Enhanced CO₂ outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode

Abstract

We investigate the interannual variability in the flux of CO 2 between the atmosphere and the Southern Ocean on the basis of hindcast simulations with a coupled physical‐biogeochemical‐ecological model with particular emphasis on the role of the Southern Annular Mode (SAM). The simulations are run under either pre‐industrial or historical CO 2 concentrations, permitting us to separately investigate natural, anthropogenic, and contemporary CO 2 flux variability. We find large interannual variability (±0.19 PgC yr −1 ) in the contemporary air‐sea CO 2 flux from the Southern Ocean (<35°S). Forty‐three percent of the contemporary air‐sea CO 2 flux variance is coherent with SAM, mostly driven by variations in the flux of natural CO 2 , for which SAM explains 48%. Positive phases of the SAM are associated with anomalous outgassing of natural CO 2 at a rate of 0.1 PgC yr −1 per standard deviation of the SAM. In contrast, we find an anomalous uptake of anthropogenic CO 2 at a rate of 0.01 PgC yr −1 during positive phases of the SAM. This uptake of anthropogenic CO 2 only slightly mitigates the outgassing of natural CO 2 , so that a positive SAM is associated with anomalous outgassing in contemporaneous times. The primary cause of the natural CO 2 outgassing is anomalously high oceanic partial pressures of CO 2 caused by elevated dissolved inorganic carbon ( DIC ) concentrations. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. The secular, positive trend in the SAM has led to a reduction in the rate of increase of the uptake of CO 2 by the Southern Ocean over the past 50 years.