FY 2022

Autonomous wintertime observations of air-sea exchange in the Gulf Stream reveal a perfect storm for ocean CO2 uptake

Nickford, S., J.B. Palter, K. Donohue, A.J. Fassbender, A.R. Gray, J. Long, A.J. Sutton, N.R. Bates, and Y. Takeshita

Geophys. Res. Lett., 49(5), e2021GL096805, doi: 10.1029/2021GL096805, View online (2022)

A scarcity of wintertime observations of surface ocean carbon dioxide partial pressure (pCO2) in and near the Gulf Stream creates uncertainty in the magnitude of the regional carbon sink and its controlling mechanisms. Recent observations from an Uncrewed Surface Vehicle (USV), outfitted with a payload to measure surface ocean and lower atmosphere pCO2, revealed sharp gradients in ocean pCO2 across the Gulf Stream. Surface ocean pCO2 was lower by ~50 µatm relative to the atmosphere in the subtropical mode water (STMW) formation region. This undersaturation combined with strong wintertime winds allowed for rapid ocean uptake of CO₂, averaging -11.5 mmol m^-2 day^-1 during the February 2019 USV mission. The unique timing of this mission revealed active STMW formation. The USV proved to be a useful tool for CO₂ flux quantification in the poorly observed, dynamic western boundary current environment.

Plain Language Summary.The North Atlantic Ocean absorbs more atmospheric carbon dioxide (CO₂) than most regions of the global ocean. Using an ocean drone in the Gulf Stream region during the winter of 2019, we measured the air-sea CO₂ difference, calculated the air-sea CO₂ exchange, and compared our results to previous wintertime ship-based measurements. We find that the region south of the Gulf Stream can absorb vast amounts of atmospheric CO₂, owing both to surface ocean properties and the strong wintertime winds. Because of extremely sparse wintertime observations in this region, we hypothesize that ocean uptake of CO₂ may be underestimated. Our work suggests that ocean drones can help close the observational gaps that create uncertainty in ocean carbon uptake in challenging regions and seasons, if these vehicles and the sensors they carry can be made robust to large breaking waves.