Physical controls on alkalinity variability in Halifax Harbour: The roles of wind and tides

Ocean Alkalinity Enhancement (OAE) is a promising method for marine carbon dioxide removal. By artificially increasing ocean alkalinity, OAE triggers chemical reactions within the carbonate system that reduce oceanic pCO₂ levels, thereby inducing an uptake of atmospheric CO₂ by the ocean. However, as alkalinity concentrations at the point of release can reach high levels, alkalinity addition is limited for environmental safety to ensure pH < 9. Pronounced alkalinity variability was observed in the Halifax Harbour (Canada), an operational OAE site since 2023. This variability is characterized by alternating high and low values with substantial differences in magnitude. Understanding the processes that generate this variability is essential for controlling its intensity and advancing toward optimized dynamic dosing strategies to maximize dosing while remaining within safe regulatory limits.

Observations of carbonate system parameters collected during and outside dosing periods in the Halifax Harbour provide a unique dataset to determine which factors control the occurrence and magnitude of the alkalinity variability. We combine in situ measurements with numerical modeling using the Regional Ocean Modeling System (ROMS), customized for the Halifax Harbour and implemented with a nested grid configuration (50–900 m horizontal resolution). We use a series of numerical simulations under different wind scenarios to examine the coupled effects of winds and tides on alkalinity dispersion.

Our results show that the concentrations of added alkalinity are primarily controlled by tidal variability on daily and monthly timescales. Wind effects act as a secondary control, modulating tidal patterns and causing notable deviations, particularly during neap tides. Winds directed toward the open ocean enhance dispersion, whereas winds blowing into the basin tend to retain alkalinity near the release site, leading to higher local concentrations.