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Spatiotemporal variations of fCO2 in the North Sea
1Geophysical Institute, University of Bergen, Allégaten 70, 5007 Bergen, Norway
2Bjerknes Centre for Climate Research, University of Bergen, Allégaten 55, 5007 Bergen, Norway
3Alfred Wegener Institute for Polar and Marine Research, Climate Sciences Department, P.O. Box 120161, 27515 Bremerhaven, Germany
4Dalhousie University, Department of Oceanography, Halifax, Canada
5Royal Netherlands Institute for Sea Research, Den Burg, Texel, The Netherlands
6Chemical Oceanography Unit, University of Liège, Liège, Belgium
Abstract. Data from two Voluntary Observing Ship (VOS) (MS Trans Carrier and MV Nuka Arctica), acquired along one zonal and one meridional transect (2005–2007) augmented with data subsets from ten cruises (1987–2005) were used to investigate the spatio-temporal variations of the CO2 fugacity in seawater (fCO2sw) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO2sw variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO2 exchange. Over the study period, the seasonal amplitude in fCO2sw induced by SST changes was 0.4–0.75 times those resulting from variations in biology plus mixing. Along the meridional transect, fCO2sw normally decreased northwards (−12 μatm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO2sw in southern parts of the North Sea. Along the zonal transect, a weak gradient (−0.8 μatm per degree longitude) was observed in the mean annual fCO2sw. Annually and averaged over the study area, surface waters of the North Sea were CO2 undersaturated and thus a sink of atmospheric CO2 throughout the year. However, during summer, surface waters in the region 55.5–54.5° N were CO2 supersaturated and, hence, a source for atmospheric CO2. Comparison of fCO2sw data acquired within two 1°×1° regions in the northern and southern North Sea during different years (1987, 2001, 2002, and 2005–2007) revealed large interannual variations, especially during spring and summer when year-to-year fCO2sw differences (≈160–200 μatm) approached seasonal changes (≈200–250 μatm). The springtime variations resulted from changes in magnitude and timing of the phytoplankton bloom, whereas changes in SST, wind speed, and total alkalinity may have contributed to the summertime interannual fCO2sw differences. The lowest interannual variation (10–50 μatm) was observed during fall and early winter. The comparison with data reported in October 1967 suggests that the fCO2sw growth rate in the central North Sea is similar to that in the atmosphere.
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Citation: Omar, A. M., Olsen, A., Johannessen, T., Hoppema, M., Thomas, H., and Borges, A. V.: Spatiotemporal variations of fCO2 in the North Sea, Ocean Sci. Discuss., 6, 1655-1686, 2009. Bibtex EndNote Reference Manager
