Dissolved organic carbon dynamics in a changing ocean: A COBALTv2–ESM2M coupled model analysis

Abstract. Dissolved organic carbon (DOC) constitutes a major component of the marine carbon cycle, yet its present contributions to carbon export, and the response to future climate change remain poorly constrained. Using COBALTv2–ESM2M – GFDL's ocean biogeochemistry model COBALTv2 coupled to the ESM2M Earth System Model – we evaluate present-day DOC distribution and export and project their responses to a high-emission future scenario RCP8.5 to the year 2100.

Our model reproduces well the observed large-scale DOC patterns, with highest concentrations (~70–80 μmol C kg^-1) in subtropical gyres and lower values (~40–50 μmol C kg^-1) in subpolar and equatorial upwelling regions. Biological DOC production and remineralization rates are highest in nutrient-rich upwelling zones. The net DOC produced is then transported to the stratified oligotrophic gyres where DOC accumulates, thereby forming the observed global DOC distribution. Present-day global DOC export at 100 m is estimated at 1.6 PgC yr^-1, accounting for about 25% of the total organic carbon (TOC) export modeled at that depth. By 1000 m, DOC export decreases sharply to 0.09 PgC yr^-1, solely because microbial remineralization removes a significant fraction of DOC as it descends deeper into the water column. At 100 m, globally integrated mixing-mediated export is nearly twice that of advection, especially in boundary current regions and subpolar gyres where strong seasonal mixing occurs, whereas advection dominates in subtropical gyres via large-scale subduction of accumulated DOC. At 1000 m, however, advection dominates, particularly in the North Atlantic where deep-water formation facilitates DOC export. Under future warming, intensified stratification and reduced nutrient supply drive a net decline in global DOC production. Nevertheless, upper-ocean DOC concentrations increase slightly, underscoring the continued importance of physical transport in redistributing DOC. The model projects a ~6% reduction in DOC export at 100 m, driven primarily by weakened mixing, and a 25% reduction in advection-dominated deep export at 1000 m depth.