Enhanced gas fluxes in small sea ice leads and cracks: Effects on CO2 exchange and ocean acidification

Earth system models generally partition grid cells into an ice-covered and an open-water area and view the ice-covered area as a barrier to gas fluxes. However, observations suggest that exchange in cracks and small leads can be much higher in localized areas than expected under similar conditions in open water. While these models project a significant retreat in sea ice cover, affecting air-sea CO2 exchange and consequently ocean acidification, the simple grid cell partitioning might underestimate the actual CO2 exchange. A sensitivity study with the Canadian Earth System Model (CanESM2) shows that enhanced CO2 exchange in sea ice areas in the Arctic Ocean increases the uptake in fall and winter, allowing more continuous equilibration and hence reduced uptake in summer. The reduction in summer also hints at a limited CO2-uptake capacity of Arctic surface waters. Retreating sea ice in the future leads to a similar shift in the seasonal cycle. The annual mean carbon uptake of the Arctic Ocean north of 68 degrees N changes only slightly with the enhanced flux parameterization (<3%). For the central Arctic (north of 80 degrees N) the change is up to 21% and accelerates ocean acidification, e. g. locally, surface waters could reach aragonite undersaturation 1-2 decades earlier. The differences between standard and enhanced exchange test runs become less pronounced in the future due to the reduced ice cover and extended open water season. For the Antarctic sea ice zone where the CO2 flux is out of the ocean, the enhanced exchange increases outgassing, slightly slowing down acidification.