On the wind-driven European shelf sea-level variability and the associated oceanic circulation

The shelf to the west of Ireland, France and the United Kingdom is a region where currents and sea level respond to the wind activity in a remarkable manner throughout a range of timescales. Using altimetry-obtained measurements and a wind reanalysis, we demonstrate in the present contribution how the sub-annual sea-level variability can be understood as a response to the wind action. The winds drive water towards (away from) the coastline through Ekman transport, yielding sea-level changes coherent along and across the shelf and with maximum amplitude at the coast. The alignment of the winds with the isobaths determines the magnitude of sea-level changes. To investigate the impacts of these changes on the circulation variability, we bring together a comprehensive dataset of 30+ in-situ observations of recent current changes. Using these measurements, we show that sub-annual changes in the shelf-edge circulation from the Goban Spur to the Faroe-Shetland Channel arise from the geostrophic adjustment to shelf sea-level variations induced by the Ekman-driven accumulation of water towards the coastline. Our analysis suggests that the along-isobath current generated through this mechanism are primarily found over the shelf, only impinge on the upper slope, and do not affect the circulation above greater depth (>500 m). Nonetheless, important slope circulations such as the Rockall Slope Current are substantially influenced on their shoreward side by this simple geostrophic adjustment process. Because sea-level changes co-vary over large distances on the shelf, there also is remarkable along-isobath coherence in the associated current changes but we warn against concluding this is evidence for the continuity of an ‘European Slope Current’ circumnavigating the European slope from Portugal to Norway.