To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

for the AMOC deep limb. In order to assess the impact of the interior pathway relative to the DWBC pathway, this work seeks to quantify the AMOC deep limb pathways in ocean circulation models, compare the pathway signatures of these models to observations, and identify a mechanism driving the interior pathway. The partitioning of the AMOC deep limb into interior and DWBC pathways is observed in several ocean models. Furthermore, there is a good agreement between the structure of the export pathways in models and observations. Both Eulerian and Lagrangian techniques, in models and observations, are used to identify the DWBC and interior pathways and these two perspectives are shown to be compatible with one another.Finally, deep, eddy-driven, recirculation gyres are shown to be a mechanism driving the interior pathway and the existence of the interior pathway is consistent with the vorticity balance at depth. The interior pathway makes a significant contribution to the total transport of the deep limb of the AMOC. Since the interior pathway is much broader and slower than the DWBC pathway, the large-scale transport of climate signals, heat, and anthropogenic CO 2 associated with the AMOC are slower and mixed more broadly throughout the ocean than once thought. iv

Though critically important in sustaining the ocean's biological pump, the cycling of nutrients in the subtropical gyres is poorly understood. The supply of nutrients to the sunlit surface layer of the ocean has traditionally been attributed solely to vertical processes. However, horizontal advection may also be important in establishing the availability of nutrients. Here we show that the production and advection of North Atlantic Subtropical Mode Water introduces spatial and temporal variability in the subsurface nutrient reservoir beneath the North Atlantic subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients.

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