| Summary: | 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 that air-sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. Further we show that the seasonal to interannual variability of the overturning streamfunction at OSNAP cannot be linked to changes in the transformation by air--sea fluxes and mixing. This implies that water mass transformation anomalies in the region to the north of the OSNAP array do not manifest as overturning variability at the latitude of the OSNAP array on seasonal to interannual timescales. This is likely linked to the contrasting transport time-scales and export pathways of the subpolar basins and Nordic Sea. This study therefore highlights that for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small scale processes and circulation pathways that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.
|
|---|