title: Distribution, drivers, and impacts of compound marine heatwave and low net primary productivity extreme events in the ocean
Creators: name: Le Grix de la Salle, Natacha
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 530 Physics

description: Ocean extreme events can severely impact marine organisms and ecosystems. Of particular concern are multivariate compound events, namely when conditions are simultaneously extreme for multiple ocean ecosystem stressors. In 2013-2015, for example, an extensive marine heatwave (MHW), known as the ‘Blob’, co-occurred locally with extremely low net primary productivity (NPPX) and negatively impacted marine life in the northeastern Pacific. Yet, little is known about the distribution, drivers, and impacts of such multivariate compound MHW-NPPX events.
The introduction of this thesis (Chapter 1) motivates the study of compound MHW-NPPX events by first giving an overview of how sea surface temperature (SST) and net primary production (NPP) are changing under climate change, with repercussions on climate and marine ecosystems. Long-term changes in SST and NPP are associated with a rise in extremely high SST and low NPP events, which may co-occur during compound MHW-NPPX events and drive severe impacts on marine ecosystems.
The Methods section (Chapter 2) then introduces the tools used in this thesis to study these potentially harmful compound MHW-NPPX events. We specifically use a combination of satellite-derived SST, chlorophyll concentration, and NPP observations, and large ensemble simulations by two Earth system models and by one global marine fish model.
Chapter 3, published in Biogeosciences (Le Grix et al., 2021), characterizes the distributions of MHW-NPPX events over the satellite period using satellite-based SST and chlorophyll concentration as a proxy for NPP. From 1998 to 2018, we find compound events to be frequent in the low latitudes, especially in the center of the equatorial Pacific, in the Arabian Sea and along the borders of the subtropical gyres. In contrast, compound events are rare in the high latitudes, where MHWs rarely co-occur with NPPX events. The frequency of compound MHW-NPPX events also varies across seasons, with most events occurring in spring in the mid-latitudes and in summer in the high latitudes. At the interannual time scale, large-scale modes of climate variability seem to modulate the frequency of compound events. For example, in the eastern equatorial Pacific, compound event likelihood is multiplied by a factor of 4 during El Niño events. Climate modes are associated with particular ocean conditions, which can favor or prevent the occurrence of compound MHW-NPPX events. Surface warming and reduced upwelling of nutrient-rich waters in the eastern equatorial Pacific during El Niño events may, for instance, contribute to driving compound MHW-NPPX events (Le Grix et al., 2022).
Earth system models (ESMs) can help identify the exact drivers of compound MHW-NPPX events. In Chapter 4, also published in Biogeosciences (Le Grix et al., 2022), we use large ensemble simulations by two ESMs: the GFDL ESM2M and the CESM2. Their representation of compound MHW-NPPX events is evaluated against satellite-derived observations. Both models correctly simulate frequent compound MHW-NPPX events in the low latitudes. There, MHWs are associated with nutrient limitation on phytoplankton growth and a relative increase in phytoplankton loss (e.g., grazing) compared to phytoplankton production, which drive low phytoplankton NPP and result in frequent compound MHW-NPPX events. In the high latitudes, however, models disagree on the likelihood and drivers of compound MHW-NPPX events. There, improved understanding of the factors controlling phytoplankton NPP is key to improving model representation of NPPX events and, ultimately, of compound MHW-NPPX events.
Lastly, in Chapter 5 (Le Grix et al., 2023 (submitted)), we evaluate the actual threat posed by compound MHW-NPPX events on marine ecosystems using a global marine fish model, the DBEM, forced by a large ensemble simulation from an ESM, the GFDL ESM2M. We first identify events of extremely low biomass of pelagic fish, and then look back at the ocean conditions that may have driven these events. We find that MHWs and NPPX events are both drivers of extreme impacts on pelagic fish biomass in the equatorial Atlantic, the central and eastern equatorial Pacific, in the northern part of the Indian Ocean and in the northeastern Pacific. There, pelagic marine ecosystems may be particularly vulnerable to compound MHW-NPPX events. We also identify regions where impacts may be caused by ocean conditions other than compound MHW-NPPX events, and by ocean conditions that do not necessarily correspond to a compound extreme event. Moderate anomalies in one ocean ecosystem stressor were found to never drive extreme impacts on pelagic fish. In contrast, over 78% of the global ocean, a combination of anomalies in multiple ocean ecosystem stressors is necessary to experience extreme impacts. Overall, Chapter 5 highlights the key role of ocean compound events in driving extreme impacts on marine ecosystems.
The Discussion (Chapter 6) summarizes our results and puts them into the larger perspective of ocean and compound event research. Overall, this thesis provides a first understanding of the distribution, drivers, and impacts of compound MHW-NPPX events. We highlight the need to improve their representation in ESMs, to further study their impacts, and to extend compound event research to other types of oceanic compound events. We also inform potential strategies to predict, prevent and moderate compound MHW-NPPX events impacts on marine ecosystems.