Sea ice and its effect on CO2 flux between the atmosphere and the Southern Ocean interior

The advance and retreat of sea ice produces seasonal convection and stratification, dampens surface waves and creates a separation between the ocean and atmosphere. These are all phenomena that can affect the air‐sea gas transfer velocity (k660), and therefore it is not straightforward to determine how sea ice cover modulates air‐sea flux. In this study we use field estimates k660 to examine how sea ice affects the net gas flux between the ocean and atmosphere. An inventory of salinity, 3He, and CFC‐11 in the mixed layer is used to infer k660 during the drift of Ice Station Weddell in 1992. The average of k660 is 0.11 m d−1 across nearly 100% ice cover. In comparison, the only prior field estimates of k660 are disproportionately larger, with average values of 2.4 m d−1 across 90% sea ice cover, and 3.2 m d−1 across approximately 70% sea ice cover. We use these values to formulate two scenarios for the modulation of k660 by the fraction of sea ice cover in a 1‐D transport model for the Southern Ocean seasonal ice zone. Results show the net CO2 flux through sea ice cover represents 14–46% of the net annual air‐sea flux, depending on the relationship between sea ice cover and k660. The model also indicates that as much as 68% of net annual CO2 flux in the sea ice zone occurs in the springtime marginal ice zone, which demonstrates the need for accurate parameterizations of gas flux and primary productivity under partially ice‐covered conditions. Key Points There are very few estimates of gas transfer velocity in the sea ice zone The transport of CO2 to the deep ocean is sensitive to k in the ice zone The net CO2 flux in the sea ice zone depends on k and on phyto blooms