A note on the potential AMOC collapse for nonexperts

As the Earth warms due to anthropogenic climate change, sea- and land-based ice melts, adding buoyant freshwater to the surface of the high-latitude North Atlantic and thus decreasing salinity. This makes the surface water lighter reducing the sinking of the surface water and thus the AMOC. However, the amount of added freshwater is not enough to directly shut down the AMOC.

Since the AMOC transports warm and salty water to the high-latitude North Atlantic, the reduced AMOC will decrease both the surface water temperature and salinity in the sinking regions. While the reduced salinity further makes the surface water lighter (as known as “salt feedback”), the cooling makes the surface water heavier (as known as “temperature feedback”). Thus, the AMOC salt feedback destabilizes the AMOC, while the AMOC temperature feedback stabilizes the AMOC. In the modern climate, the latter is much stronger than the former; thus the AMOC is stable to freshwater perturbations. However, if Arctic sea ice & Greenland ice sheet melting becomes strong and persistent enough to reach a tipping point, the salt feedback may become stronger than the temperature feedback, and thus weaken the AMOC until it collapses eventually.

In summary, the AMOC in the modern climate is stable to freshwater perturbations. To destabilize and shut down the AMOC system, the freshwater flux must be strong and persist long enough to change the mean state of the North Atlantic Ocean to the extent that the salt feedback wins the competition.

Figure. Idealized schematic of the AMOC salt feedback loop.

Stommel, H. 1961. Thermohaline Convection with Two Stable Regimes of Flow. Tellus, 13: 224-230. https://doi.org/10.1111/j.2153-3490.1961.tb00079.x