Hydrography of the East Shetland Basin in relation to decadal North Sea variability

Abstract

Three hydrographic sections running east from the coast of Shetland into the central northern North Sea have been surveyed since 1989. These sections cross the East Shetland atlantic Inflow, which is one of the three northerly oceanic inflows to the North Sea. The East Shetland Atlantic Inflow has previously been discounted as of great importance to the hydrography and ecology of the North Sea. However, results from the hydrographic sections, current meter deployments and acoustic Doppler current profiler surveys reveal that the current is a persistent feature of North Sea circulation during the summer months. This inflow may form the western boundary of a cyclonic gyre in the bottom waters east of Shetland. When considering the ecological significance of the inflow it is evident that there have been substantial changes in the salinity of the northern North Sea, and of the adjacent oceanic waters during the past decades. A 20-year record of hydrographic parameters along the standard JONSIS section (59°17'N) suggests that these changes are related to decadal variability of the two western North Sea inflows; the Fair Isle Current and the ESAI. These changes are in turn related to changes in the oceanic waters at the entrance to the North Sea, as demonstrated by long-term time series generated from two standard sections in the Faroe Shetland Channel. It is directly via the ESAI, and indirectly via the Fair Isle Current, that oceanic variability is communicated to the North Sea.