Surface and subsurface compound events under climate change

Marine species are increasingly threatened by extreme and compound events driven by climate change, as warming, deoxygenation, and acidification unfold. Yet, the surface and especially the subsurface distribution and evolution of such compound events remain poorly understood. We present the current and projected distributions of compound marine heatwave (MHW), low oxygen (LOX), and high acidity (OAX) events throughout the water column, using observation-based data from 2004-2019 and large ensemble simulations from 1890-2100 based on the Earth system model GFDL-ESM2M. Our findings reveal that compound MHW-OAX and OAX-LOX events are prevalent in low to mid latitude at the ocean surface. At 200m and 600m, MHW-LOX and MHW-OAX events are relatively frequent in high latitudes and parts of the tropics, while OAX-LOX events occur globally. These subsurface events are often associated with vertical water mass displacements. Projections show a strong rise in compound event frequency over the historical period and under continued global warming, primarily driven by shifts in mean oceanic conditions. However, localized decreases in subsurface events may occur due to complex physical and biogeochemical interactions. The portion of the top 2000m affected by extreme or compound events rises from 20% to 99% by the end of the 21^st century under a high emissions scenario using a preindustrial baseline, and to 35% using a shifting baseline. This increase poses a major threat to marine ecosystems, potentially disrupting food webs and biodiversity.