The main basin of the Bering Sea is a climatological CO2 outgassing region, where wintertime mixed-layer deepening entrains dissolved inorganic carbon (DIC)-rich water, enhancing air-sea CO2 release. However, since 2014, observations show an abrupt, basin-wide subsurface warming. The suppression of wintertime mixed-layer development has limited the cold-water supply to deeper layers, forming a subsurface warming layer (26.4–26.7 σθ) and enhancing the stratification. How this weakened vertical exchange affects subsurface carbon storage and subsequent CO2 outgassing remains unclear.
To investigate changes in vertical DIC distribution associated with subsurface warming, this study analyzed repeat biogeochemical sections along the P14 line across the Bering Sea from late summers in 1993, 2007, and 2023. The Transit Time Distribution (TTD) method by using CFC-12 and SF6 as tracers separated anthropogenic carbon from variations due to natural climate variability. Physical drivers were examined using an oceanic objective analysis product (ProjD, 1990–2024), a machine-learning DIC reconstruction (MOBO-DIC, 2004–2019), and wind stress curl calculated from the JRA-3Q reanalysis (1998–2022). Satellite-based regional CO2 flux estimates (1998–2024) were used to compare interior carbon changes with surface outgassing variability.
As results, the 2023 observations reveal a pronounced subsurface warming layer between 26.4–26.7 σθ, where potential temperature increased by ~1.2 °C and salinity-normalized DIC (nDIC) rose by ~52 µmol kg-1 relative to the 1993 baseline. The TTD analysis indicates that about 70% of this DIC increase along the 26.5 σθ surface is driven by natural climate variability rather than anthropogenic uptake. Comparing the apparent oxygen utilization, the increase of DIC is attributed to accumulation of biological activities.
Furthermore, from time-series change of data-constrained reanalysis, this accumulated DIC becomes a potential carbon pool, increasing CO2 outgassing from the Bering Sea. While stratification has enhanced since 2014, JRA-3Q data shows that wind stress curl has intensified since 2018, strengthening Ekman-driven upwelling. Comparisons with MOBO-DIC time-series profiles suggest that the weakening of vertical water exchange led to DIC accumulation within the subsurface warming layer. Subsequently, strengthened Ekman upwelling induced the shoaling of these isopycnals, allowing the wintertime mixed-layer to intersect this accumulated carbon reservoir, accelerating CO2 outgassing. Consequently, winter CO2 outgassing increased by ~1.7 times after 2018 compared to the 1998–2017 baseline. These results suggest that the interplay between isopycnal shoaling and wintertime mixed-layer deepening can re-entrain the accumulated subsurface carbon reservoir, significantly amplifying regional CO2 outgassing in this upwell-dominated subpolar region.

 

CO2 outgassing,bering sea,subsurface warming,DIC