Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP-Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014-2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region. Western boundary current variability in the subpolar North Atlantic is thought to reflect interior convection changes and determine Atlantic Meridional Overturning Circulation variability. Here, the authors show with an extended OSNAP time series that neither linkage is robust due to the complex dynamics in the region.

Automated summary

The study using trans-basin observation data found that deep water formation in the subpolar North Atlantic has minimal impact on the overturning circulation, contrary to previous modeling studies. The authors revealed that changes in western boundary currents do not significantly influence the variability of the subpolar overturning circulation over observational time scales.