Much has been written about a shutdown of the AMOC (the Atlantic Meridional Overturning Circulation) which some writers claim is already underway. Curiously, much less is written about what happens when it does. This is unfortunate because it has led to a lot of confusion and misunderstanding about the nature of the AMOC and AMOC change. The AMOC in its ‘on’ state is the flow poleward of warm salty water, on the one hand, and the return south at depth of cold fresher water, on the other. While the transports must be the same, the flow patterns are distinctly and functionally quite different.
Off the US east coast and Canada all warm water flows poleward along the path of the Gulf Stream. The Gulf Stream also serves to close the wind-driven subtropical gyre. At the Tail of the Grand Banks the wind-driven water continues east and south while the AMOC portion turns NE as the North Atlantic Current (NAC). This branch shuts down when the AMOC is ‘off’ but the wind-driven branch never does. Continuing north into the subpolar North Atlantic the NAC splits in two directions, one that wraps around the Iceland Basin and the Reykjanes Ridge into the Irminger Sea, and one that continues into the Nordic Seas between Iceland, the Faroes and Scotland.
While the two branches transport similar volumes of water, the fate of these is strikingly different. As the Irminger Sea branch loses heat to the atmosphere in winter these waters convect and mix to shallow depths, but at most to 2 km. For reasons I do not understand the AMOC literature has focused on the ‘warming hole’ SE of Greenland as a sensitive indicator of the state of the AMOC. I find this obsession perplexing for this is not where the Irminger Sea convection takes place. Further, the dense water produced here does not flow south as a deep western boundary current but as a broader flow south and west, some along the continental slope but only as far as to Cape Hatteras where it is blocked by the Gulf Stream and forced to turn east - as it does today. The Gulf Stream acts as a barrier, specifically a potential vorticity barrier for the water flowing west cannot reduce its height and slip to the south underneath the Gulf Stream. It is only thanks to the meandering Gulf Stream well east of Cape Hatteras that water can be ‘stirred’ across the stream - the Gulf Stream as a blender – and continue west and south along the southern Gulf Stream recirculation gyre. NB this shallow overturning branch does not shut down during an AMOC shutdown, this is well-known even if not fully appreciated. While this shallow overturning water reaches into the South Atlantic, it does not appear to participate in the global overturning of the oceans.
Instead, it is the cessation of warm water flow toward the Nordic Seas that will lead to the expected cooling if the AMOC were to shut-down. While the transport of water is comparable to the Irminger Sea branch, the heat loss is more than a factor 2 greater (Chafik and Rossby, 2019) leading to the production and pooling of very dense water north of the Greenland-Iceland-Scotland ridge. This water spills across the ridge into the deep North Atlantic as a stratified layer of cold water that continues south along the continental slope as a deep western boundary current (DWBC, see my April 24, 2025 post). The export of cold water allows for the import of warm NAC water. This is the ‘on-state’ of the AMOC.
The AMOC turns ‘off’ when the overflow water no longer has a density extremum that enables it to slide along the bottom as a DWBC. This can result from either the warming of the Nordic Seas or reduced salinity due to ice melt or both. As of now the rate of outflow of dense water and its density remain stable. But let’s suppose the density of the overflow water decreases and loses its identity such that it mixes with subpolar water instead of sliding to greater depths as a flow along the bottom. As with water from the Irminger Sea, it will be blocked at Cape Hatteras. This the key point, unless the overflow is sufficiently dense that it can leave the subpolar North Atlantic by sliding along the bottom under the Gulf Stream there will be no demand for warm water entering the Nordic Seas to replace the overflow. To put it differently, the nearest source of warm salty water is the Gulf Stream. But it isn’t available unless we export a matching amount of cold water at depth that isn’t blocked by the Gulf Stream, i.e., as a DWBC.
A well-documented consequence of shutting down the supply of water from the Nordic Seas is that the entire deep half of the Atlantic from the far south to the Greenland-Iceland-Scotland ridge is filled with dense water from the Antarctic.
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