• We analysed the CO₂ system in an important region of the Southern Ocean.

• The Gerlache Strait acts as a stronger CO₂ sink than nearby open ocean areas during the austral summer.

• We identified both strong and near-equilibrium sink scenarios for FCO₂ from 1999 to 2017.

• The pattern of variability of FCO₂ has changed since 2012 to a higher frequency of years with a strong CO₂ sink.

Towards an intensified summer CO₂ sink behaviour in the Southern Ocean coastal regions

Abstract - https://doi.org/10.1016/j.pocean.2020.102267

The Southern Ocean is a globally important carbon sink region. However, the austral coastal zones are usually not considered in global estimations due to their general undersampling and large regional dynamics. Thus, estimations of carbon uptake in the Southern Ocean may differ considerably from current values, i.e., without accounting for coastal regions. Here, we conducted a case study in the Gerlache Strait, an ecologically important Antarctic coastal zone. We show that the net sea–air CO₂ flux (FCO₂) in the strait may reach the same or greater magnitudes than those in large open sea regions around Antarctica during summer, despite having a much smaller area. A large mean FCO₂ of –31 ± 19 mmol m⁻² d^–1 was observed in the strong CO₂ sink years (i.e., FCO₂ < –12 mmol m⁻² d^–1), in contrast to –1 ± 7 mmol m⁻² d^–1 in CO₂ near-equilibrium conditions (i.e., CO₂ sea–air difference ≈ 0). This variability is mainly modulated by phytoplankton activity and likely upwelling processes. We also identified two cycles of variability with 2-year and 4-year periodicities from 1999 to 2017. The 2-year periodicity becomes stronger after 2012, intensifying the strong CO₂ sink scenario in the Gerlache Strait. Our findings reinforce the importance of polar coastal zones as CO₂ sinks during the austral summer and the need to broaden our understanding of the role of these regions at other time scales.