Formation of dense water over the North Atlantic subpolar gyre in a hierarchy of climate models

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate. Recent observations have highlighted the dominant role of the buoyancy forcing in the transformation of surface waters to the AMOC lower limb at subpolar latitudes. The short (4 years) length of the OSNAP timeseries, however, limits conclusions over longer time scales. To investigate a wide range of temporal scales, we use three 100-years long coupled simulations of HadGEM3-GC3.1, at resolutions ranging from ~130 km atmosphere and 1° ocean to 25 km atmosphere and 1/12° ocean. In line with observations, the models show that the mean overturning and buoyancy-induced transformation are concentrated in the eastern subpolar gyre rather than in the Labrador Sea.

However, the horizontal resolution of the models impacts the formation of dense water over the subpolar gyre. An unrealistically large sea ice extent induces a weak buoyancy-induced transformation over the western subpolar gyre at low resolution, while a bias in surface density produces too dense water at high resolution. These biases are associated with a shift in the location of dense water formation. The transformation is mainly localized in the interior of the Irminger and Labrador seas at low resolution, and over the boundary current at high resolution. The interannual variability of the transformation is thus driven by different mechanisms between the simulations. In contrast with observations, the interannual variance in air-sea fluxes plays a more prominent role in the variance of transformation along the boundary current at high resolution.