Drivers of Compound Marine Heatwaves and Acidity Extremes in the Southern Ocean

Our previous work revealed that between 1980 and 2019, Column-Compound eXtremes (CCX) of Marine Heatwaves (MHW) and Ocean Acidity eXtremes (OAX) increased in frequency and intensity in the Southern Ocean (Wong et al., 2025, https://doi.org/10.1029/2025gb008630), disproportionately affecting the Antarctic zone and its Marine Protected Areas (MPAs). These extremes were identified using a fixed baseline in hindcast simulations with the Regional Ocean Modeling System (ROMS) coupled with the Biological Elemental Cycling (BEC) model. Extremes were classified as CCX when at least 50 m of each extreme was present in the upper 300 m. Here, we identify the driving processes of these CCX, using a detrended baseline to better separate these processes from the trend. We group these detrended CCX into four clusters, according to the locations of MHW and OAX within the water column. Surface CCX are primarily a response of MHW thermodynamically increasing acidity, with increased stratification reducing productivity. Subsurface CCX, particularly the most extreme ones around Antarctica, are mostly caused by the upwelling of warm and acidic Upper Circumpolar Deep Water. The CCX characterized by surface MHW and subsurface OAX tend to be initiated by MHW, which stimulates higher productivity and export, thereby enhancing remineralization at depth. The occurrence of these clusters is modulated interannually by large-scale climate variability that influences Ekman divergence and upper-ocean stratification. Improving our understanding of the links between large-scale climate variability and local CCX processes provides a foundation for improving the predictability of CCX and assessing their potential impact on marine ecosystems, particularly within MPAs.