I’m not sure where this fascination with a possible AMOC collapse started. Perhaps with Broecker’s widely cited paper in Science 1997? I bring this up because a number of publications have been appearing lately about the slow-down and eventual collapse of the AMOC. There seems to be almost an axiomatic belief that because there is increasing ice melt, this fresh water must inhibit the production of deep water and thus weaken the AMOC, perhaps to the point of collapse. I’ve always found the notion that as the globe warms the Nordic countries will head toward cooler times rather implausible.
The purported AMOC collapse would be driven by the global warming that increases Greenland ice melt which in turns freshens the salty water from the North Atlantic Current to the point that when cooled it can’t sink and flow back south at depth, thus shutting down the AMOC. Superficially this sounds plausible, and 25 years ago perhaps even convincing. But today we know so much more about the AMOC overturning that we can with confidence say that an imminent shutdown of the AMOC isn’t possible, why?
First, there are two overturning paths, a shallow overturning in the subpolar gyre, what we might or should call the production of the North Atlantic Intermediate Water (NAIW), and a deep overturning path through the Nordic Seas. The significance of these two distinct paths wasn’t recognized 25 years ago, and even today it isn’t always fully appreciated. The deep overturning path depends upon the cooling and pooling of very dense water in the Nordic Seas north of the Greenland-Iceland-Scotland ridge. As more cold water is produced in winter some will spill across the ridge sills to O(3) km depths to flow unrestricted out into the global abyss as North Atlantic Deep Water (NADW). The shallow overturning is driven by wintertime convection in the cyclonic circulation of the Irminger and Labrador Seas to O(1) km, max 2 km depths. This NAIW water also spreads south as the shallow branch of the AMOC. But this water doesn’t have an open path to the global abyss, instead it must cross the east-streaming Gulf Stream. The meandering of the current allows NAIW (often referred to as Labrador Water) to stir or mix across it.
Comparable amounts of AMOC water, 8+ Sv, overturn along the shallow and deep paths. But the heat loss to the atmosphere is significantly greater for the Nordic Seas branch which is why this branch produces denser water. Further, and perhaps just as important, the Greenland ice melt water remains largely trapped to flow south along the east Greenland shelf. Some of this water leaks into the cold Greenland Sea, but the Mohn and Knipovich Ridges (that separate the western and eastern halves of the Nordic Seas) prevent this low-salinity water from spreading east into the Norwegian Sea. Which means that the warm water flowing into the Norwegian Sea from the Atlantic remains salty and preconditioned for deep convection when cooled in winter. The Greenland ice melt will flow around and into the Labrador and Irminger Seas, but as near as we can tell it has little impact on the salty water entering the Nordic Seas.
It’s not clear that one can turn off the production of NAIW, there will always be wintertime convection to some depth. Production of NAIW appears to have been to similar depths during glacial times. We know this because NAIW was present in the South Atlantic in glacial times. Similarly, there will be wintertime cooling and convection to some depth in the Nordic Seas. But unless the Nordic Seas is fed with warm salty water from the North Atlantic Current, the northeastern extension or branch of the Gulf Stream, wintertime convection can’t produce dense water which means that exchange with the North Atlantic will be limited to shallower depths than today. If no warm salty water is imported, then no deep dense water will be exported and the deep AMOC turns off. Consequently, little heat is transferred to the atmosphere and northern Europe turns cold. The point is that the major climate variations around the North Atlantic depend upon the state of the deep branch of the AMOC, not what is happening in the subpolar North Atlantic.
Broecker, W. (1997). Thermohaline circulation, the Achilles heel of our climate system: Will man‐made CO2 upset the current balance? Science, 278, 1582–1588.