Abstract
AbstractThe influence of large‐scale oceanic circulation on salinity in the northern North Sea has lead to the hypothesis that nutrient concentrations in this region are also driven by remote oceanic anomalies. Here, using a newly established biogeochemical data set of the North Sea, we show that interannual to decadal variability in winter nutrient concentrations exhibits distinct phase deviations from salinity. The variability in salinity is explained by zonal shifts in the position of the subpolar front (SPF) in the eastern North Atlantic and the associated advective delay. However, the high correlation and absence of advective delay between the position of the SPF and winter nutrient concentrations in the Shetland region (59–61°N, 1°W to 3°E) point to the role of atmospheric variability in driving concurrent changes in winter nutrient concentrations and the SPF position. Our analysis suggests that the prevailing wind direction and local distribution of winter nutrient concentrations together determine the interannual to decadal variability in winter nutrient concentrations in this region. In the analyzed observations, we find a strong spatial gradient in mean winter nutrient concentrations northwest of the Shetland region, which is absent in salinity. The horizontal shift of this spatial gradient, forced by changes in wind direction, has a larger influence on winter nutrient concentration in the Shetland region than the nutrient signal in oceanic anomalies originating from the eastern subpolar North Atlantic. Overall, we conclude that interannual to decadal variability in the observed nutrient concentrations is mainly driven by atmospheric variability here expressed as wind direction.
Highlights
The North Sea is flushed mainly by North Atlantic water via its northern boundaries (Pätsch et al, 2017; Winther & Johannessen, 2006)
The influence of large‐scale oceanic circulation on salinity in the northern North Sea has lead to the hypothesis that nutrient concentrations in this region are driven by remote oceanic anomalies
Our analysis suggests that the prevailing wind direction and local distribution of winter nutrient concentrations together determine the interannual to decadal variability in winter nutrient concentrations in this region
Summary
The North Sea is flushed mainly by North Atlantic water via its northern boundaries (Pätsch et al, 2017; Winther & Johannessen, 2006). The nutrient loads of this North Atlantic water have a large biogeochemical impact on the northern and central North Sea (Leterme et al, 2008). Unlike the southern and coastal areas, primary production in the northern and central part of the North Sea is limited by the nutrients nitrate and phosphate PÄTSCH ET AL. Besides local processes like stratification and vertical mixing, winter concentrations of these nutrients quantitatively govern annual primary production in the northern and central North Sea (Pätsch & Kühn, 2008). Understanding the mechanisms driving the variability of winter nutrient concentrations in the northern and central North Sea is crucial for describing the variability of the most important biogeochemical processes in this area
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