Improving Antarctic Bottom Water precursors in NEMO for climate applications

Abstract

Abstract. The world’s largest ice shelves are found in the Antarctic Weddell and Ross Seas 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 forced configuration of NEMO (eORCA1) to explicitly simulate circulation beneath Filchner-Ronne (FRIS), Larsen C (LCIS), and Ross (RIS) ice shelves. 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 net melt rates of FRIS (117 ± 21 Gt/yr), LCIS (36 + 7 Gt/yr) and RIS (112 + 22 Gt/yr) 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 re-organises the shelf circulation to bring the regional model hydrography closer to observations. A change in AABW within the Weddell and Ross Seas 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.

GOSI9: UK Global Ocean and Sea Ice configurations

“…For climate applications requiring open ice cavities, such as UK Earth System Model (UKESM) configurations (Smith et al, 2021), improving water mass properties on the shelf is essential. Using a 1 • forced configuration, Hutchinson et al (2023) show that explicitly simulating the circulation beneath the largest ice shelves improves the realism of the Antarctic continental shelf circulation. However, they note that the impact on the representation of the Antarctic Bottom Water is limited by the absence of realistic overflows.…”

“…Grid space dependent values have been tested and found to give small improvements in model fidelity. A modification to the input file similar to that applied by Hutchinson et al (2023) will be necessary for applications in which the Antarctic ice shelf cavities are opened.…”

Emulating Present and Future Simulations of Melt Rates at the Base of Antarctic Ice Shelves With Neural Networks

“…However, currently, these cavities are only resolved by ocean models on rare occasions. We advocate to strongly push efforts toward resolving these two cavities in ocean models by default, when possible, even at the lower resolution of 1°, as was already done for NEMO in Smith et al (2021) or Hutchinson et al (2023).…”