More Than Marine Heatwaves: A New Regime of Heat, Acidity, and Low Oxygen Compound Extreme Events in the Gulf of Alaska

Recent marine heatwaves in the Gulf of Alaska have had devastating impacts on species from various trophic levels. Due to climate change, total heat exposure in the upper ocean has become longer, more intense, more frequent, and more likely to happen at the same time as other environmental extremes. The combination of multiple environmental extremes can exacerbate the response of sensitive marine organisms. Our hindcast simulation provides the first indication that more than 20% of the bottom water of the Gulf of Alaska continental shelf was exposed to quadruple heat, positive hydrogen ion concentration [H+], negative aragonite saturation state (omega arag), and negative oxygen concentration [O2] compound extreme events during the 2018-2020 marine heat wave. Natural intrusion of deep and acidified water combined with the marine heat wave triggered the first occurrence of these events in 2019. During the 2013-2016 marine heat wave, surface waters were already exposed to widespread marine heat and positive [H+] compound extreme events due to the temperature effect on the [H+]. We introduce a new Gulf of Alaska Downwelling Index (GOADI) with short-term predictive skill, which can serve as indicator of past and near-future positive [H+], negative omega arag, and negative [O2] compound extreme events near the shelf seafloor. Our results suggest that the marine heat waves may have not been the sole environmental stressor that led to the observed ecosystem impacts and warrant a closer look at existing in situ inorganic carbon and other environmental data in combination with biological observations and model output. The Gulf of Alaska supports a rich ocean ecosystem and valuable fisheries. Climate change and ocean acidification threaten to disrupt marine life in the region from plankton to fish, marine mammals, and sea birds. The gradual build-up of these environmental pressures can be exacerbated further by short-term extreme events, such as marine heat waves, that can temporarily push ocean conditions beyond physiological and ecological thresholds for some organisms. The problem is worsened by the co-occurrence of extreme events for multiple factors, for example, heat and acidity. Our analysis using a regional ocean model indicates that such compound extreme events have become more frequent and intense with time in the Gulf of Alaska, raising concerns for vulnerable parts of the ecosystem. Improvements in model forecasts and observing systems may help by providing advanced warning of compound extreme events and be useful to fisheries and marine resource managers as they develop climate adaptation strategies. 20% of the shelf bottom water was exposed to quadruple heat, positive [H+], negative omega arag, and negative [O2] compound extreme events in 2019Interaction of marine heat waves and local natural variability of deep-water intrusion triggered quadruple compound extreme events on shelfNew Gulf of Alaska Downwelling Index presented as indicator for environmental conditions on continental shelf