Freshwater Displacement Effect on Weddell Gyre Carbon Transport

The Weddell Gyre region mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate on millennial timescales. The Weddell also features enormous and highly variable freshwater fluxes due to seasonal sea ice, glacial melt, and net precipitation; however, the impact of these freshwater fluxes on the regional carbon cycle has not been fully appreciated. Using a novel budget analysis of dissolved inorganic carbon (DIC) mass in the Biogeochemical Southern Ocean State Estimate, we highlight two key freshwater-driven transports. When freshwater with minimal DIC enters the ocean, it displaces seawater with a large background DIC concentration leading to an export of 73 Tg DIC/year; on the other hand, sea ice export requires a compensating import of seawater, which carries 43 Tg DIC / year into the region. Together, these transports help set the modelled 26 Tg/year air-sea carbon flux into the gyre. Regrouping of published transport estimates to isolate this freshwater displacement effect reveals its central but previously unrecognized role in the observed Weddell carbon export.