The global carbon cycle is integral to the Earth System. Its three major reservoirs (land, atmosphere, and ocean) exchange carbon with the ocean containing the vast majority (> 90 %) of the carbon. In the past two centuries, the anthropogenic tapping into fossil carbon in the geologic reservoir has dramatically altered global carbon cycling. As a result, over time, the ocean has switched from being a small net source of carbon to the atmosphere to become a significant sink. The excess anthropogenic carbon that has been accumulating in the ocean is altering its chemistry and marine ecosystems. At the same time, climate change is modifying ocean physics, with consequences for the transport of carbon into the ocean. Changing ocean circulation is similarly important for global oxygen cycling. Approximately half of oxygen production occurs in the ocean, but changes in ocean transport, mixing, and biological production are changing oxygen production and cycling. Earth System Models (ESMs) are critical tools to explore these changes as they represent underlying physical and biochemical processes and their interactions in the context of changing global climate. The realism of these models improved significantly in the last two decades, however, the models still underestimate the amplitude of the changes in ocean carbon uptake and dissolved oxygen content on the timescales of tens of years. Additionally, they disagree over the sign of predicted changes in dissolved oxygen in the tropical oceans under global warming scenarios. These disagreements hinder the use of ESM-based predictions and limit their application to the study of ecosystem health and fisheries research.
What sets the amplitude of the changes in ocean carbon uptake on the timescales of tens of years? Why do models predict diverging trajectories of oxygen levels in the tropical oceans under global warming scenarios? This project has three main objectives related to the understanding of the processes and feedbacks regulating ocean carbon and oxygen cycling and their representation in ESMs. The first objective is to evaluate the model’s ability of the U.S. Department of Energy (DOE) Energy Exascale Earth System Model (E3SM) to represent the interactions between physical and biogeochemical processes in the ocean. A growing amount of observational data now enables us to evaluate the model’s ability to represent the air-sea exchange of carbon dioxide and dissolved oxygen contents on the timescale of years to decades. Furthermore, we will compare E3SM against other ESMs participating in the Coupled Model Inter-comparison Project Phase 6 (CMIP6). The second objective is to characterize the patterns of physical and biogeochemical variability and their spatio-temporal linkages in the oceans. We will characterize key physical and biological processes and their sensitivities to modes of climate variability using both standard statistical analysis and complex network analysis tools. The third objective is to test several hypotheses behind the three key mechanisms that regulate the upper ocean carbon and oxygen cycling, namely, water mass distribution, ventilation, and biological production. We hypothesize that, together, these processes control mean states and changes in the oceanic inventories of carbon and oxygen, and explain the two major issues that are common among state-of-the-art ESMs around the world mentioned earlier. In summary, this project will improve the scientific understanding of physical-biogeochemical interactions focusing on two key shortcomings of ESMs, and will deliver an analysis framework that can be applied to observational and model-derived physical and biogeochemical data. All analysis tools resulting from this project will be shared in the public domain.