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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-412
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-412
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Nov 2020

14 Nov 2020

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This preprint is currently under review for the journal BG.

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

Natacha Le Grix1,2, Jakob Zscheischler1,2,3, Charlotte Laufkötter1,2, Cécile S. Rousseaux4,5, and Thomas L. Frölicher1,2 Natacha Le Grix et al.
  • 1Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 3Department for Computational Hydrosystems, Centre for Environmental Research – UFZ, Leipzig, Germany
  • 4Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, USA
  • 5Universities Space Research Association, Columbia, USA

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.

Natacha Le Grix et al.

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Natacha Le Grix et al.

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Short summary
Marine ecosystems could suffer severe damage from the co-occurrence of a marine heatwave with extremely low chlorophyll concentration. Here, we provide a first assessment of compound marine heatwave and low chlorophyll events in the global ocean from 1998 to 2018. We reveal hotspots of these compound events in the equatorial Pacific and in the Arabian Sea, show that they mostly occur in summer at high latitudes, and that their frequency is modulated by large-scale modes of climate variability.
Marine ecosystems could suffer severe damage from the co-occurrence of a marine heatwave with...
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