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.

中文翻译：

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卫星期间海洋中的高温和低叶绿素极端复合

摘要。极端事件严重影响海洋生物和生态系统。特别令人关注的是复合事件，即，当多种潜在生态系统压力源（例如温度和叶绿素）处于极端条件时。然而，人们对这种复合高温（又名海洋热浪-MHWs）和低叶绿素（LChl）极端事件的发生，强度和持续时间知之甚少，它们的分布在过去几十年中是否发生了变化，以及潜在的驱动因素是什么。在这里，我们使用基于卫星的海面温度和叶绿素浓度估算值来提供此类复合极端事件的首次评估。我们揭示了赤道太平洋，亚热带环流边界，印度洋北部和南极洲周围的MHW和LChl复合事件的热点。在这些地区，在假定MHW和LChl事件之间具有独立性的情况下，通常持续一周的复合事件发生的频率是预期的三到七倍。MHW和LChl复合事件的发生在季节到年际时间尺度上都不同。在季节时间尺度上，大多数复合事件都发生在两个半球的夏季。在每年的时间尺度上，MHW和LChl复合事件的发生频率受到厄尔尼诺-南方涛动等大规模自然气候变化模式的强烈调制，其正相位与东赤道地区复合事件发生的增加有关太平洋和印度洋的乘积系数高达四倍。我们的结果提供了对MHW和LChl复合事件发生地点，时间和原因的初步了解。