Collaborative Research: Forced drivers of trends in ocean biogeochemistry: Volcanos and atmospheric carbon dioxide

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The world’s oceans play an important role in the global carbon and oxygen cycles. In addition to their importance in the natural cycling of carbon, the oceans have absorbed approximately 40% of the carbon dioxide that has been emitted by fossil fuel burning. Understanding the processes that cause spatial and temporal variations of ocean carbon and oxygen concentrations is critical to predicting how these ocean cycles will develop into the future. Recent measurement-based estimates and computer models agree that ocean carbon uptake increased significantly in the early 1990s and then slowed over the rest of the decade. Observations and models also indicate significant oxygen variations. One possible driver of these patterns that has not been explored is the influence of large volcanic eruptions, specifically Mount Pinatubo in 1991. With the eruption, small particles were forced to great altitude where they spread through the upper atmosphere, reflected sunlight back to space, and led to a temporary cooling of global climate. This project will explore how this temporary cooling influenced ocean circulation, and air-sea carbon and oxygen exchange, by comparing Earth system model simulations that do and do not include the effects of Mt. Pinatubo’s eruption. Within the framework of NCAR’s Community Earth System Model Large Ensemble (CESM-LE) effort, the team will complete a new experiment that explicitly excludes forcing from Mt. Pinatubo (CESM-LE-NoVolc). By difference from the existing CESM-LE that includes all forcing, the investigators will directly identify the effects of Mt. Pinatubo and put these effects in context with observed carbon and oxygen change. Specifically, they propose to address two hypotheses: Hypothesis 1: Trends in surface fluxes and interior distributions of anthropogenic carbon and oxygen since the 1990s have been significantly impacted by Mt. Pinatubo.Hypothesis 2: After the initial uptake anomaly due to Mt. Pinatubo, thermocline anomalies that are cool and anomalously high in tracers return to the surface. These anomalies suppress air- to-sea fluxes for up to a decade after the eruption. Recent work has also indicated that variability in the growth rate of atmospheric carbon dioxide has a first-order effect on variability of ocean carbon uptake. The investigators will also do preliminary analysis using one new run of the CESM ocean-ice hindcast and a stratified analysis of CMIP6 models to further explore this issue. Specifically, they propose a third hypothesis: Hypothesis 3: By including the seasonal cycle and latitudinal distribution of atmospheric carbon dioxide in simulations, variability of the globally integrated air-sea carbon flux is increased and becomes more comparable to observationally-based estimates. By creating CESM-LE-NoVolc, this work will allow for investigation of the forced impact of Mt. Pinatubo on a wide range of ocean biogeochemical and physical processes. Model runs will be made easily available under the CESM-LE project umbrella. Under this research project, graduate and undergraduate research will be supported. The team will continue to our long-standing efforts to attract underrepresented students to science, and to explain ocean and carbon cycle science to the general public and K-12 students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.