Assessment of austral autumn air–sea CO₂ exchange in the Pacific sector of the Southern Ocean and dominant controlling factors

The factors that control the partial pressure of carbon dioxide (pCO₂) in the Pacific sector of the Southern Ocean were investigated in April 2018, onboard the icebreaker, ARAON. The mean (± 1σ) of the sea surface pCO₂ was estimated to be 431 ± 6 μatm in the north of the Ross Sea (NRS), 403 ± 18 μatm in the Amundsen–Bellingshausen Sea (ABS), and 426 ± 16 μatm in the western Antarctic Peninsula and Weddell Sea (WAP/WS). The controlling factors for pCO₂ in the NRS appeared to be meridionally different based on the southern boundary of the Antarctic Circumpolar Current (SB; ~62.5°S in the Ross Sea). The sea surface pCO₂ exhibited a strong correlation with salinity and the difference between the O₂/Ar (ΔO₂/Ar) values of the sample and air-saturated water in the north and south of the SB, respectively. The pCO₂ in the ABS and western WAP/WS displayed a strong correlation with salinity. Furthermore, ΔO₂/Ar and sea ice formation appear to be the dominant factors that control pCO₂ in the Confluence Zone (CZ) and northern parts of WAP/WS. The estimated air–sea CO₂ fluxes (positive and negative values indicate the source and sink for atmospheric CO₂, respectively) range from 3.1 to 18.8 mmol m⁻² d⁻¹ in the NRS, −12.7 to 17.3 mmol m⁻² d⁻¹ in the ABS, and −59.4 to 140.8 mmol m⁻² d⁻¹ in the WAP/WS. In addition, biology-driven large variations in the air–sea CO₂ flux were observed in the CZ. Our results are the most recent observation data acquired in austral autumn in the Southern Ocean.