Dense water formation in the Nordic Seas diagnosed from sea surface buoyancy fluxes

The relationship between the surface water mass transformation over the Nordic Seas and the large-scale diapycnal overturning flow through the same region was examined. The surface transformation was estimated from climatological and reanalyzed surface hydrography and air–sea buoyancy fluxes. Ocean currents and transports were assembled from climatological wind stresses and modern hydrographic data with reference level velocities for thermal wind calculations gathered from independent studies and adjusted in a linear inverse model. The data reveal a close match between the surface transformation and the magnitude of the large-scale diapycnal overturning. This result confirms an existing hypothesis that turbulent fluxes of buoyancy and nonlinearities in the equation of state are of secondary importance in high-latitude dense water production regions. Fields from a coupled numerical ocean–ice model also suggest that the seasonal cycle of freezing and melting within the Nordic Seas has a small net effect on the surface transformation, particularly at high densities (<10%)(<10%). Finally, the data confirm that the bulk of the dense water with eventually feeds the North Atlantic Ocean via the Greenland–Scotland Ridge is formed within the Nordic Seas. Most of this dense water production takes place east of the Mohn–Knipovich ridge system with particularly high production over the Norwegian and Lofoten basins.