ALBERT

Description: 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.

Description: Stratospheric warming events have been extensively studied on the Northern Hemisphere. The interest in stratospheric warming events on the Southern Hemisphere (SH) increased recently after a minor stratospheric warming (MSW) in 2019 had significant surface impacts. These were especially large over Australia: the strong negative phase of the Southern Annular Mode (SAM) favored extreme hot and dry conditions and thereby the occurrence of severe wildfires over the eastern part of the continent. Here, we use a large ensemble of AMIP-like simulations with ECHAM6 as the atmosphere component to investigate the effect of different global warming levels relative to pre-industrial conditions (+1.5K, +2K, +3K and +4K) on the frequency and surface impact of SH MSWs. We find that increasing global warming levels leads to a decrease in the number of MSWs that have an impact on the tropospheric SAM, consistent with previous studies. The coupling between stratosphere and troposphere starts to decrease at +2K warming. Nevertheless, also under +4K conditions, MSWs with a significant surface impact can still occur. The modeled surface response is close to that observed in 2019 with warmer and dryer conditions over eastern Australia for most of the warming levels. Furthermore, we evaluate the surface impact of MSWs on South Africa, South America and New Zealand. At these regions, we find different responses under increasing global warming conditions. Over South Africa, for example, the largest surface response, also characterized by dry and warm conditions, is found for +2K warming.