Observed and simulated variations of marine partial pressure of CO2 within the central North Sea: The choice of the bulk air-sea flux formula matters

The partial pressure of carbon dioxide (pCO2) in surface waters is influenced by physical, chemical and biological variables. The interplay of these environmental variables is shown along a section in the central North Sea between Immingham (UK) and Halden (Norway) for the year 2013. Both, underway measurements by ships of opportunity and the results of a high resolution 3D – biogeochemical model show very high pCO2 values near the UK coast and reduced values near the Norwegian coast. This pattern is due to high UK river loads at the western part and the outflow of Baltic water along the Norwegian coast at the eastern part of the section. Seasonal variations within the central North Sea off the coastal waters exhibit both the influence of temperature and biological activities. During the phytoplankton growth seasons, the pCO2 is considerably lowered by the biological uptake of dissolved inorganic carbon and concurrently increased by higher sea surface temperatures.The model is able to reproduce observed winter pCO2 values using a variety of different bulk air-sea flux formulas. However, during spring and summer, the use of these formulas results in overestimated simulated pCO2 values. Only the application of a formula which takes the presence of surfactants into consideration improved the simulated pCO2 values. The latter formula exhibits lower piston velocities for the exchange of CO2 between atmosphere and ocean than the common formulas.