Towards detecting biogeochemical compound extremes in the surface ocean

Long-term changes in ocean biogeochemistry that are projected under an evolving climate in the 21^st century are superimposed by short-term extreme events. Of particular interest are compound events, where such extreme events occur successively or simultaneously, combining or amplifying the impact of multiple stressors on ocean ecosystems. The resilience of marine species to the simultaneous exposure of extremely high temperature, low pH and low oxygen concentration presumably depends on the magnitude and variability of the perturbation, which is likely to increase and intensify in response to rising global mean temperatures. However, changes in marine heat waves, ocean acidification and deoxygenation extremes, remain to be detected, in order to quantify their combined impact. Here, we use the Grand Ensemble of the fully coupled Max Planck Institute Earth System Model (MPI-GE) that consists of 100 members forced by historical CO₂ emissions and those according to the Representative Concentration Pathway 4.5 (RCP4.5). The daily frequency of the simulation output for sea surface temperature, hydrogen ion concentration and oxygen concentration allows analysing spatio-temporal changes of marine extreme events between 1850 and 2100. We assess the number, duration, and intensity of extreme states using a moving threshold criterion, and aim to identify concurrent and consecutive driving mechanisms for such events in the surface ocean in order to evaluate potential risks for the marine ecosystem.