Impact of grid resolution on the abyssal ocean representation in numerical models

Accurately representing abyssal water masses and their inter-basin pathways remains a challenge for global ocean models and reanalyses. In this study, we investigate how horizontal and vertical resolution influence the simulation of abyssal waters by first evaluating three ocean reanalyses and one forward ocean model, focusing on Antarctic Bottom Water pathways from the Weddell Sea to the Argentine and Brazil basins and through the Vema Channel. Model outputs are evaluated against WOA18 climatology and in situ observations from moorings and hydrographic sections. Building on the recurring limitations identified in this intercomparison, we conduct four targeted experiments with the Finite-Volume Sea Ice–Ocean Model (FESOM 2), modifying horizontal and vertical grid resolution while keeping all other model components unchanged. The experiments show that increasing vertical resolution substantially improves the representation of cold and dense abyssal waters and their inter-basin connectivity, whereas horizontal refinement alone does not systematically improve the representation of abyssal properties and can even degrade it when mixing parameterizations are not adequately tuned. Combining vertical and horizontal refinement improves specific local features, including the structure of the abyssal flow and the realistic eastward deflection of the Antarctic Bottom Water core within the Vema Channel, but does not outperform vertical refinement alone at the basin scale.