The Southern Ocean Carbon Cycle 1985–2018: Mean, Seasonal Cycle, Trends, and Storage

Abstract

We assess the Southern Ocean CO2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr−1) and pCO2‐observation‐based products (0.73 ± 0.07 PgC yr−1). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present‐day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2‐products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr−1 decade−1, respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2‐products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non‐thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle.

Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

“…In the Southern Ocean, the F SimB estimate of −0.04 ± 0.27 Pg C year −1 of the GOBMs is twice as uncertain as in the other basins (Table S1 in Supporting Information ). The relatively large uncertainty in the Southern Ocean may partly be the result of large inter‐model differences in the simulated F nat ss fluxes, as dynamically complex regions like the Southern Ocean are difficult to simulate (Hauck, Gregor, et al., 2023; Sallée et al., 2013). Inter‐model differences in F drift+bias likely also play a role for the uncertain F SimB estimate as the Southern Ocean is the region where most of the oldest water masses are upwelled to the ocean surface (Caldeira & Duffy, 2000), which have not been in contact with the atmosphere during the spin up and would hence presumably cause a larger disequilibrium and a larger F drift+bias than in other ocean basins with less upwelling.…”

“…The underestimation of the contemporary C ant ‐storage change by GOBMs is likely smaller than the underestimation of C ant ‐storage changes since 1800 because the later starting date of several GOBMs (Section 3.3.2) has a smaller effect on contemporary C ant ‐storage changes. Regionally, differences between the GOBM mean and the eMLR( C *)‐estimate (Figure 5) are most prominent in the Atlantic and Southern Ocean (Hauck, Gregor, et al., 2023; Hauck, Nissen, et al., 2023). The eMLR( C *)‐estimate indicates an anomalously high rate of C ant ‐change in the South Atlantic for the period from 1994 to 2007 and an anomalously low rate of C ant ‐change in the subpolar North Atlantic and the Indian and Pacific sectors of the Southern Ocean (Gruber, Clement, et al., 2019), which was attributed to a temporary slow‐down and reorganization of the North Atlantic overturning circulation (Fröb et al., 2016; Pérez et al., 2013; Steinfeldt et al., 2009) and changes in the Southern Ocean meridional overturning circulation and ventilation of water masses (Tanhua et al., 2017; Waugh et al., 2013).…”

“…The eMLR( C *)‐estimate indicates an anomalously high rate of C ant ‐change in the South Atlantic for the period from 1994 to 2007 and an anomalously low rate of C ant ‐change in the subpolar North Atlantic and the Indian and Pacific sectors of the Southern Ocean (Gruber, Clement, et al., 2019), which was attributed to a temporary slow‐down and reorganization of the North Atlantic overturning circulation (Fröb et al., 2016; Pérez et al., 2013; Steinfeldt et al., 2009) and changes in the Southern Ocean meridional overturning circulation and ventilation of water masses (Tanhua et al., 2017; Waugh et al., 2013). The GOBMs do not exhibit the regionally anomalous accumulation of C ant that is apparent in the eMLR( C *)‐estimate so that the GOBM ensemble mean is smaller than the eMLR( C *)‐estimate in the South Atlantic and subtropical North Atlantic and larger than the eMLR( C *)‐estimate in the subpolar North Atlantic and the Indian and Pacific sectors of the Southern Ocean (Hauck, Gregor, et al., 2023; Hauck, Nissen, et al., 2023). However, the eMLR( C *)‐estimate might also overestimate the strength of these anomalies, due to structural biases in the reconstructed changes of C ant (Clement & Gruber, 2018; Gruber, Clement, et al., 2019).…”

An Assessment of CO2 Uptake in the Arctic Ocean From 1985 to 2018

“…In the remaining regions, the climate effect is of similar magnitude or even larger than the CO 2 effect. In contrast to the Arctic Ocean, the CO 2 effect is much larger than the climate effect in the Southern Ocean (the other polar ocean; Hauck et al, 2023) and the global ocean where the CO 2 flux via the CO 2 effect is 2.1 PgC yr −1 of uptake and the flux from the climate effect is 0.2 PgC yr −1 of outgassing, respectively (DeVries et al, 2023). The Arctic Ocean is thus a unique ocean basin where climate change plays a role of similar magnitude as the increase in atmospheric CO 2 in controlling the sea-air fluxes of CO 2 .…”

Drivers of Air‐Sea CO2 Flux in the Subantarctic Zone Revealed by Time Series Observations

“…Despite its apparent recovery after that, it seems to have weakened again since 2011 (Keppler & Landschützer, 2019). While the spatial coverage of in situ measurements has increased dramatically (Bushinsky et al., 2019; Gray et al., 2018; Takahashi et al., 2009), disagreements among the global carbon models (Hauck et al., 2023; Lenton et al., 2013) remain, and the magnitude of the Southern Ocean carbon sink is still being refined (Bushinsky et al., 2019; Gray et al., 2018).…”