Drivers of column-compound extremes in the Southern Ocean

Vertically compounded marine heatwaves (MHW) and ocean acidity extremes (OAX) impact marine biota and contract habitable space in the epipelagic zone. On a climate trend of warming, acidification, and sea ice reduction, these extreme events pose additional risks to Antarctic and Southern Ocean ecosystems. Anomalously low seasonal sea ice diminishes a critical habitat and grazing area for zooplankton and fish larvae. Co-occurring MHW and OAX impose further thermal and physiological stress on organisms that are typically adapted to narrow environmental conditions. Using a regional ocean model hindcast (1980-2019), we analysed column-compound extreme (CCX) events in temperature and the hydrogen ion concentration. Results indicate an increasing frequency of warmer and more acidic events, particularly in the Antarctic Marine Protected Areas (MPAs). These events can span over 200 000 km² in area and persist for more than 500 days, occurring during periods of low sea ice and the positive phase of the Southern Annular Mode (SAM). Through driver attribution, we identified two main processes driving CCX. In the Antarctic zone, buoyancy changes driven by increased Ekman drift or reduced sea ice concentration, drive CCX at varying depths. In the Subantarctic and Northern zones, surface MHW can drive co-occurring OAX at or below the surface by influencing primary production, which is modulated by nutrient limitations. Overall, CCX is the Southern Ocean is found to be predominantly driven by dynamical and biogeochemical changes. These analyses elucidate the processes leading to CCX in the Southern Ocean, establishing a basis for their future predictability.