Mixing and air-sea buoyancy fluxes drive the overturning circulation in the subpolar North Atlantic

Abstract

<p>The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm/salty Atlantic Water into cold/fresh North Atlantic Deep Water (NADW). The magnitude and variability of the overturning at the OSNAP  mooring array can therefore be linked to the water mass transformation by air-sea buoyancy fluxes and mixing in the region to the north of the OSNAP array. Here, we estimate these water mass transformations using a combination of observational-based, reanalysis-based and model-based datasets. Our results highlight the complementary roles of air-sea buoyancy flues and mixing in setting the time-mean magnitude of the overturning at OSNAP. A cooling by air-sea heat fluxes and a mixing-driven freshening in the Nordics Seas, Iceland basin and Irminger basin, precondition the warm/salty Atlantic Water, forming subpolar mode water classes. Mixing in the interior of the Nordic Seas, over the Greenland-Scotland ridge and along the boundaries of the Irminger and Iceland basins drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. In contrast to the time-mean magnitude, the seasonal variability of the overturning streamfunction at OSNAP cannot be linked to changes in the transformation by air-sea fluxes and mixing. The seasonal variability of air-sea fluxes and mixing greatly exceed the seasonal variability measured in the overturning at OSNAP. This implies that water mass transformation anomalies in the region to the north of the OSNAP array don't manifest as overturning variability at the latitude of the OSNAP array on seasonal timescales. Air-sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the overturning within the subpolar North Atlantic and Nordic Seas. Further, this study highlights that for climate models to realistically simulate the overturning circulation in the North Atlantic, the small scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.</p>