The Irminger Gyre as a key driver of AMOC variability in the subpolar North Atlantic

The Atlantic meridional overturning circulation (AMOC) is key in regulating the global climate system through a large-scale system of currents transporting warm waters northward and cooler waters southward. The Overturning in the Subpolar North Atlantic Program (OSNAP) has been measuring the AMOC directly since 2014, demonstrating that water mass transformation within the eastern subpolar North Atlantic and Nordic Seas dominate AMOC variability in the subpolar North Atlantic. Here, we use OSNAP data to further analyse the AMOC in this region. We find that the North Atlantic Current (NAC) accounts for over 72% of the variability in the upper limb of the AMOC. The easternmost branches of the NAC (over the Rockall Plateau and Trough) account for the majority of the AMOC variability (~38%), even though the westernmost branches account for more than half the mean transport (~10 Sv). The lower limb of the AMOC is found to have a statistically meaningful connection to the circulation in the interior of the Irminger basin, i.e. the Irminger Gyre, accounting for ~38% of the AMOC variability. During the OSNAP time period, a prominent feature of the Irminger basin is a layer of low potential vorticity (PV) in the intermediate water density classes. Further observations (ARMOR3D) show that changes in intermediate water thickness in the Irminger basin are connected to AMOC variability (r = 0.60). We hypothesise a buoyancy-driven mechanism connecting the Irminger Gyre with AMOC variability, where an increase in intermediate water layer thickness in the Irminger basin inhibits the northward recirculation of the Irminger Gyre, leading to a strengthening of the subpolar AMOC.