The role of the Subpolar Gyre in the future of the AMOC

A key question for the future ocean is what will happen to the Atlantic Meridional Overturning Circulation (AMOC). In climate models that are run beyond 2100 for a high emission scenario it shuts down in the majority, where the shutdown is preceded by a ceasing of convection in the North Atlantic subpolar gyre (SPG). As some climate models already show a collapse of SPG convection around 2040, it is key to know what this means for the AMOC. What is the interaction between SPG and AMOC? If convection in the SPG stops, is the AMOC bound to shut down as well? Or will other regions or processes take over?

For deep water formation both deep convection in the gyre centre, as well as densification in the boundary current play a role. Climate models do not resolve the SPG boundary current and eddies due to their coarse resolution, meaning key processes for deep water formation are parametrised. I will discuss the relative role of densification in the boundary current and deep convection in the SPG gyre centre for the AMOC in both CMIP6 models and ocean reanalyses. Using causal inference the importance of the two processes for the AMOC is investigated, distinguishing the respective roles of the Labrador and Irminger seas.

Differences between CMIP6 models and reanalyses are discussed, and put in the context of the recent OSNAP results on the relative importance of the eastern and western SPG for the AMOC. This sheds light on the representation of the process of deep water formation that are relevant for the AMOC in climate models, and aids in understanding the impact a collapse of SPG convection would have on the AMOC.