N. L. Williams

Biogeochemical sensor performance in the SOCCOM profiling float array

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long-term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1±1%, nitrate to within 0.5±0.5 µmol kg−1, and pH to 0.005±0.01, where the error limit is 1 standard deviation of the fleet data. The bio-optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m−3 or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.

Calculating surface ocean pCO2 from biogeochemical Argo floats equipped with pH: An uncertainty analysis

U.S. National Science Foundations Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project under the NSF [PLR-1425989]; NASA [NNX14AP49G]; U.S. Argo through NOAA/JISAO grant [NA17RJ1232]; Ocean Observations and Monitoring Division, Climate Program Office, National Oceanic and Atmospheric Administration, U.S. Department of Commerce; David and Lucile Packard Foundation; NOAA Climate and Global Change postdoctoral fellowship; ARCS Foundation Oregon Chapter

Empirical algorithms to estimate water column pH in the Southern Ocean

Empirical algorithms are developed using high-quality GO-SHIP hydrographic measurements of commonly measured parameters (temperature, salinity, pressure, nitrate, and oxygen) that estimate pH in the Pacific sector of the Southern Ocean. The coefficients of determination, R2, are 0.98 for pH from nitrate (pHN) and 0.97 for pH from oxygen (pHOx) with RMS errors of 0.010 and 0.008, respectively. These algorithms are applied to Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) biogeochemical profiling floats, which include novel sensors (pH, nitrate, oxygen, fluorescence, and backscatter). These algorithms are used to estimate pH on floats with no pH sensors and to validate and adjust pH sensor data from floats with pH sensors. The adjusted float data provide, for the first time, seasonal cycles in surface pH on weekly resolution that range from 0.05 to 0.08 on weekly resolution for the Pacific sector of the Southern Ocean.

Assessment of the Carbonate Chemistry Seasonal Cycles in the Southern Ocean From Persistent Observational Platforms

U.S. National Science Foundations Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project under the NSF [PLR-1425989]; NASA [NNX14AP49G]; U.S. Argo through NOAA/JISAO [NA17RJ1232]; Pacific Marine Environmental Laboratory of NOAA; Ocean Observations and Monitoring Division, Climate Program Office, National Oceanic and Atmospheric Administration, United States; ARCS Foundation Oregon Chapter

Autonomous Biogeochemical Floats Detect Significant Carbon Dioxide Outgassing in the High‐Latitude Southern Ocean

Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased toward summer. Here we present new estimates of Southern Ocean air-sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling project during 2014–2017. Compared to ship-based CO2 flux estimates, the float-based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon-rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship-based observations in this high-latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year-round biogeochemical observations in the Southern Ocean. Plain Language Summary The Southern Ocean absorbs a great deal of carbon dioxide from the atmosphere and helps to shape the climate of Earth. However, we do not have many observations from this part of the world, especially in winter, because it is remote and inhospitable. Here we present new observations from robotic drifting buoys that take measurements of temperature, salinity, and other water properties year-round. We use these data to estimate the amount of carbon dioxide being absorbed by the Southern Ocean. In the open water region close to Antarctica, the new estimates are remarkably different from the previous estimates, which were based on data collected from ships. We discuss some possible reasons that the float-based estimate is different and how this changes our understanding of how the ocean absorbs carbon dioxide.