Abstract
Abstract
For the North Sea, a semienclosed shelf sea in the northeastern North Atlantic, the seasonal and annual CO2 air-sea fluxes (ASF) had been estimated for 2001 and 2002 in earlier work. The underlying observations, pCO(2), salinity, and temperature had been combined with 6-hourly wind data derived from ERA40 reanalysis. In order to assess the impact of different wind data products on the computation of CO2 ASF, we compared ERA40 wind data with coastDat data derived from the nonhydrostatic regional climate model COSMO-CLM. From the four observational months September, November, February, and May all but the May data show higher wind speeds for coastDat than for ERA40, especially off the Norwegian, UK, and continental coasts. Largest differences occur in the northern offshore areas. The comparison with observed wind data supports this feature generally: At Helgoland, an island in the German Bight, and at the Belgium pile Westhinder the ERA40 data underestimate both, the coastDat data and the observations. Wind observations for two Norwegian North Sea platforms were available: At the northern station Troll off the Norwegian coast the coastDat data overestimate the observations in winter. At Ekofisk in the central North Sea the ERA40 data fit the observations well, while the coastDat data slightly overestimate the observational data in all months but in May. The corresponding CO2 ASF estimates show strongest deviations off the Norwegian coast. Using different bulk formulas for determining the net annual ASF resulted in differences due to different wind products of up to 34%.
Plain Language Summary Climate change is induced by gases like carbon dioxide, which are added to the atmosphere. The increase of the concentration in the atmosphere is dampened by the uptake of this gas by land and ocean. Especially, the coastal ocean is able to efficiently absorb CO2. To calculate the North Sea-wide uptake of CO2, simulated wind speed data were used. The formerly used model data cover the total Earth and thus have a less fine resolution. Especially near the coast this effect becomes dominant, as wind over land is more efficiently retarded than over sea. A new wind product (coastDat) with a refined grid was established especially for coastal applications. We compare the old and the new data with observational data sets. It has shown that the coastDat data are closer to observations near the coast. The old data set significantly underestimates the observational data there. At the open sea the new data set slightly overestimates the observations. The comparison of the mean flux of CO2 from the atmosphere into the ocean revealed an increase of 34% when using the new wind data instead of the old one.
