The impact of changes in North Atlantic Gyre distribution on water mass characteristics in the Rockall Trough

Regular time-series of temperature and salinity in the upper waters of the northern Rockall Trough can be traced back to 1948, when Ocean Weather Ships on passage to the west began to record them. Since 1975, these data have been complemented by, then replaced by, dedicated Conductivity, Temperature, and Depth sections along 57.5°N. Today, they are enhanced with real-time observations of the upper 1000 m made by an underwater glider. Since 1995, there has been a steady increase in both temperature (from 9.1 to 10°C) and salinity (from 35.32 to 35.41) as the Subpolar Gyre has retreated west. The historical record suggests that such increases could be compatible with decadal scales of natural variability. Nutrient (nitrate, phosphate, and silicate) concentrations have been measured systematically since 1996 and exhibit interannual variability, particularly in nitrate, the causes of which are not immediately clear. Average phosphate concentrations in the upper 800 m declined from 0.80 to 0.63 μM by 2009, consistent with the recent incursion of depleted Subtropical Gyre water. The Ellett Line dataset contains a unique and essential archive of observations that can be used to place in context the recent changes in local ocean climate.