Freshwater Displacement Effect on the Weddell Gyre Carbon Budget

The Weddell Gyre mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate. This region also features large and highly variable freshwater fluxes due to seasonal sea ice, net precipitation, and glacial melt; 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 freshwater-driven transports. Where freshwater with minimal DIC enters the ocean, it displaces DIC-rich seawater outwards, driving a lateral transport of 75 & PLUSMN; 5 Tg DIC/year. Additionally, sea ice export requires a compensating import of seawater, which carries 48 & PLUSMN; 11 Tg DIC/year into the gyre. Though often overlooked, these freshwater displacement effects are of leading order in the Weddell Gyre carbon budget in the state estimate and in regrouped box-inversion estimates, with implications for evaluating basin-scale carbon transport. The ocean surrounding Antarctica plays a key role in the global carbon cycle because it is one of the few places where carbon-enriched abyssal waters can exchange with the atmosphere. Here we focus on the Weddell Gyre that covers the Atlantic sector poleward of 60 & DEG;S; this region features enormous exchanges of freshwater through the surface, via sea ice that covers the region each winter, frequent precipitation, and land ice melting into the ocean. Using a model that assimilates available observations, we explore how this freshwater affects the region's ability to transport carbon between the atmosphere and deep ocean. Whereas previous studies have focused on how springtime sea ice melt dilutes surface ocean carbon concentrations, we emphasize that throughout the year, adding water with minimal carbon (from rain, snow, or land ice) to the ocean displaces carbon-rich seawater. Second, sea ice melt and formation alone do not displace seawater, but when sea ice flows out of the region, carbon-rich seawater responds by flowing in the opposite direction. These freshwater displacement effects are leading order terms of the regional carbon budget in our model analysis and in our reconsideration of past observational estimates. Displacement of seawater by net precipitation and glacial melt is a leading order term in Weddell Gyre dissolved inorganic carbon (DIC) budgetEvidence is provided by quantifying the DIC mass budget in Biogeochemical Southern Ocean State Estimate (B-SOSE) and regrouping published box inversion resultsIn the B-SOSE mean, freshwater displaces 75 Tg DIC/yr, whereas sea ice export drives a 48 Tg DIC/yr lateral import