Understanding the Sensitivity of the North Atlantic Subpolar Overturning in Different Resolution Versions of HadGEM3-GC3.1

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate but is not simulated consistently across models or model resolutions. Here, we use a hierarchy of the global coupled model HadGEM3-GC3.1, with ocean resolutions of 1 & DEG;, 1/4 & DEG;, and 1/12 & DEG;, to evaluate the subpolar AMOC and its sensitivity to horizontal resolution. In line with observations, the models show that the mean overturning and surface forced water mass transformation (SFWMT) are concentrated in the eastern subpolar gyre rather than in the Labrador Sea. However, the magnitude of the overturning along the OSNAP line at medium and high resolutions is 25% and 40% larger than in the observations, respectively. This disagreement in overturning strength is noted for both OSNAP East and OSNAP West, and is mainly due to anomalously large SFWMT rather than anomalously large interior mixing or overflow transport from the Nordic Seas. Over the Labrador Sea, the intensification of SFWMT with resolution is explained by a combination of two main biases. Anomalously warm surface water enhances heat loss and reduces the extension of marginal sea ice, which increases the surface density flux over the boundary of the basin. A bias in salinity leads to anomalously dense surface water that shifts the outcropping area of the AMOC isopycnal and results in intense dense water formation along the boundary of the basin at medium and high resolutions. Thus, our analysis sheds light on a range of model biases responsible for large overturning over the Labrador Sea in climate models. The circulation and formation of dense water over the North Atlantic plays a key role for the Northern Hemisphere climate as it influences sea surface temperature, the formation of hurricane and the intensity of rainfall over Europe. It is thus important to evaluate how this circulation will change in a context of global climate change. However, the simulated overturning is not necessarily consistent with observations and differs across models or model resolutions. In this work, we evaluate the impact of horizontal resolution for the overturning over the subpolar gyre in the climate model HadGEM3-GC3.1. We find that its magnitude at medium and high resolutions is 25% and 40% larger than in the observations, respectively. The water mass transformation induced by the air-sea exchanges (SFWMT) over the subpolar gyre mainly explains this disagreement. Over the Labrador Sea, the relatively strong SFWMT in these models is related to anomalously small sea ice coverage as well as anomalously warm and salty surface water. The subpolar overturing is 25% and 40% too strong in HadGEM3-GC3.1 at medium and high resolutions compared to OSNAP observationsThe anomalously large overturning at OSNAP is consistent with anomalously large surface forced transformations over the subpolar gyreLarge surface forced transformation over the Labrador Sea is explained by anomalously warm and salty water over the boundary of the basin