Irminger Sea Is the Center of Action for Subpolar AMOC Variability.

Significant societally important climate impacts can be caused by changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) at higher latitudes. Focusing on variability and long‐term change of the subpolar North Atlantic (SPNA)—a key AMOC action center—and using eastern OSNAP array observations, we identify a distinct density and sea‐surface height signature of the AMOC strength in the Irminger Sea (2014–2018), reinforced and extended with an ocean reanalysis (1993–2018). Reconstruction of AMOC variability using Irminger Sea density shows strong control by the North Atlantic Oscillation on subpolar overturning on multiple timescales, achieved via the gyre circulation and waters from the Labrador Sea. Furthermore, the observed decrease of Irminger Sea density since the mid‐twentieth century (1950–2019) is suggestive of a long‐term AMOC weakening of 2.2 Sv or 13%, however, this trend remains statistically insignificant due to the large interannual and decadal variability of the SPNA. Plain Language Summary: The subpolar North Atlantic (SPNA) Ocean between Greenland and Scotland is a region that is key to the health of the Gulf Stream system. Here, the warm currents carrying heat from the subtropics become denser along their way from Scotland toward Greenland; a process that has a strong imprint on the strength of this vital ocean current system. In this work, we find that ocean density in the Irminger Sea east of Greenland in the SPNA explains most of the variability of the directly measured strength of the Gulf Stream system. Based on this relationship, we reconstruct density variations in this region from historical hydrographic observations to reveal whether the Gulf Stream system in the SPNA has been changing over the past 70 years. While there is suggestive evidence for a slow‐down of this system in the SPNA in recent decades there is no consensus regarding its relevance to longer timescales. Key Points: Irminger Sea is the center of action for subpolar Atlantic Meridional Overturning Circulation (AMOC) variability from interannual to multidecadal time scalesIrminger Sea density variations are controlled by the North Atlantic Oscillation through its subpolar gyre connection to the Labrador SeaIrminger Sea density trends suggests uncertain weakening of the subpolar AMOC since 1950 [ABSTRACT FROM AUTHOR]