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

Abstract. Surface ocean alkalinity enhancement (OAE), through the release of alkaline materials, is an emerging marine carbon dioxide removal technology that could increase the storage of anthropogenic carbon in the ocean. Observations collected during recent and on-going field trials will provide important information on the feasibility and effects of alkalinity additions on carbon cycling and study ecological responses. However, given the scales involved (24/7 continuous addition, meters to hundreds/thousands of kilometers and minutes to months for alkalinity dispersion) observations, even with the use of autonomous platforms, will remain inherently sparse and limited. Alone, they cannot provide a comprehensive quantification of the effects of OAE on the carbonate system, and ultimately of the net air-sea CO2 fluxes. Numerical models, informed and validated by field observations, are therefore essential to OAE deployments and the measurement, reporting, and verification (MRV) of any resulting carbon uptake. They can help guide fieldwork design, including optimal design of measurement monitoring networks, provide forecasts of the ocean state, simulate the effects of alkalinity additions on the seawater carbonate system, and allow one to quantify net CO2 uptake. Here we describe a coupled physical-biogeochemical model that is specifically designed for coastal OAE. The model is an implementation of the Regional Ocean Modelling System (ROMS) in a nested grid configuration with increasing spatial resolution from the Scotian Shelf to Halifax Harbour (coastal fjord, eastern Canada), a current test site for operational alkalinity addition. The biogeochemical model simulates oxygen dynamics, carbonate system processes (including air-sea gas exchange), and feedstock properties (dissolution, sinking). We present a multi-year hindcast validated against the long-term weekly time series available for a long-term monitoring station at the deepest part of Halifax Harbour, as well as alkalinity addition simulations at various locations inside and outside the harbour to show the model's capabilities for assessing the effects of OAE at this coastal site. We found that release locations had a strong effect on the outcome of the addition, whereas feedstock types influenced the distribution of the signal. Up to 69 % of the net CO2 uptake was realized within the model domain, most of it inside the harbour. These results stress the importance of operational design, as well as the use of high-resolution regional models when quantifying additionality.