Improving regional and global air-sea CO2 flux estimates

Dong, Yuanxu (2023) Improving regional and global air-sea CO2 flux estimates. Doctoral thesis, University of East Anglia.

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The global oceans are a major carbon sink accounting for approximately a quarter of carbon dioxide (CO2) emissions by human activities. Accurate quantification of ocean CO2 uptake is critical to the assessment of the global carbon budget and to the projection of the future climate. The air-sea CO2 flux is often estimated by the bulk method using sea surface CO2 fugacity (fCO2w) measurements combined with a wind speed-dependent gas transfer velocity (K660). However, there are large uncertainties in bulk CO2 flux estimates due to uncertainties in K660, upper ocean gradients in fCO2w and in temperature. In this thesis, I use direct air-sea CO2 flux observations by the eddy covariance (EC) technique to improve CO2 flux estimates over the high-latitude oceans. Upper ocean temperature gradients and their impact on CO2 flux estimates are further assessed to update our understanding of global ocean CO2 uptake.

Here I first make a comprehensive analysis of the uncertainties in ship-based EC air-sea CO2 flux measurements to better understand the EC observations and to optimise the EC-based studies of K660. Second, the impact of shallow stratification due to sea-ice melt is investigated using the EC CO2 flux and fCO2w measurements in the Arctic Ocean. Additional analysis of EC CO2 fluxes from seven cruises in the Southern Ocean helps to improve our understanding of Southern Ocean CO2 flux estimates. Finally, I reassess two temperature effects (the warm bias in the shipboard temperature dataset and the cool skin effect) and update their impact on global ocean CO2 flux estimates.

My uncertainty analysis suggests that the state-of-the-art EC system is well suited for air-sea CO2 flux measurements and that the EC flux can be considered a reference for evaluating indirect fluxes in strong flux signal regions. The Arctic study shows a clear underestimation of the bulk CO2 flux in sea-ice melt regions estimated from subsurface fCO2w observations (made on water from typically 5 m depth). The EC CO2 flux indicates strong CO2 uptake in the summertime Southern Ocean, which supports the shipboard fCO2w observation (from SOCAT dataset)-based flux products after correcting for the temperature effects but suggests that the float observation (from SOCCOM dataset)-based CO2 sink estimate is too weak. The impact of the temperature effects is even more significant for global ocean CO2 flux estimates, increasing the global ocean CO2 uptake by ~35% (0.6 Pg C yr-1). The K660−wind speed relationships based on EC observations agree well with the widely used K660 parameterisations, especially at intermediate wind speeds. In summary, this thesis advances our understanding of oceanic CO2 uptake and contributes to reducing the uncertainties in air-sea CO2 flux estimates.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Chris White
Date Deposited: 02 Aug 2023 08:32
Last Modified: 02 Aug 2023 08:32

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