Although long-term observational data of the strength of the AMOC doesn’t exist, the AMOC leaves “fingerprints” in sea-surface temperature and salinity patterns of the Atlantic ocean.
“A detailed analysis of these fingerprints in eight independent indices now suggests that the AMOC weakening during the last century is indeed likely to be associated with a loss of stability,” says Niklas Boers.
“The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse.”
A number of factors are likely important for the phenomenon, that add to the direct effect that the warming of the Atlantic ocean has on its circulation.
These include freshwater inflow from the melting of the Greenland ice sheet, melting sea-ice, increasing precipitation and river run-off. Freshwater is lighter than saltwater and reduces the tendency of the water to sink from the surface to greater depths, which is one of drivers of the overturning.
Niklas Boers added: “I wouldn’t have expected that the excessive amounts of freshwater added in the course of the last century would already produce such a response in the overturning circulation.
“We urgently need to reconcile our models with the presented observational evidence to assess how far or how close the AMOC really is to its critical threshold”.
Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation is published in Nature Climate Change on August 5th 2021.