Compound high temperature and low net primary production extremes in the ocean over the satellite period

Extreme events, such as marine heatwaves (MHWs), severely impact marine ecosystems. Of particular concern are compound events, i.e. situations when conditions are extreme for multiple ecosystem stressors, such as temperature and net primary productivity (NPP). In 2013-2015 for example, an extensive MHW, known as the Blob, cooccurred with low NPP and severely impacted marine life in the northeast Pacific, with cascading impacts on fisheries. Yet, little is known about the distribution and drivers of compound MHW and low NPP extreme events. We use satellite-based sea surface temperature and NPP estimates to provide a first assessment of these compound events. We reveal hotspots of compound MHW and low NPP events in the equatorial Pacific, along the boundaries of the subtropical gyres, and in the northern Indian Ocean. 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 low NPP events. At the seasonal timescale, most compound events occur in summer in both hemispheres. At the interannual time-scale, their frequency is strongly modulated by large-scale modes of climate variability such as the El Niño-Southern Oscillation, whose positive phase is associated with increased compound event occurrence in the eastern equatorial Pacific by a factor of up to four. Using large ensemble simulations of two Earth system models, we then investigate the exact physical and biological drivers of these compound events. We find that both models suggest that MHWs in the low latitudes are often associated with low surface ocean nutrient concentrations due to enhance stratification and/or reduced upwelling, which limits the growth of phytoplankton resulting in extremely low NPP. However, the models show large disparities in simulated compound events and its drivers in the high latitudes. This identifies an important need for improved process understanding for high latitude compound MHW and low NPP events.