Strong sensitivity of Antarctic dense water formation to surface vertical resolution

Many models fail to accurately represent the formation of dense waters over the Antarctic continental shelf with a sufficiently high density to overflow into the abyss. This study probes a suite of 0.1deg MOM5- and MOM6-based ocean model configurations that have a good representation of Antarctic dense water formation. We find a strong sensitivity of dense water formation and transport to the thickness of the top vertical layer in the ocean model. Coarser resolution in the top vertical layer decreases the density and rate of Antarctic dense water formation, with a 60% reduction in dense water formation when the upper layer thickness is increased from 1m to 5m. This result is insensitive to the choice of ocean surface boundary layer mixing scheme and occurs with both the KPP and ePBL parameterisations. The sensitivity is a result of the salinity-driven dense water formation around Antarctica, and the interaction of sea ice freshwater fluxes only with the top vertical layer. Whereas we find that North Atlantic convection is largely insensitive to upper ocean vertical resolution, because it is driven primarily by heat fluxes.