Compound Marine Heatwaves and Acidity Extremes in the Southern Ocean

Compound extremes of temperature and acidity that extend over substantial fractions of the water column can be particularly damaging to marine organisms, as they experience not only additional stress by the potentially synergistic effects of these two stressors, but also a reduction in habitable vertical space. Here, we detect and analyze such column‐compound extremes (CCX) in the Southern Ocean between 1980 and 2019, and characterize their duration, intensity, and spatial extent. To this end, we use daily output from a hindcast simulation of the Regional Ocean Modeling System (ROMS), coupled with the Biological Elemental Cycling (BEC) model. We first detect extremes in temperature and acidity ([]) within the top 300 m using a relative threshold of 95% and then identify CCX where conditions are extreme for both stressors for at least 50 m of the water column. When analyzed on a fixed baseline, positive trends in ocean warming and acidification caused CCX to last longer, intensify, and expand throughout the Southern Ocean. In the Antarctic zone, CCX expanded between 1980 and 2019 more than ten times in volume, lasted up to 120 days longer, and doubled in anomaly. Some of the largest and longest events occurred in Antarctic Marine Protected Areas (MPAs), covering more than 200,000 km 2 and persisting for over 500 days. CCX in the Subantarctic and Northern zones quadrupled in volume and increased by more than 30% in anomaly. Across the Southern Ocean, the increasing occurrence of CCX exacerbates the risks to marine ecosystems from warming and acidification.