Compound high temperature and low chlorophyll extremes in the ocean over the satellite period

Abstract

Abstract. Extreme events severely impact marine organisms and ecosystems. Of particular concern are compound events, i.e., when conditions are extreme for multiple potential ecosystem stressors such as temperature and chlorophyll. Yet, little is known about the occurrence, intensity and duration of such compound high temperature (aka marine heatwaves – MHWs) and low chlorophyll (LChl) extreme events, whether their distributions have changed in the past decades and what the potential drivers are. Here we use satellite-based sea surface temperature and chlorophyll concentration estimates to provide a first assessment of such compound extreme events. We reveal hotspots of compound MHW and LChl events in the equatorial Pacific, along the boundaries of the subtropical gyres, in the northern Indian Ocean, and around Antarctica. In these regions, compound events that typically last one week occur three to seven times more often than expected under the assumption of independence between MHWs and LChl events. The occurrence of compound MHW and LChl events varies on seasonal to interannual timescales. At the seasonal timescale, most compound events occur in summer in both hemispheres. At the interannual time-scale, the frequency of compound MHW and LChl events is strongly modulated by large-scale modes of natural climate variability such as the El Niño-Southern Oscillation, whose positive phase is associated with increased compound event occurrence in the eastern equatorial Pacific and in the Indian Ocean by a factor of up to four. Our results provide a first understanding of where, when and why compound MHW and LChl events occur. Further studies are needed to identify the exact physical and biological drivers of these potentially harmful events in the ocean and their evolution under global warming.

Marine high temperature extremes amplify the impacts of climate change on fish and fisheries

“…The occurrence of extreme high-temperature events across many regions of the ocean is often associated with the dominant climate modes. The occurrence of extreme high-temperature events is especially elevated during El Niño years (31,32) and conversely reduced during La Niña years. Marine annual extreme high-temperature events add to decadalmean warming impacts on fisheries stocks in the world's EEZs through declines in biomass (Fig.…”

“…The occurrence of extreme high-temperature events across many regions of the ocean is often associated with the dominant climate modes. The occurrence of extreme high-temperature events is especially elevated during El Niño years ( 31 , 32 ) and conversely reduced during La Niña years.…”

“…Hence, our findings provide a foundation for future research to build upon to gain better understanding of the effects of marine temperature extremes on marine fisheries social-ecological systems and inform approaches to support the sustainability of global fisheries. Such research should include the following: examining the effects of temperature extremes at higher temporal frequencies using daily or monthly climate model outputs, as well as the impacts of subsurface temperature extremes; exploring extreme events of other ecosystem drivers ( 25 , 32 ) and their cumulative impacts, and the oceanographic, biological, and social-economic mechanisms underpinning the spatial and temporal variations of these impacts; examining the effects of temperature extremes on additional ecological and social-economic dynamics, such as trophic interactions and fisheries management; and evaluating a wider range of possible fisheries management and adaptation interventions to identify more effective measures to reduce risks and impacts on fisheries under climate change. Moreover, our findings show that marine cold spells (low-temperature extremes) could affect fisheries in ways other than the inverse of marine high-temperature extremes and that are presently poorly understood.…”

Compound marine heatwaves and ocean acidity extremes

“…The predominantly harmful impacts of such individual extreme events on marine ecosystems 10-12 may become more severe when two extreme events occur together [13][14][15] . These so-called compound events 16,17 have been vastly studied on land 18,19 , whereas marine compound events, such as compound MHW-OAX events (events of unusually high temperature and hydrogen ion concentration, [H + ]), are just starting to receive more attention 20,21 .The impact of compound MHW-OAX events on marine biota may exceed the impact from individual MHW or OAX events since cooccurring extremes can interact synergistically 22 . For example, the combination of high temperature and acidity conditions negatively impacted pteropods across cellular, physiological, and population levels in the California Current System in 2016 13,15 , and some of the devastating impacts of the Northeast Pacific 2013-2015 MHW 23 may have been amplified by the co-occurring extreme OA conditions 21 .…”

“…The predominantly harmful impacts of such individual extreme events on marine ecosystems 10-12 may become more severe when two extreme events occur together [13][14][15] . These so-called compound events 16,17 have been vastly studied on land 18,19 , whereas marine compound events, such as compound MHW-OAX events (events of unusually high temperature and hydrogen ion concentration, [H + ]), are just starting to receive more attention 20,21 .…”