The Effect of Antarctic Sea Ice on Southern Ocean Carbon Outgassing: Capping Versus Light Attenuation

Abstract

We examine the role of sea ice in controlling air-sea carbon fluxes around Antarctica using numerical simulations and idealized theory. Upwelling of carbon and nutrient-rich deep waters in the Southern Ocean promotes outgassing of CO₂ and fuels the biological flux of sinking organic particles. Sea ice inhibits outgassing, by presenting a physical barrier to air-sea exchange (capping), and decreases biological uptake by reducing the flux of photons to the ocean surface (light attenuation). These two compensating effects suggest that changes in sea ice may have a modest impact on the air-sea flux of CO₂ in the region. Numerical simulations support this inference, showing that the net integrated flux remains nearly constant for a large range of sea ice fractions when the ice cover is uniform and time-invariant. Consequently, the outgassing flux is only significantly capped when the ice cover is nearly complete. A simple analytical model shows that the compensation strength can be uniquely characterized by a single parameter that depends on the flow residence time scale under ice, relative to the air-sea equilibration and biological time scales. When the ice is seasonal, compensation between capping and light attenuation is weakened, but still significant. The spring months are particularly important due to the co-occurrence of an extended sea ice cover and the presence of sunlight.