Spatiotemporal variations of fCO₂ in the North Sea

Abstract. 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 (fCO₂^sw) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO₂^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 fCO₂^sw induced by SST changes was 0.4–0.75 times those resulting from variations in biology plus mixing. Along a meridional transect, fCO₂^sw normally decreased northwards (−12 μatm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO₂^sw in southern parts of the North Sea. Along a zonal transect, a weak gradient (−0.8 μatm per degree longitude) was observed in the annual mean fCO₂^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° N were CO₂ supersaturated and, hence, a source for atmospheric CO₂. Comparison of fCO₂^sw 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 fCO₂^sw 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 fCO₂^sw differences. The lowest interannual variation (10–50 μatm) was observed during fall and early winter. Comparison with data reported in October 1967 suggests that the fCO₂^sw growth rate in the central North Sea was similar to that in the atmosphere.