty subtropical water northward. This salty water cools in the North Atlantic, becoming dense enough to sink. The sinking drives the overturning, which continues to import salt — a self-reinforcing positive feedback (more circulation → more salt → more sinking → more circulation). In the 'off' state, no subtropical water is imported. Without the salty inflow, the North Atlantic surface freshens over time from precipitation and river input. Fresh water is less dense than salty water, so there is no longer enough density contrast to drive sinking. No sinking → no circulation → no salt import → the surface stays fresh → still no sinking. Both states are thus self-reinforcing: the 'on' state maintains the salinity gradient that drives it, and the 'off' state maintains the freshness that prevents it from restarting.
The bistability arises from the salt-advection feedback operating as a positive feedback in both directions: it amplifies both the 'on' state (keeping it going) and the 'off' state (keeping it from restarting). The threshold between states is where the freshwater forcing is large enough to overwhelm the salt import and tip the balance from the positive feedback maintaining the 'on' state to the positive feedback maintaining the 'off' state.