Rapid fCO2 rise in the northern Barents Sea and Nansen Basin

Maps of surface water fugacity of CO₂ (fCO₂) over eastern Fram Strait, south-western Nansen Basin, and the north-western Barents Sea (73–84°N, 5–46°E) from September 1997 to December 2020 were made and used to investigate seasonal and temporal trends. The mapping utilized a neural network technique, the self-organizing map (SOM), that was trained with different combinations of satellite/observational/model data of sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), chlorophyll a (Chl a), sea ice concentration, and atmospheric mole fraction of CO₂ (xCO₂). The trained SOM was labelled with available surface ocean fCO₂ data, and the labelled SOM was subsequently used to map the fCO₂. The produced maps reveal that fCO₂ in northern Barents Sea, at the border of the Nansen Basin, has increased significantly over the last decades by between 4.2 and 5.5 ± 0.6–1.1 µatm yr⁻¹ over the winter to summer seasons. These rates are twice the rate of atmospheric CO₂ increase, which was about 2 µatm yr⁻¹. The spatial pattern coincides with the strongest decreases in sea ice concentration as well as with a salinification of the surface water. The former allows for a prolongation of the air-sea CO₂ flux with resultant oceanic CO₂ uptake in previously ice-covered waters, and the latter is caused by a shift from Arctic Water dominance to more saline waters containing more dissolved inorganic carbon, most likely of Atlantic Water origin although brine-release influenced deep water may also contribute.