Abstract
Marine extreme events such as marine heatwaves, ocean acidity extremes
and low oxygen extremes can pose a substantial threat to marine
organisms and ecosystems. Such extremes might be particularly
detrimental (i) when they occur compounded in more than one stressor,
and (ii) when the extremes extend substantially across the water column,
restricting the habitable space for marine organisms. Here, we use daily
output from a hindcast simulation (1961-2020) from the ocean component
of the Community Earth System Model (CESM) to characterise such
column-compound extreme events (CCX), employing a relative threshold
approach to identify the extremes and requiring them to extend
vertically over at least 50m. The diagnosed CCXs are prevalent,
occupying worldwide in the 1960s about 1% of the volume contained
within the top 300m. Over the duration of our simulation, CCXs become
more intense, last longer, and occupy more volume, driven by the trends
in ocean warming and ocean acidification. For example, the triple CCX
have expanded 24-fold, now last 3-times longer, and have become 6-times
more intense since the early 1960s. Removing this effect with a moving
baseline permits us to better understand the key characteristics of the
CCXs. They last typically about 10 to 30 days and predominantly occur in
the tropics and the high latitudes, regions of high potential biological
vulnerability. Overall, the CCXs fall into 16 clusters, reflecting
different patterns and drivers. Triple CCX are largely confined to the
tropics and the North Pacific, and tend to be associated with the El
Nino-Southern Oscillation.