The ACCESS-AM2 climate model underestimates aerosol concentration in the Southern Ocean; improving aerosol representation could be problematic for the global energy balance

Abstract. The interaction of natural marine aerosol with clouds and radiation is a significant source of climate model uncertainty. The Southern Ocean represents a key area to understand these interactions, and a region where significant model biases exist. Here we provide an evaluation of the Australian Community Climate and Earth System Simulator atmosphere model which includes a double-moment aerosol scheme. We evaluate against measurements of condensation nuclei (N10) and cloud condensation nuclei (CCN) number from seven ship campaigns and three terrestrial locations, spanning the years 2015–2019. We find that N10 is heavily underestimated in the model across all regions and seasons by more than 50 % and in some cases by over 80 % at higher latitudes. CCN is also strongly underestimated over marine and Antarctic regions, often by more than 50 %. We then perform seven sensitivity tests to explore different aerosol configurations. We find that updating the dimethyl sulfide climatology and turning on the primary marine organic aerosol flux marginally improves marine CCN by between 4 %–9 %. N10 was reduced by between 3 %–9 %. The Southern Ocean radiative bias is also reduced by this combination of changes, with limited adverse effects. We also test altering the sea spray flux to use wind gust instead of mean wind speed. This significantly improved CCN in the marine regions, but resulted in detrimental impacts on the region's radiation budget, indicating that drastically improving the Southern Ocean's CCN budget may lead to poorer simulations of the global climate.