Improving Antarctic Bottom Water precursors in NEMO for climate applications

Abstract

The world's largest ice shelves are found in the Antarctic Weddell Sea and Ross Sea where complex interactions between the atmosphere, sea ice, ice shelves and ocean transform shelf waters into High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW), the parent waters of Antarctic Bottom Water (AABW). This process feeds the lower limb of the global overturning circulation as AABW, the world's densest and deepest water mass, spreads outwards from Antarctica. None of the coupled climate models contributing to CMIP6 directly simulated ocean-ice shelf interactions, thereby omitting a potentially critical piece of the climate puzzle. As a first step towards better representing these processes in a global ocean model, we run a 1° resolution Nucleus for European Modelling of the Ocean (NEMO; eORCA1) forced configuration to explicitly simulate circulation beneath the Filchner-Ronne Ice Shelf (FRIS), Larsen C Ice Shelf (LCIS) and Ross Ice Shelf (RIS). These locations are thought to supply the majority of the source waters for AABW, and so melt in all other cavities is provisionally prescribed. Results show that the grid resolution of 1° is sufficient to produce melt rate patterns and total melt fluxes of FRIS (117±21 Gt yr-1), LCIS (36±7 Gt yr-1) and RIS (112±22 Gt yr-1) that agree well with both high-resolution models and satellite measurements. Most notably, allowing sub-ice shelf circulation reduces salinity biases (0.1 psu), produces the previously unresolved water mass ISW and reorganizes the shelf circulation to bring the regional model hydrography closer to observations. A change in AABW within the Weddell Sea and the Ross Sea towards colder, fresher values is identified, but the magnitude is limited by the absence of a realistic overflow. This study presents a NEMO configuration that can be used for climate applications with improved realism of the Antarctic continental shelf circulation and a better representation of the precursors of AABW.