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The Atlantic Meridional Overturning Circulation (AMOC) transports heat to high latitudes and carbon to the deep ocean. Paleoceanographic observations have led to the widely held view that the strength of the AMOC was significantly reduced at two intervals during the most recent glacial-to-interglacial transition, with global climate impacts. Climate models predict that the AMOC may decline in the future due to anthropogenic forcing, but the time periods for modern observations are too short to detect recent trends with high confidence. To understand the likelihood of future changes in the AMOC, it is important to understand the mechanisms that drove past changes in AMOC strength. In this paper we review (1) the paleoceanographic proxy data that have led to the widespread view that the AMOC sharply decreased for periods of several thousand years during the last deglaciation, (2) climate model simulations of the last deglaciation, with particular attention to their use of fresh water to alter the AMOC, (3) the physical mechanisms that could have driven past changes in the AMOC, and (4) how insights from past ocean change can inform our understanding of what may happen in the future. 

Abstract. In the coming decades increasing amounts of freshwater are predicted to enter the subpolar North Atlantic from Greenland and the Arctic. If this additional freshwater reaches the regions where deep convection occurs, it could potentially dampen ventilation and the formation of deep waters. In this study, we use a surface drifter dataset spanning the period 1990–2023 to investigate the pathways followed by waters originating from Davis Strait and Hudson Strait on the Labrador shelf and into the interior subpolar North Atlantic. Recent drifter deployments in the region allow for an improved understanding of the circulation on the Labrador shelf, in particular its northern part, where prior data were sparse. We show that waters originating from Davis Strait and Hudson Strait remain on the shelf as they flow downstream until they reach the Newfoundland shelf. This confirms that very little exchange takes place between the Labrador shelf and the interior Labrador Sea. Decomposing the Labrador shelf into five regions, we further describe typical pathways for these waters and show that extensive exchanges take place between the coastal and shelf-break branches of the Labrador Current. Our results suggest that if an increasing amount of freshwater reaches the Labrador shelf, it would not directly affect the Labrador Sea convection region; instead, it would lead to the formation of a salinity anomaly off the Grand Banks, which could then circulate around the subpolar North Atlantic. 

Quantifying the strength of the Atlantic Meridional Overturning Circulation (AMOC) involves separating the northward and southward limbs and calculating their volume transports. The limbs can be distinguished either by depth level or by density class, but recent results have indicated that this choice of coordinate system leads to divergent results, both in terms of the AMOC mean state and its variability. Here, we demonstrate that the AMOC in density coordinates is more informative of the large-scale, three-dimensional AMOC structure, is more closely aligned with the AMOC’s climatic impact via oceanic meridional heat transport, and retains more information about future AMOC pathways than the depth space definition. Adopting a commonly accepted definition of the AMOC in density coordinates will unify a divided literature and promote progress in the field. This commentary thus highlights that the coordinate system used to define the AMOC matters, not only for understanding physical processes and past variations that remain elusive, but also for physically appropriate monitoring of its future evolution. 

In the 2010s, a large freshening event similar to past Great Salinity Anomalies occurred in the Iceland Basin that has since propagated into the Irminger Sea. The source waters of this fresh anomaly were hypothesized to have come from an eastward diversion of the Labrador Current, a finding that has since been supported by recent modeling studies. In this study, we investigate the pathways of the freshwater anomaly using a purely observational approach: particle tracking using satellite altimetry-derived surface velocity fields. Particle trajectories originating in the Labrador Current and integrated forward in time entered the Iceland Basin during the freshening event at nearly twice the frequency observed prior to 2009, suggesting an increased presence of Labrador Current-origin water in the Iceland Basin and Rockall Trough during the freshening. We observe a distinct regime change in 2009, similar to the timing found in the previous modeling papers. These spatial shifts were accompanied by faster transit times along the pathways which led to along-stream convergence and more particles arriving to the eastern subpolar gyre. These findings support the hypothesis that a diversion of relatively fresh Labrador Current waters eastward from the Grand Banks can explain the unprecedented freshening in the Iceland Basin.