Spatiotemporal variations of <i>f</i>CO<sub>2</sub> in the North Sea
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 CO<sub>2</sub> fugacity in seawater (<i>f</i>CO<sub>2</sub><sup>sw</sup>) in...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2010-01-01
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| Series: | Ocean Science |
| Online Access: | http://www.ocean-sci.net/6/77/2010/os-6-77-2010.pdf |
| Summary: | 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 CO<sub>2</sub> fugacity in seawater (<i>f</i>CO<sub>2</sub><sup>sw</sup>) in the North Sea at seasonal and inter-annual time scales. The observed seasonal <i>f</i>CO<sub>2</sub><sup>sw</sup> variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO<sub>2</sub> exchange. Over the study period, the seasonal amplitude in <i>f</i>CO<sub>2</sub><sup>sw</sup> 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>CO<sub>2</sub><sup>sw</sup> normally decreased northwards (−12 μatm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of <i>f</i>CO<sub>2</sub><sup>sw</sup> 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 <i>f</i>CO<sub>2</sub><sup>sw</sup>. Annually and averaged over the study area, surface waters of the North Sea were CO<sub>2</sub> undersaturated and, thus, a sink of atmospheric CO<sub>2</sub>. However, during summer, surface waters in the region 55.5–54.5° N were CO<sub>2</sub> supersaturated and, hence, a source for atmospheric CO<sub>2</sub>. Comparison of <i>f</i>CO<sub>2</sub><sup>sw</sup> 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 <i>f</i>CO<sub>2</sub><sup>sw</sup> 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 <i>f</i>CO<sub>2</sub><sup>sw</sup> 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 <i>f</i>CO<sub>2</sub><sup>sw</sup> growth rate in the central North Sea was similar to that in the atmosphere. |
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| ISSN: | 1812-0784 1812-0792 |