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.

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC.

Abstract. Wintertime convection in the North Atlantic Ocean is a key component of the global climate as it produces dense waters at high latitudes that flow equatorward as part of the Atlantic Meridional Overturning Circulation (AMOC). Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of North Atlantic Deep Water. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the AMOC lower limb. Air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation. We analyze these contributions to the transformation in order to better understand the connection between atmospheric forcing and the densification of surface water. More precisely, we study the impact of air–sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that the variance in SPMW transformation is mainly influenced by the variance in density at the ocean surface. This surface density is set by a combination of advection, wind-driven upwelling and surface fluxes. Our study shows that the latter explains ∼ 30 % of the variance in outcrop area as expressed by the surface area between the outcropped SPMW isopycnals. The key role of the surface density in SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–2015 over the Iceland Basin.

The Reykjanes Ridge is a major topographic feature located south of Iceland in the North Atlantic Ocean that strongly influences the subpolar gyre circulation. Based on velocity and hydrographic measurements carried out along the crest of the Reykjanes Ridge from the Icelandic continental shelf to 50°N during the RREX cruise in June-July 2015, we derived the first direct estimates of volume and water mass transports over the Reykjanes Ridge. North of 53.15°N, circulation was mainly westward; south of this latitude it was mainly eastward. The westward transport was estimated at 21.9 ± 2.5 Sv (Sv = 10 6 m 3 s À1 ) and represents the subpolar gyre intensity. The westward flows followed two main pathways at 57°N near the Bight Fracture Zone and at 59-62°N. We argue that those pathways were connected to the northern branch of the North Atlantic Current and to the Sub-Arctic Front, respectively, which were both intersected by the southern part of the section. In addition to this horizontal circulation, mixing and bathymetry shaped the water mass distribution. Water mass transformations in the Iceland Basin lead to the formation of weakly stratified Subpolar Mode Water. We explain why Subpolar Mode Water, the main water mass contributing to the westward flow, was denser at 57°N than at 59-62°N. At higher densities, both Intermediate Water and Icelandic Slope Water contributed more to the westward transport across the Reykjanes Ridge than the sum of Labrador Sea Water and Iceland-Scotland Overflow Water.Plain Language Summary The Reykjanes Ridge, the northern section of the Mid-Atlantic Ridge, strongly influences the cyclonic circulation of the North Atlantic subpolar gyre, a major component of the climate system. Up to now, no dedicated data set was available to describe the circulation across this ridge. To fill this gap, surface-to-bottom measurements of flow velocity and water mass properties were carried out along the crest of the ridge, from Iceland to 50°N, in 2015. North of 53.15°N, the flow was mainly westward. It defines the westward branch of the subpolar gyre, and our study provides the first direct estimate of its intensity. The westward flow followed two main pathways related to specific bathymetry features: at the Bight Fracture Zone (57°N), which is a deep opening in the ridge, and at 59-62°N where the bathymetry rapidly deepens southward. The horizontal circulation of the Iceland Basin connects these pathways to the North Atlantic Current flowing eastward south of 53.15°N. Knowledge of the westward cross-ridge flows is a prerequisite for understanding the northward evolution of the Irminger Current, a major conduit for the subtropical waters toward the deep convection regions in the Irminger and Labrador Seas.