The concern about the state of the AMOC continues today. It is appropriate to be concerned, but fears that it will shut down are greatly exaggerated, and there is no evidence that it has slowed in recent times. First, the presumed mechanism for shutting it down, fresh-water run-off from Greenland, besides being a gradual process, takes a circuitous route around Greenland and the Labrador Sea and gains salinity such that when it meets with the warm, salty North Atlantic Current, the northern branch of the Gulf Stream, the dilutionary impact is muted. Second, the AMOC involves two overturning pathways, one is via the Irminger Sea, and the other through the Nordic Seas. We all know this, but it is a crucial distinction that needs to be better recognized because they play quite different roles in major climate change.
The Irminger and Nordic Sea branches are comparable in terms of volume transport, but the latter branch loses vastly more heat in the overturning (see reference below). The Irminger Sea overturning produces what we might call North Atlantic Intermediate Water (NAIW). It is the result of progressive cooling as some of the North Atlantic Water weaves its way around the Reykjanes Ridge from east to west. As it does so it increases in density gradually passing the σt~27.55 kgm-3 density and in effect leaves the upper branch of the AMOC. It is pooled in the Irminger Sea (more generally in the subpolar gyre) where it will be further cooled by wintertime convection, but at most will be densified to ~27.8 kgm-3 and a maximum depth of 2 km. It leaves the subpolar gyre flowing south along the Canadian margin to the Tail of the Grand Banks where some of it turns the corner and continues along the continental slope and the rest stirs across the Gulf Stream at depth and continues west as part of the Gulf Stream recirculation – its southern recirculation gyre – toward Cape Hatteras where it continues south into the South Atlantic.
The Nordic Seas branch, after passing the Faroes to either side, continues in two parallel branches north along the coast of Norway to the Lofoten Basin and Barents Sea, gradually losing its heat to the atmosphere and becoming increasingly dense. Somewhat simplified all water will eventually end up the Greenland Sea where the last and greatest densification takes place. This dense water mass is pooled behind (north of) the Greenland-Iceland-Scotland ridge over which it spills back into the deep North Atlantic. It will entrain North Atlantic Intermediate water and in so doing will lose some of its extreme properties but remains dense enough to gradually sink to 3-4 km depths. It continues south at depth along the continental margin as North Atlantic Deep Water (NADW) passing underneath the Gulf Stream out into the global abyssal ocean.
From a glacial-interglacial point of view this distinction of pathways is crucial because in glacial times the Nordic Seas branch shuts down, but not the shallow overturning. We know this because during glacial times the entire Atlantic is filled with Southern Ocean Water (from the Antarctic) up to nearly two km depths, but not shallower because production of NAIW continued. This paleo-NAIW can be traced into the South Atlantic. Thus, the shallow overturning AMOC branch continued, but not the deep branch. It hardly has to be mentioned that a shutdown of the Nordic Seas branch and its supply of heat would have enormous consequences to European climate. While there is no evidence that this branch is slowing at present, it is the one to be concerned about with respect to major climate change in the North Atlantic. The shallow branch will vary over time as weather and climate change, but there is to my knowledge no indication that it has ever shut down.
Chafik, L. and T. Rossby (2019). Volume, heat, and freshwater divergences in the subpolar North Atlantic suggest the Nordic Seas as key to the state of the meridional overturning circulation. Geophys. Res. Lett.,46. https://doi.org/10.1029/2019GL082110