Central Arctic Ocean surface-atmosphere exchange of CO₂ and CH₄ constrained by direct measurements

Abstract. The Central Arctic Ocean’s (CAO) current and future role in the exchange of climate-forcing trace gases with the atmosphere is highly uncertain, in particular the effect of sea ice on the exchange. There are no previously reported direct air-sea methane (CH₄) flux estimates from the CAO. We present direct measurements of the air-sea flux of CH₄ and carbon dioxide (CO₂), as well as air-ice fluxes of CO₂ in the summertime CAO North of 82.5 N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021.

Measurements of air-sea CH₄ and CO₂ flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden. Fluxes and dissolved gas concentrations from surface water were used to determine gas transfer velocities that exhibited a weaker wind speed dependence than existing parameterisations, with a median gas transfer rate in sea-ice leads 2.5 cm hr⁻¹. Average observed CO₂ air-sea flux were −7.6 mmol m⁻² day⁻¹, and the average air-snow flux −1.1 mmol m⁻² day⁻¹. Extrapolating these fluxes and the corresponding sea ice concentrations gives an August and September flux for the CAO of −1.75 mmol m⁻² day⁻¹ and a corresponding annual CAO uptake of 126.6 Tg yr⁻¹.

Average observed air-sea CH₄ flux of 3.5 μmol m⁻² day⁻¹, accounting for sea ice concentration, equates to a CAO flux of 0.35 μmol m⁻² day⁻¹, or 9.2 Gg yr⁻¹, lower than previous estimates and implying that the CAO is a very small, << 1 %, contributor to the flux of CH₄ to the atmosphere from the Arctic.