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Title: Effects of near surface ocean gradients upon shelf sea air-sea gas exchange estimates
Author: Sims, R.
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2019
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The seawater CO2 partial pressures (pCO2) which are used in flux calculations are measured in the mixed layer at ~5m. If the surface ocean is not mixed, the pCO2 measured at 5m is not representative of the interfacial value and the calculated flux is incorrect. The objective of this work was to measure near surface pCO2 gradients in the mixed layer to discern their occurrence, size and effect on the flux. A Near Surface Ocean Profiler (NSOP) was designed to precisely measure vertical gradients in the top 5 m of the ocean. Vertical profiles of pCO2 were possible due to a fast equilibrating membrane equilibrator. Vertical profiles of temperature, salinity and pCO2 were collected with NSOP during 4 research cruises at different times of the year in Shelf Seas and as part of a seasonal study at the Western Channel Observatory Site L4. Differences in pCO2 of < 2.5μatm and < 5μatm over the surface 5m of the ocean were calculated using the near surface profiles made in the Shelf Seas and at L4 respectively. The largest differences in interfacial and subsurface CO2 were > 4μatm and were observed only when there were strong temperature gradients (> 0.05°C) and low wind speeds (< 6ms-1). There was no apparent relationship between the local meteorology and differences in the interfacial and subsurface pCO2. Theoretical calculations of the formation of near surface layers with distinctly different pCO2 to the subsurface pCO2 indicate that the air-sea flux, biological production and rainfall could change the pCO2 in a surface layer equal to the magnitude of the pCO2 changes observed during the cruises. The largest differences in interfacial and subsurface CO2 were observed when the wind speeds were low, this means that they have a relatively large effect on a small air sea fluxes over short periods of time but do not substantially alter the absolute size of flux which is dominated by strong winds over long timescales.
Supervisor: Bell, T. ; Schuster, U. ; Watson, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Ocean ; CO2 ; Profiles ; Gradients ; fluxes