On the oceanographic variability of the North-West European Shelf to the West of Scotland

Abstract

Temperature, salinity and flow variability on the north-west European Shelf west of Scotland are examined. Analyses are derived from a range of marine and atmospheric data sources which together comprise a “proto-observatory” of the European Shelf to the west of the Scottish mainland. The main time series are from a current meter mooring which has been maintained on the northwest European continental shelf since June 1981, and a hydrographic section repeated at least annually across the shelf at ~ 56°N since 1975. With the exception of occasional episodes the water column at the mooring site is well mixed or weakly stratified throughout the year. The mean water temperature was 10.1 °C and the dominant mode of variance in the temperature record is the seasonal cycle (SA), with amplitude of 3.2 °C. The currents are constrained by a passage and are dominated by the semi-diurnal tidal species. There is also significant energy in SA in the along channel velocity but not in the across channel velocity. The along channel residual has mean value of 10.8 cm s − 1 directed towards the north, a clear manifestation of the coastal current, with very few, short duration periods of flow reversal. The monthly temperature anomaly time series are compared with the NE Atlantic upper layer heat content anomalies. Both anomaly time series show highs in the late 1980s and 1990s and lows in the early 1980s and mid 1990s. The overall trend is of warming at a rate of + 0.57 °C per decade, with the timing of maximum annual temperature receding by 12 days per decade through the time series. A simple barotropic dynamic balance for the along channel flow in the passage demonstrates that the near surface wind dominates the variability of the northward flow, and that both the north/south gradient in sea level slope and atmospheric pressure are significant in driving the coastal current northwards. This balance reveals a bias is previous summer-only estimates of the flow rate of the coastal current, and a revised figure approximately double that of previous estimates is suggested. Residual winter flows correlate significantly with the North Atlantic Oscillation index ( r = 0.59). In contradiction to previous published analyses, shelf salinities exhibit only weak seasonality. No single determining factor is found for the longitudinal excursion of surface isohalines across the shelf, however prolonged periods of high NAO index do coincide with a raised salinity of shelf waters.