CO2 AIR-SEA EXCHANGE IN THE NORDIC SEAS - AN ATTEMPT TO MAKE AN ESTIMATE BASED ON DATA

The net air-sea exchange of CO2 in the ocean area enclosed by the Fram Strait in the north and by the Greenland-Scotland ridge in the south is estimated on the basis of oceanic pCO(2) data from three different expeditions. The data comprise observed pCO(2) as well as pCO(2) calcu lated from data on dissolved inorganic carbon (C-t), total alkalinity (TA) and pH. Data on atmospheric content of CO2 are available from two of the expeditions and are supplemented with monthly mean values from ocean station ''M'' (66 degrees N, 2 degrees E). Wind speed drawn fro m a data set with a spatial resolution of 75 km and a temporal resolut ion of six hours are used in the calculation. The pCO(2) data are inte rpolated to the same spatial resolution as that of the wind speed data , before calculating the net flux. The objective of this work is to su pplement earlier estimates of the net CO2 flux between atmosphere and ocean in the Nordic Seas, and to find a plausible shape of the seasona l variation of the net CO2 air-sea exchange. The calculations, per for med with the parameterization of the gas exchange coefficient accordin g to Liss and Merlivat (1986), give a net uptake by the Nordic Seas of atmospheric CO2 amounting to approximately 2.4 moles C m(-2) year(-1) . This is equivalent to 0.05 Gtonnes C year(-1), if the ice-free area is assumed to be 1.710(12) m(2). The estimate is in the lower range o f the earlier estimates. Sensitivity tests of the calculated fluxes ar e performed as to the influence on the results by the choice of parame terization of the gas exchange coefficient, errors of observation in t he pCO(2) data, choice of time scale of wind speed data and choice of interpolation method. The sensitivity tests indicate that the largest variations of the individual net fluxes arise from the uncertainty of the parameterization of the gas transfer and in the case of pCO(2) cal culated from C-t, TA and pH, also from errors of observation. The resu lts from three different equations of gas transfer (Liss and Merlivat, 1986; Tans et al., 1990; Wanninkhof, 1992) are compared. The paramete rizations by Tans et al. and by Wanninkhof result in an increase of th e estimated net fluxes of approximately 75 % compared to the net fluxe s obtained by using the equations by Liss and Merlivat. Regarding the situation examined here, this uncertainty corresponds to an error of o bservation in oceanic pCO(2) of 3-11 %. The choice of time scale of th e wind speed data and the choice of interpolation method do not seem t o be of the same significance as far as the uncertainty of the result is concerned. Another uncertainty of the estimated yearly net flux ari ses from the fact that the pCO(2) data are very sparse in time, but th e magnitude of this uncertainty is difficult to estimate.