Reducing Southern Ocean biases in the FOCI climate model

• Main Authors: Kjellsson, J., Wahl, S., Ödalen, M., Bischof, S., Harlass, J., Kummer, L., Martin, T., Pilch-Kedzierski, R., Zeller, M., Park, W.
• Format: Conference Object
• Language: English
• Published: 2023
• Subjects: Antarctic; Southern Ocean; The Antarctic; Weddell; Antarc*; Sea ice
• Online Access: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5021782

Biases in mid- to high-latitude Southern Hemisphere ocean and atmosphere temperatures, winds, currents, and other properties are a common issue in climate models. FOCI is a fully coupled climate model employing a 1/2° NEMO3.6 ocean and a T63 ECHAM6.3 atmosphere as default, including modules representing sea ice (LIM2) and land surface (JSBACH) processes. Similar to some CMIP models, FOCI has a warm bias in the surface and intermediate ocean across the Southern Ocean, and jet stream winds in the Southern Hemisphere are displaced equatorward. This wind bias is theorized to be partly due to biases in sea surface properties. The Antarctic Circumpolar Current (ACC) is weak in FOCI compared to observations, which is common in models of intermediate resolution. In this study, we test approaches of improving the above-mentioned biases. Using AGRIF nesting, we run additional simulations where ocean resolution south of 28°S is increased from 1/2˚ to 1/10°, which yields a stronger ACC transport, as discussed by Martin et al. in this session. Shortening the ocean-atmosphere coupling time step, from 3-hourly to hourly, clearly reduces the warm bias, and in consequence, improves the representation of sea ice and water mass properties at depth. This links to a weaker Weddell Gyre. The change in coupling frequency is effective at default as well as at nest resolution. Further improvements in simulated surface temperature and sea ice are found with reduced iso-neutral diffusion. The jet stream position remains unchanged, suggesting that it is insensitive to the surface temperature and sea ice biases.