Double and triple whammies: Compound extremes in ocean biogeochemistry

Extremes in sea-surface temperature, aka marine heat waves, have received considerable attention in the last few years. However, we know very little about the characteristics of corresponding extremes in ocean biogeochemistry, especially those associated with deoxygenation, nutrient stress, and ocean acidification. Of particular concern are the compound events, i.e., those where more than one of these stressors are concurrently or consecutively outside the norm. Here we investigate such events from the global to the regional scale. To this end, we are combining analyses of in-situ and remote observations with model output from regional high-resolution models and define extremes using a combination of relative (temperature, nutrient stress) and absolute (oxygen and saturation state with respect to aragonite) thresholds. Singular events, i.e., those that are extreme with respect to only one variable are identified using a simple peak over threshold method, while we employ multi-variate methods for identifying compound events. The preliminary global-scale analyses suggest that the eastern North Pacific represents a hotspot of “hot” and “sour” double whammies, i.e., events when both temperature and pH are outside the norm. The frequency and intensity of these events has been increasing rapidly, largely caused by the long-term ocean acidification-driven trend in the extremes characterized by anomalously low pH conditions. These “hot”-“sour” compound events occur in summer, when temperatures are maximum, and the offshore transport brings anomalously low pH waters from the California Current System into the eastern North Pacific. We find triple whammies (“hot”, “sour”, & “no breath”) along the flanks of the oxygen minimum zones in the eastern tropical and subtropical Pacific, largely associated with mesoscale eddies creating local conditions when all three parameters are extreme.