The Northwest European Shelf Temperature and Salinity Variability

From long temperature and salinity time series of the Northwest European shelf and 39 years of numerical model transport calculations, it is possible to derive the long-term hydrographic variability of the North Sea system and its possible causes. This is shown by means of trend analyses, cross-correlations, and power spectra. Initially there is a weak linear long-term positive trend in the SST series, which is in agreement with the global warming trend. In all series investigated, several periodicities are found on corresponding band widths, and we argue that this is due to long-term oscillations in the North Atlantic circulation system and interaction with the atmosphere. Cross-correlations between Sea Surface Temperature (SST)/ Sea Surface Salinity (SSS) and some transports are significant, with time lags of about 5 to 6 years. These time lags are unexplained; we assume that differences in the subpolar and subtropical gyre circulation might be responsible. A two-year time lag between the SSS in the Rockall Channel and east of Scotland points to lateral exchanges by shear dispersion, not to advective transports. It seems that the North Sea SST longterm fluctuations are coupled to the Atlantic subpolar gyre SST variability. In contrast, the Bay of Biscay behaves very similarly to the Atlantic subtropical gyre system. The ratio between the inflow of saline Atlantic water into the North Sea and continental run-off seems to have been rather constant in the last 100 years.