Antarctic Dense Water Formation Sensitivity to Ocean Surface Cell Thickness

Dense water formation on the Antarctic continental shelf is the main process by which Antarctic Bottom Waters form and is fundamental to the abyssal overturning circulation. However, most ocean models fail to simulate Antarctic dense water formation on the continental shelf and flow down the continental slope (i.e., overflow) due to resolution constraints. While the impact of horizontal and vertical resolution on the overflows has been previously studied, the effect of surface vertical resolution on dense water formation remains unexplored. To address this gap, we vary the surface ocean grid cell of two dense water-forming models from 1.1 to 5.1 m thickness. We used two ocean and sea ice models, each employing a different boundary layer parameterization scheme. In one model, thickening the surface cell to 5.1 m reduced dense water formation by 64% and led to the complete cessation of the overflow after 10 years of simulation. In the other, the same thickening decreased dense water formation by 32% and overflow by 67% over the same period. The dense water formation reduction in the experiments with thicker surface grid cells is explained by a southward shift in the surface Ekman transport, which brings light offshore waters to the coast and limits dense water formation at the continental shelf. Although dense water formation responds to surface layer thickening in both models, differences in sea ice production contribute to greater sensitivity in one case, where a weaker sea ice formation in the 5.1 m configuration further decreases dense water production. These results highlight that a high vertical resolution at the ocean surface is required to form Antarctic dense waters.