Ocean Science www.ocean-sci.net 1812-0784 1812-0792 6 1 2010 10.5194/os-6-77-2010 http://www.ocean-sci.net/6/77/2010/ http://www.ocean-sci.net/6/77/2010/os-6-77-2010.html http://www.ocean-sci.net/6/77/2010/os-6-77-2010.pdf 77 89 2010-01-29 Spatiotemporal variations of <i>f</i>CO₂ in the North Sea A. M. Omar abdirahman.omar@gfi.uib.no A. Olsen T. Johannessen M. Hoppema H. Thomas A. V. Borges Geophysical Institute, University of Bergen, Allégaten 70, 5007 Bergen, Norway Bjerknes Centre for Climate Research, University of Bergen, Allégaten 55, 5007 Bergen, Norway Alfred Wegener Institute for Polar and Marine Research, Climate Sciences Department, Postfach 120161, 27515 Bremerhaven, Germany Dalhousie University, Department of Oceanography, Halifax, Canada Royal Netherlands Institute for Sea Research, Den Burg, Texel, The Netherlands Chemical Oceanography Unit, University of Liège, Liège, Belgium Data from two Voluntary Observing Ship (VOS) (2005–2007) augmented with data subsets from ten cruises (1987–2005) were used to investigate the spatiotemporal variations of the CO₂ fugacity in seawater (<i>f</i>CO₂^sw) in the North Sea at seasonal and inter-annual time scales. The observed seasonal <i>f</i>CO₂^sw 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 <i>f</i>CO₂^sw induced by SST changes was 0.4–0.75 times those resulting from variations in biology plus mixing. Along a meridional transect, <i>f</i>CO₂^sw normally decreased northwards (&minus;12 &mu;atm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of <i>f</i>CO₂^sw in southern parts of the North Sea. Along a zonal transect, a weak gradient (&minus;0.8 &mu;atm per degree longitude) was observed in the annual mean <i>f</i>CO₂^sw. Annually and averaged over the study area, surface waters of the North Sea were CO₂ undersaturated and, thus, a sink of atmospheric CO₂. However, during summer, surface waters in the region 55.5–54.5&deg; N were CO₂ supersaturated and, hence, a source for atmospheric CO₂. Comparison of <i>f</i>CO₂^sw data acquired within two 1&deg;&times;1&deg; 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 <i>f</i>CO₂^sw differences (&asymp;160–200 &mu;atm) approached seasonal changes (≈200–250 &mu;atm). 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 <i>f</i>CO₂^sw differences. The lowest interannual variation (10–50 &mu;atm) was observed during fall and early winter. Comparison with data reported in October 1967 suggests that the <i>f</i>CO₂^sw growth rate in the central North Sea was similar to that in the atmosphere. % vor jede Referenz Beaugrand, G.: The North Sea regime shift: Evidence, causes, mechanism and consequences, Prog. Oceanogr., 60, 245–262, 2004. 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