Direct Wintertime pCO2 Observations From the Saildrone Gulf Stream Mission Reveal Higher Ocean Carbon Uptake in 2019 than in Climatologies

Sarah Nickford1, Jaime B Palter1, Nicholas Robert Bates2, Kathleen A Donohue3, Andrea J Fassbender4, Alison R Gray5, Jacqueline Long6, Stacy Maenner7, Adrienne J Sutton8 and Yuichiro Takeshita9, (1)University of Rhode Island, Graduate School of Oceanography, Narragansett, RI, United States, (2)Bermuda Institute of Ocean Sciences, St. George's, Bermuda, (3)Univ Rhode Island, Narragansett, RI, United States, (4)Monterey Bay Aquarium Research Institute, Moss Landing, United States, (5)University of Washington, School of Oceanography, Seattle, WA, United States, (6)University of South Florida, College of Marine Science, St. Petersburg, FL, United States, (7)NOAA Seattle, Seattle, WA, United States, (8)NOAA Pacific Marine Environmental Laboratory, Seattle, WA, United States, (9)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States
Western boundary currents, such as the Gulf Stream, are hot spots of ocean carbon dioxide (CO2) uptake. In these regions, the maximum sea-air CO2 flux occurs during wintertime, when in-situ observations are sparse due to intense weather conditions that challenge ship-based sampling. Moreover, the short spatio-temporal scales of variability require dense sampling to resolve the influence of the current on gas exchange. We show the capability of an Autonomous Surface Vehicle (ASV), Saildrone, for collecting transformative measurements in the Gulf Stream region during a February 2019 deployment. The Saildrone is a high-endurance (>3 months), fast-moving (1-8 kts) ASV that carries a large payload of sensors. For our deployment, it was equipped with the PMEL-designed ASVCO2 system, which measures pCO2 in both the atmosphere and the ocean with climate-quality (2 μatm) accuracy using 2-point calibrations before every measurement. With pCO2, sea surface temperature, salinity, and near-surface wind measurements from Saildrone, we calculate CO2 fluxes in the cold, nutrient-rich Slope Sea, across the Gulf Stream, and into the warm, nutrient-poor Sargasso Sea during active convection. We find that atmospheric pCO2 varies by 18 μatm while oceanic pCO2 varies by 43 μatm, with the largest sea-air pCO2 gradients (ΔpCO2) on the subtropical side of the Gulf Stream. The measured ΔpCO2 averages -51atm, which is 46% and 58% greater in magnitude compared to the February climatologies of Takahaski et al. (2009) and Landschützer et al. (2013), respectively, for the region surrounding the Gulf Stream. Thus, the observations reveal higher ocean CO2 uptake than is inferred from climatologies based on shipboard data. We test the hypothesis that the ΔpCO2 in such climatologies are biased due to chronic under-sampling in winter when heat loss drives a strong increase in oceanic CO2solubility and increased vertical nutrient fluxes support enhanced phytoplankton productivity.