Evidence from the Baltic Sea for an enhanced CO2 air—sea transfer velocity

Surface water total CO₂ concentrations (C_T) and the CO₂ partial pressure of the surface water and in the atmosphere were measured in the eastern Gotland Sea at approximately monthly intervals during five cruises in the winter of 1999/2000. Taking into account vertical/lateral exchange processes and the decomposition of organic matter, the monthly changes in C_T were used to determine CO₂ evasion fluxes. In addition, the CO₂ fluxes were calculated on the basis of the CO₂ partial pressure differences using local wind speed (u) records and different currently applied parametrizations of the gas exchange transfer velocity (k). The latter resulted in substantially lower monthly fluxes that indicated a considerable underestimation of k from the k(u) functions used. To achieve an optimal agreement between the flux calculations and the balance-derived CO₂ fluxes, the coefficients of both a simple quadratic and cubic function k(u) were iterated using a least-squares fitting procedure. The resulting equations, which refer to short-term wind data and to the CO₂ exchange at 20 °C, were k = (0.45 ± 0.10)u² and k = (0.037 ± 0.008)u³ (k, cm h⁻¹; u, m s⁻¹). These yielded higher k values than most of the previously proposed parametrizations. Unfortunately, our data did not allow us to decide whether the quadratic or cubic function is more appropriate to describe the gas exchange dynamics.