Natural processes behind the CO2 sink variability in the Southern Ocean during the last three decades

Anthropogenic activities during the past two centuries have caused an increase in atmospheric CO₂ which has driven a linear increase in oceanic CO₂ uptake. The Southern Ocean (SO, < 35ᵒS) is one of the major uptake areas for anthropogenic CO₂, responsible for ~40% of ocean CO₂ sink. Apart from the linear increase in the CO₂ sinking trend, in the SO pronounced variations have been observed in recent decades, driven by natural processes, but the exact mechanisms behind them are still debated. Aiming to fill this knowledge gap, we investigated the natural drivers of CO₂ flux variations in the SO using existing observation-based datasets between the years 1982-2019. We removed the long-term linear trend in the time series of CO₂ flux and other indexes to focus on decadal variations. We found that two mechanisms explain the interannual to decadal variations in the SO: Ekman upwelling and eddy kinetic energy, by their controls on different components of surface pCO₂ variations. The pattern of variability in Ekman upwelling during the time period studied was markedly circumpolar, and the time series of its 1^st principal component was strongly correlated with the detrended SAM Index (r=0.81, p<0.05). Similarly, leading EOF maps of CO₂ flux anomalies and the components of surface pCO₂ changes (i.e., nonthermal and thermal) show that their variations were dominantly symmetric. As previously shown, weakening of SO CO₂ sink in the 1990s coincides with intense positive SAM episodes. Following the late 1990s, the intensity of SAM decreased, which strengthened the CO₂ sink in the early 2000s. At the same time, the relative contribution of the thermal component grew south of the Polar Front, indicating positive temperature anomalies during this period. Such warming events, following intense and recursive SAM episodes were reported before and were attributed to the increased mesoscale eddy activity in the region. In agreement with these studies, our results show that eddy kinetic energy increased after intense SAM periods with a lagged response of ~2 years, and a positive temperature anomaly in low frequency was observed following these peaks. This warming prevented the CO₂ uptake rate from reaching immediately to its potential strength in the absence of strong westerlies, and explains the growing effect of the thermal pCO₂ component.