Air-sea carbon dioxide exchange in the Southern Ocean and Antarctic Sea ice zone

Abstract

The Southern Ocean is an important part of the global carbon cycle, responsible for roughly half of the carbon dioxide (CO₂) absorbed by the global ocean. The air-sea CO₂ flux (F_c) can be expressed as the product of the water-air CO₂ partial pressure difference (ΔpCO₂) and the gas transfer velocity ( k), an exchange coefficient which represents the efficiency of gas exchange. Generally, F_c is negative (a sink) throughout the Southern Ocean and Antarctic sea ice zone (SIZ), but uncertainty in k has made it difficult to develop an accurate regional carbon budget. Constraining the functional dependence of k on wind speed in open water environments, and quantifying the effect of sea ice on k, will reduce uncertainty in the estimated contribution of the Southern Ocean and Antarctic SIZ to the global carbon cycle.

To investigate F_c in the Southern Ocean, a ruggedized, unattended, closed-path eddy covariance (EC) system was deployed on the Antarctic research vessel Nathaniel B. Palmer for nine cruises during 18 months from January 2013 to June 2014 in the Southern Ocean and coastal Antarctica. The methods are described and results are shown for two cruises chosen for their latitudinal range, inclusion of open water and sea ice cover, and large ΔpCO₂. The results indicated that ship-based unattended EC measurements in high latitudes are feasible, and recommendations for deployments in such environments were provided.

Measurements of F_c and ΔpCO₂ were used to compute k. The open water data showed a quadratic relationship between k (cm hr^–1) and the neutral 10-m wind speed (U_10n, m s ^–1), k=0.245 U_10n ²+1.3, in close agreement with tracer-based results and much lower than previous EC studies. In the SIZ, it was found that k decreased in proportion to sea ice cover. This contrasted findings of enhanced F_c in the SIZ by previous open-path EC campaigns. Using the NBP results a net annual Southern Ocean (ocean south of 30°S) carbon flux of –1.1 PgC yr^–1 was calculated.