The impact of model resolution on the North Atlantic multidecadal response to anthropogenic sulphate aerosols
Abstract. Anthropogenic sulphate aerosol is a primary forcing agent in the historical period, and its exact impact on North Atlantic SST variability remains an open question. A set of novel, idealised, single-forcing experiments was performed to isolate the physical processes. The medium-resolution (60 km atmosphere, 0.25° ocean) and low-resolution (135 km atmosphere, 1° ocean) of the HadGEM3-GC3.1 model were used to investigate the impact of resolution on the forced response. The SST response at both resolutions is timescale dependent: a fast, large-scale surface cooling is followed by a slow, small-scale subpolar warming lagging by 15–20 years. Warming of the subpolar North Atlantic is due to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) and is stronger at the medium resolution. This resolution difference is because sea ice growth in the Labrador Sea (LS) is stronger at low resolution, which inhibits air-sea interaction and reduces surface forced dense water formation, leading to a weaker AMOC response. There is also evidence of a stronger AMOC positive feedback involving salt-advection at medium-resolution. These results show that the large-scale North Atlantic response to external forcing can be sensitive to regional differences, such as model climatology of LS sea ice and its response to aerosol cooling.