Interannual variability in the physical and biological drivers of carbon sequestration in the southeast Pacific Subantarctic Mode Water Formation region

The uptake of carbon by the Southern Ocean plays a critical role in mitigating atmospheric CO₂ increases, but its magnitude and temporal and spatial variability are still subject to large uncertainties due to the scarcity of observations, and model disagreements. The Southeast Pacific is one such region where deep, nutrient and carbon-rich circumpolar deep waters upwell, but also where Subantarctic Mode Water and Antarctic Intermediate Water are formed and subducted, carrying with them high loadings of anthropogenic carbon dioxide into the ocean interior. The processes driving the upper ocean carbon levels are a balance of biological activity and heat-flux driven solubility effects in response to changing physical dynamics. While the former is thought to drive the region being a net annual carbon sink, the depth at which exported organic matter is remineralised will have a large effect on whether it remains there on climatically-important timescales. Here we present a multi-year biogeochemical timeseries from the OOI mooring located in the region, combined with observations from profiling BGC-Argo floats, a 6-week process cruise in austral summer 2019-2020 (as part of the UK CUSTARD programme), and outputs from data assimilation models to investigate the effects of interannual variability in mixed layer dynamics on primary production, carbon export and long-term carbon sequestration. We find a strong relationship between winter mixed layer depths and densities, biological activity the following summer, and impacts on the magnitude and distribution of subsurface remineralisation, providing insight into the controls on carbon uptake in a region of global significance for climate regulation