Climate-driven variability of the Southern Ocean CO2 sink

Abstract

The Southern Ocean is a major sink of atmospheric CO₂, but the nature and magnitude of its variability remains uncertain and debated. Estimates based on observations suggest substantial variability that is not reproduced by process-based ocean models, with increasingly divergent estimates over the past decade. We examine potential constraints on the nature and magnitude of climate-driven variability of the Southern Ocean CO₂ sink from observation-based air-sea O₂ fluxes. On interannual time scales, the variability in the air-sea fluxes of CO₂ and O₂ estimated from observations is consistent across the two species and positively correlated with the variability simulated by ocean models. Our analysis suggests that variations in ocean ventilation related to the Southern Annular Mode are responsible for this interannual variability. On decadal time scales, the existence of significant variability in the air-sea CO₂ flux estimated from observations also tends to be supported by observation-based estimates of O₂ flux variability. However, the large decadal variability in air-sea CO₂ flux is absent from ocean models. Our analysis suggests that issues in representing the balance between the thermal and non-thermal components of the CO₂ sink and/or insufficient variability in mode water formation might contribute to the lack of decadal variability in the current generation of ocean models. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Keywords: Southern Ocean; carbon sink; climate; decadal; interannual; oxygen.

Figure 1.

Southern Ocean CO₂ sink from 1985 to 2019. Positive values denote a sink for CO₂. The GOBM mean estimate (solid black line) is the average of 10 GOBMs with ±1 standard deviation of the model ensemble (grey shading). The dashed line represents the effect of increasing atmospheric CO₂ only estimated from the GOBM mean. The pCO₂ product mean estimate (blue line) is the average of eight pCO₂ products with ±1 standard deviation of the product ensemble (blue shading). For the pCO₂ product estimates, a river flux adjustment term of 31.6 Tmol yr⁻¹ was added to be comparable with the GOBM estimates (see reference [15] for more details). The seasonal cycle was removed from all estimates with a 12-month moving average (see Methods). (Online version in colour.)

Figure 2.

Comparison of the climate-driven air–sea CO₂ (left) and O₂ (right) fluxes estimated by GOBMs and observation-based products. (a,b) Climate-driven air–sea CO₂ and O₂ fluxes from GOBMs (black) and observation-based products (blue and magenta, see legend), decomposed into their (c,d) decadal and (e,f) interannual components, showing the mean across estimates and the ±1 standard deviation across the ensembles (shading). Fluxes are defined as positive from the atmosphere into the ocean. (Online version in colour.)

Figure 3.

Standard deviation of the decadal (left) and interannual (right) variability of CO₂ and O₂ air–sea fluxes. For CO₂ (a,b and d,e), the averaged maps from GOBMs and from pCO₂ products are shown, while for O₂ (c and f) only GOBMs could be used to study the spatial patterns. Note the differences in units and colour scales between O₂ and CO₂ maps. The black lines represent the average location of the Subantarctic Front (northern line) and of the September extent of sea ice (southern line). Maps from individual GOBM and pCO₂ product are available in the electronic supplementary material (figures S1–S6). (Online version in colour.)

Figure 4.

Comparison of the variability in CO₂ and O₂ air–sea fluxes. The (a) decadal and (b) interannual components of CO₂ and O₂ air–sea fluxes are from the observation-based products (i.e. pCO₂ products and atmospheric inversion with five or nine stations). The y-axes on the right for O₂ air–sea fluxes are inverted. Fluxes are defined as positive from the atmosphere into the ocean. (Online version in colour.)

Figure 5.

Comparison of the SAM index and the climate-driven variability of CO₂ and O₂ air–sea fluxes from the GOBM mean, pCO₂ product mean and APO inversion. (a) The detrended time series of the SAM index, which has been decomposed into (b) a decadal component and (c) an interannual component, and compared with the decadal and interannual components from CO₂ (d and e, the right-hand axes are inverted) and O₂ (f and g) air–sea fluxes. Fluxes are defined as positive from the atmosphere into the ocean. (Online version in colour.)

Figure 6.

Correlation maps between the interannual components of the CO₂ and O₂ air–sea fluxes, and with the interannual components of wind speed and SST. The black lines represent the averaged location of the Subantarctic Front and of the September extent of sea ice. Fluxes are defined as positive from the atmosphere into the ocean. (Online version in colour.)