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Browsing by Author "Wickström, Siiri"

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  • Wickström, Siiri (2015)
    This study aims to connect air-sea ice turbulent carbon dioxide (CO2) exchange and the surface energy balance over melting fjord ice. Recent studies have shown that sea ice melt might act as a significant CO2 sink in Arctic waters. Melt process have been suggested to dilute both brine and surface water partial pressures of carbon dioxide ( pCO2). Also biological activity and carbonate chemistry changes the air-ocean CO2 concentration gradient. Even small fluxes might be potentially significant as the maximum sea ice extent covers approximately 7 % of Earth's surface. As multi-year ice diminishes with the on-going climate change a bigger portion of the ice cover will experience melt in the summer season and thus the melt induced changes on the carbon cycling in the Arctic will have a greater effect. Surface energy balance consists of net radiation, turbulent fluxes of latent and sensible heat and conductive heat flux. During melt the sea ice surface transforms from a dry snow cover to melt ponds. Surface melt leads to a decrease in the surface albedo controlling the surface energy balance. Sea ice temperature affects both air-ice-ocean energy exchange and the permeability of the ice. My thesis is based on a 30 day measurement campaign in June 2014 from The Young Sound fjord, in North-East Greenland (74° 18' N, 20° 13' W). Turbulent fluxes of CO2 and H2O were measured with 10 Hz with two open-path infrared gas-analysers and two sonic anemometers at approximately 3 m height. One mast was used to measure basic meteorology (temperature, humidity, radiation, wind). Continuous measurements of surface water pCO2 were made 2.5 m below the ice. Conductive heat flux was determined from ice cores. The turbulent fluxes were calculated with the Eddy Covariance-methodology. Weak winds decreased the number of good quality measurements and created gaps in the time series. The measured CO2 flux ranged between 1.92 ja -3.2 µmol m-2 s-1 (positive fluxes being efflux) and sea ice was a net sink during the campaign. Sea ice temperature rose steadily with time, being above the permeability threshold in all measurements. Surface water pCO2 was lower than the atmospheric throughout the study and the under saturation grew from 50 µatm to 140 µatm in 10 days (13. to 23. June). Despite the strong pCO2 difference indicating ocean sink the CO2 absorption at the ice surface did not show an increase over time. A possible explanation for this is the equilibration of the air-melt pond pCO2 gradient. The strongest CO2 uptake was recorded as the melt ponds formed. Two strong efflux events ( flux > 1 µmol m-2 s-1) were recorded on June 11 and 18. No strong correlations were found between the surface energy budget and the air-surface CO2 exchange. However, the CO2 flux has a weak positive correlation both with wind speed and net radiation. The weak correlations are probably explained by the co-existence of different processes simultaneously affecting the air-ice-ocean CO2 gradients. The results support the hypothesis of melting sea ice acting as a sink for atmospheric CO2. The measurement quality could be improved by using a closed-path gas analyser. Despite the challenges Eddy Covariance is currently the state-of-art methodology for measuring ecosystem-scale turbulent exchange.