Robotic Observing System Challenges Previous Estimates of the Southern Ocean Carbon Sink

Principal Investigator

At a Glance

A robotic observation system that researchers have deployed in the Southern Ocean is providing year-round measurements of carbon fluxes and is changing our understanding of the ocean carbon sink. Previous results had revealed that significant outgassing of carbon dioxide (CO₂) in the region was occurring in wintertime, reducing the region’s net uptake of carbon. Researchers are now trying to determine if part of the change in uptake may be due to Southern Ocean circulation changes and surface warming in response to a shift in the regional climate over recent years.

Research Highlight

Jorge Sarmiento directs the National Science Foundation-funded Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project, a multi-institutional effort funded by the National Science Foundation to dramatically increase the number and variety of observations of the Southern Ocean through the world’s first large-scale deployment of biogeochemical (BGC) Argo floats – robotic floats that have been used to measure ocean temperature and salinity that can now be equipped with newly developed biogeochemical sensors to measure pH, nitrate, and oxygen. SOCCOM has 123 of these augmented Argo floats operating, which have collectively made nearly 5 million observations in the Southern Ocean (Figure 2.1) in all seasons and under ice.

Figure 2.1. Locations and trajectories of 124 SOCCOM floats operating as of January 28, 2019. White dots are locations of operating floats; cyan are inoperative floats; yellow dots are pre-SOCCOM floats deployed before 2014. Black lines indicate float trajectories since deployment. (Credit: SOCCOM)

Analyses made since the inception of the project in 2014 have suggested a significant outgassing of CO₂ in the high-latitude Southern Ocean that had escaped previous ship-based observation. This outgassing occurs during the wintertime when deep waters enriched in carbon are brought to the surface, and when hardly any ships travel into this harsh and remote region. Analyses of float data alone had indicated that outgassing cancelled CO₂ drawdown elsewhere in the Southern Ocean, resulting in a net uptake of only -0.08 Pg C yr^-1 (petagrams of carbon per year) in the Southern Ocean, a significant reduction from prior ship-based uptake estimates of over -1 Pg C yr^-1 (negative into the ocean). To put this in context, the ocean as a whole takes up approximately -2 Pg C yr^-1.

In order to reconcile the differences between ship and float-based estimates, this year SOCCOM researchers at Princeton have worked with scientists involved with the Global Carbon Project – an international research collaboration that seeks to fully understand the carbon cycle – to merge SOCCOM pCO₂ estimates with the shipboard and mooring observations that form the backbone of our understanding of the ocean’s role in carbon uptake. The group has found that the addition of SOCCOM observations reduces annual Southern Ocean carbon uptake estimates by approximately one-third relative to estimates based on ship-board observations alone, yielding an annual uptake of -0.75 Pg C yr^-1 (Figure 2.2). SOCCOM observations thus appear to be capturing an important signal previously missed by the shipboard-only dataset.

Figure 2.2. Southern Ocean (south of 35 ̊S) carbon fluxes calculated from ships and floats. Air-sea Southern Ocean CO₂ fluxes over the past 30 years are shown as calculated from the two mapping products used in the Global Carbon Project (blue line +/- 1 s.d.: Landschützer et al. 2013 neural network; green line: Rödenbeck et al. 2013 Jena CarboScope interpolation scheme). 2015-2017 mean fluxes for the three pCO2 products used in this study are overlaid. The combined SOCAT (shipboard dataset) and SOCCOM (new float observations) estimate of the Southern Ocean carbon flux is -0.75 ± 0.22 Pg C yr¹ (black triangle), a decrease in the Southern Ocean uptake of 0.4 Pg C yr^-1 from the SOCAT-only estimate of -1.14 ± 0.19 Pg C yr^-1 (orange circle). Removing shipboard measurements south of 35 ̊S and from 2014 onwards further decreases the Southern Ocean uptake to -0.35 ± 0.19 Pg C yr¹ (purple square). (Bushinsky et al., submitted)

The group is now working to understand how much of the difference between older and new estimates of uptake is due to the impact of the new observations, and whether any of the decrease may reflect a change in the Southern Ocean’s state during the observational period that could foreshadow conditions in a warmer climate.

Figure 2.3. Time-series of sea-ice area (upper panel; red) and sea-surface temperature (lower panel; black) anomalies from 1982 to 2018. The anomalies are averaged over the region south of 55° S as indicated by the gray area on the small map. The blue dashed line (2014) indicates the start year of the SOCCOM observational period during which a major shift in the regional climate occurred. (Credit: A. Haumann)

In recent decades, Southern Ocean surface waters have been cooling slightly, but this trend was abruptly and unexpectedly interrupted in 2016 and 2017, when southern hemisphere sea-ice cover shrank to a record minimum and the ocean’s surface warmed strongly (Figure 2.3). This warming occurred as a circumpolar high-latitude heat wave during austral summer 2017 that exceeded the average mean surface temperature by more than four standard deviations. Analysis of the float data shows that these events took place due to a destratification of the water column that might have switched the Southern Ocean climate system to a new state over the last few years, enhancing natural CO₂ release. Because sea ice plays an important role in controlling stratification, future changes in sea ice and ocean circulation might change these fluxes. While the anomaly in the surface ocean was only a short-time departure from the long-term mean, it may foreshadow the consequences of a potentially warmer future Southern Ocean for biogeochemistry and ecosystems.

In the coming year, the Sarmiento group will continue to scrutinize carbon fluxes in the Southern Ocean and identify causes of their variability. Work in progress includes analysis of float data to identify the subsurface source of high-CO₂ waters and the comparison of float and ship-based flux estimates with observations from NASA’s Orbiting Carbon Observatory, which measures atmospheric CO₂ concentrations from space.

Sarmiento is also involved in two promising efforts to expand the biogeochemical observing system to all the world’s oceans. The National Science Foundation has announced a competition for “mid-scale” infrastructure projects in the range of $20-70 million and Sarmiento is co-principal investigator on a proposal to develop a global biogeochemical float network of ~400 floats. He is also involved in developing the community plan for the next stage of the global Argo system, which similarly proposes incorporating biogeochemical floats as part of the existing global network.

References

Arteaga, L., M. Pahlow, and J.L. Sarmiento, 2018. Mesopelagic remineralization and Surface nutrient limitation of export production in the Southern Ocean. Geophys. Res. Let., submitted.

Bronselaer, B., J.L. Russell, M. Winton, N.L. Williams, R.M. Key, J. P. Dunne, R.A. Feely, and J.L. Sarmiento, 2018. Impact of wind and meltwater on recent observed physical and chemical evolution of the Southern Ocean. Nature, submitted.

Bushinsky, S.M., P. Landschützer, C. Rödenbeck, A.R. Gray, D. Baker, M.R. Mazloff, L. Resplandy, K.S. Johnson, and J.L. Sarmiento. Revisiting Southern Ocean air-sea CO2 flux estimates with the addition of biogeochemical float observations, submitted.

Chen, H, A.K. Morrison, C.O. DuFour, J.L. Sarmiento, S.M. Griffies, P. Zhai, and M. Winton, 2018. Deciphering patterns and drivers of anthropogenic heat and carbon storages in the Southern Ocean. Geophys. Res. Let., submitted.