At a Glance
Previous research using ocean observations and model results has suggested that the ocean around Antarctica acts as a key sink for atmospheric CO2, mitigating global temperature increases caused by increasing anthropogenic carbon emissions. However, ship-based observations to test these findings have been scarce and mostly limited to summertime measurements. New data from robotic floats that measure biogeochemistry year-round suggest that previous studies may have missed important wintertime outgassing in certain regions, resulting in overestimates of the size of the Southern Ocean sink. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) researchers are working to extend such observations throughout the global ocean
The Southern Ocean surrounding Antarctica plays a crucial role in the global carbon cycle and in the climate system as a whole. Observation-based estimates typically find that the Southern Ocean accounts for a significant portion of the contemporary oceanic uptake of CO2. However, measurements in this remote and inhospitable region have historically been made from ships and are severely limited in space as well as heavily biased towards spring and summer.
Sarmiento directs the 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. These robotic floats are equipped with newly developed biogeochemical sensors to measure pH, nitrate, and oxygen
in addition to ocean temperature and salinity.
Over 100 SOCCOM BGC floats are currently operating and have collectively made nearly 5 million observations in the Southern Ocean (Figure 1.1.1) in all seasons and under ice. A subset of these floats deployed in the Southern Ocean during 2014-2017 have been used to make new estimates of air-sea fluxes of CO2 using these year-round measurements. Figure 1.1.2 shows CO2 fluxes for five regions of the Southern Ocean derived from pH measurements and alkalinity estimates based on
regression algorithms. The results, while consistent with gridded CO2 flux products in the Subtropical, Subantarctic, Polar Front, and Seasonal Ice zones, are markedly different in the Antarctic zone where carbon-rich deep waters upwell to the sea surface, a region that is poorly sampled in ship-based datasets. Significant outgassing observed in this region leads to a net Southern Ocean CO2 sink of 0.03 PgC y-1 from the atmosphere to the ocean, a substantial departure from the most recent shipbased estimates of a mean sink of 1.0 PgC y-1. This finding has important implications for global climate models, which typically simulate a Southern Ocean CO2 sink of 0.5 to 1.3 PgC y-1 south of 35°S (relative to a global ocean sink of ~2 PgC y-1 and anthropogenic emissions of ~10 PgC y-1 from fossil fuels and cement). Furthermore, it suggests that our current understanding of the distribution of oceanic sources and sinks of CO2 may need revision and highlights the need for sustained year-round observations in the high-latitude Southern Ocean.
SOCCOM will continue to deploy BGC-Argo floats over the next three years, with a goal of roughly 200 floats operating in the Southern Ocean by 2020, which will give an even clearer picture of the distribution of pH, oxygen, and nutrients in the region. At the same time, high-resolution modeling studies are using the new data to better understand the current workings of the Southern Ocean and to compare and improve Earth System Models of the future of this region in a changing climate.
SOCCOM researchers are also reaching out to the oceanographic community to enable other researchers to use BGC-floats and expand the observing system worldwide. SOCCOM researchers held a town hall at this year’s American Geophysical Union Ocean Sciences meeting to inform other oceanographers about their efforts and the potential of float-based observations, and will also hold a float workshop at the University of Washington this summer to train other scientists, particularly early-career researchers, in how to use and process data from these new tools.