Compound High Temperature And Low Chlorophyll Extremes In The Ocean Over The Satellite Period

Abstract:

Extreme events are well known to severely impact marine organisms and ecosystems. Of particular concern are compound events, i.e. when conditions are extreme for multiple potential ecosystem stressors. In 2013-2015 for example, an extensive marine heatwave, known as the Blob, cooccurred with low chlorophyll concentration and severely impacted marine life in the northeast Pacific, with cascading impacts on fisheries. Yet, little is known how the occurrence, intensity and duration of such ocean compound events have changed in the past and what the potential drivers are. Here, we use satellite observations of sea surface temperature and chlorophyll concentration to provide a first assessment of compound hot extreme temperature (aka marine heatwaves – MHWs) and low chlorophyll (LChl) events. We reveal hotspots of compound MHW and LChl events in the Equatorial Pacific, along the boundaries of the subtropical gyres, in the Arabian Sea and in the Weddell Sea. In these regions, compound events occur 3 to 7 times more often than expected under the assumption of independence between MHWs and LChl events. The occurrence of compound MHW and LChl events also varies over time. At the annual time-scale, most compound events occur in summer. At the interannual time-scale, the frequency of compound MHW and LChl events is strongly affected by the state of large-scale modes of climate variability such as the El Niño Southern Oscillation, whose positive phase is associated with increased occurrence in the Eastern Pacific and in the Indian Ocean by a factor of up to 3. Our results provide a first understanding of where, when and why compound MHW and LChl events might occur. Further studies are needed to pin down the exact physical and biological drivers of these potentially strongly harmful events in the ocean.