A high-resolution nested model to study the effects of alkalinity additions in a mid-latitude coastal fjord

Surface ocean alkalinity enhancement (OAE), through the release of alkaline materials, is an emerging carbon dioxide removal (CDR) technology that could increase the storage of anthropogenic carbon in the ocean. Although essential, evaluating the effects of alkalinity additions on the carbonate system and ultimately on air-sea CO₂ fluxes is not straight forward. Observations, even with autonomous platforms, are inherently sparse and limited, and therefore cannot provide a comprehensive quantification of the effects of OAE. Numerical models are important complementary tools. They can help guide fieldwork design, provide forecasts of the ocean state, and simulate the effects of alkalinity additions on the seawater carbonate system. Here we describe a coupled physical-biogeochemical implementation of ROMS in a nested grid configuration that reaches a very high spatial resolution in Bedford Basin (51m), a coastal fjord in eastern Canada that is chosen as a test site for OAE. The biogeochemical model simulates oxygen dynamics and the carbonate system, including air-sea gas exchange. We present a multi-year hindcast validated against the long-term weekly time series available at the Compass Buoy station in the centre of the Basin as well as recent simulations carried out during alkalinity addition trials. We will discuss the model’s capabilities with respect to OAE and the challenges ahead.