The annual fCO2 cycle and the air–sea CO2 flux in the sub-Antarctic Ocean

The sub-Antarctic zone (SAZ) lies between the subtropical convergence (STC) and the sub-Antarctic front (SAF), and is considered one of the strongest oceanic sinks of atmospheric CO₂. The strong sink results from high winds and seasonally low sea surface fugacities of CO₂ (fCO₂), relative to atmospheric fCO₂. The region of the SAZ, and immediately south, is also subject to mode and intermediate water formation, yielding a penetration of anthropogenic CO₂ below the mixed layer. A detailed analysis of continuous measurements made during the same season and year, February — March 1993, shows a coherent pattern of fCO₂ distributions at the eastern (WOCE/SR3 at about 145°E) and western edges (WOCE/I6 at 30°E) of the Indian sector of the Southern Ocean. A strong CO₂ sink develops in the Austral summer (ΔfCO₂ μ - 50 μatm) in both the eastern (110°-150°E) and western regions (20°-90°E). The strong CO₂ sink in summer is due to the formation of a shallow seasonal mixed-layer (about 100 m). The CO₂ drawdown in the surface water is consistent with biologically mediated drawdown of carbon over summer. In austral winter, surface fCO₂ is close to equilibrium with the atmosphere (ΔfCO₂ ± 5 μatm), and the net CO₂ exchange is small compared to summer. The near-equilibrium values in winter are associated with the formation of deep winter mixed-layers (up to 700 m). For years 1992–95, the annual CO₂ uptake for the Indian Ocean sector of the sub Antarctic Zone (40°-50°S, 20°-150°E) is estimated to be about 0.4 GtC yr^-1. Extrapolating this estimate to the entire sub-Antarctic zone suggests the uptake in the circumpolar SAZ is approaching 1 GtC yr^-1.