Characteristics of Recent Column-Compound Extremes in the Global Ocean

Marine extreme events can be detrimental to organisms and ecosystems across the global ocean. Marine heatwaves (MHW), ocean acidity extremes (OAX) and low oxygen extremes (LOX) occur everywhere, superimposed on the already changing trends of temperature, pH, and oxygen. When such extremes concurrently occur in the vertical column, the habitable space for marine organisms can be severely reduced. We use daily output from a hindcast simulation with the Community Earth System Model coupled with Biological Elemental Cycling (CESM-BEC) for the period 1961-2020 to characterise such compound extremes. Extreme conditions are identified on a moving baseline with the 95th or 5th percentile, and an additional absolute threshold of 150 μM for low oxygen conditions. To investigate compound events in the vertical dimension, at least 50 m out of the top 300 m is required to be extreme with respect to each stressor. Such an event is termed a column-compound extreme event (CCX). On average, 1% of the global ocean volume is occupied by CCXs and up to 5% at maximum. On a fixed baseline these values increase to 8% and 27% respectively. CCXs decrease habitable space by up to 75% and have high intensity index of more than 2 in many regions around the globe. El Niño-Southern Oscillation events are found to be strongly associated with CCXs in the tropical and subtropical Pacific, but also other ocean basins through teleconnections. The global volume of CCXs is expected in increase in the future, exacerbating impacts and reducing habitable space of marine organisms.