EGU21-715
https://doi.org/10.5194/egusphere-egu21-715
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The impact of shallow stratification on air-sea CO2 flux in the summer Arctic Ocean

Yuanxu Dong1, Dorothee Bakker1, Thomas Bell2, Peter Liss1, Ian Brown2, Vassilis Kitidis2, and Mingxi Yang2
Yuanxu Dong et al.
  • 1School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom of Great Britain – England, Scotland, Wales
  • 2Plymouth Marine Laboratory, Plymouth, United Kingdom of Great Britain – England, Scotland, Wales

Air-sea carbon dioxide (CO2) flux is often indirectly estimated by the bulk method using the in-situ air-sea difference in CO2 fugacity and a wind speed dependent parameterisation of the gas transfer velocity (K). In the summer, sea-ice melt in the Arctic Ocean generates strong shallow stratification with significant gradients in temperature, salinity, dissolved inorganic carbon (DIC) and alkalinity (TA), and thus a near-surface CO2 fugacity  (fCO2w) gradient. This gradient can cause an error in bulk air-sea CO2 flux estimates when the fCO2w is measured by the ship’s underway system at ~5 m depth. Direct air-sea CO2 flux measurement by eddy covariance (EC) is free from the impact of shallow stratification because the EC CO2 flux does not rely on a fCO2w measurement. In this study, we use summertime EC flux measurements from the Arctic Ocean to back-calculate the sea surface fCO2w and temperature and compare them with the underway measurements. We show that the EC air-sea CO2 flux agrees well with the bulk flux in areas less likely to be influenced by ice melt (salinity > 32). However, in regions with salinity less than 32, the underway fCO2w is higher than the EC estimate of surface fCO2w and thus the bulk estimate of ocean CO2 uptake is underestimated. The fCO2w difference can be partly explained by the surface to sub-surface temperature difference. The EC estimate of surface temperature is lower than the sub-surface water temperature and this difference is wind speed-dependent. Upper-ocean salinity gradients from CTD profiles suggest likely difference in DIC and TA concentrations between the surface and sub-surface water. These DIC and TA gradients likely explain much of the near-surface fCO2w gradient. Accelerating summertime loss of sea ice results in additional meltwater, which enhances near-surface stratification and increases the uncertainty of bulk air-sea CO2 flux estimates in polar regions.

How to cite: Dong, Y., Bakker, D., Bell, T., Liss, P., Brown, I., Kitidis, V., and Yang, M.: The impact of shallow stratification on air-sea CO2 flux in the summer Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-715, https://doi.org/10.5194/egusphere-egu21-715, 2021.

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