CT data 2023-2024 NoSE - Norwegian Trench Version 1

The Norwegian Trench (NT) is the main transport pathway for sediments, nutrients and coastal waters of the North Sea into the Atlantic Ocean. Observations of the deep currents, near the bed, are crucial to understand the transport system of the NT. Here, we therefore present unique observations of one year near-bed current velocity measurements. The in-situ data was recorded from June 2023 to May 2024 with two moored ADCPs at 270 m and 370 m depth in the middle of the NT and close to the Atlantic frontier, respectively. Tides explain 19 % of the total variance in the flow at L2, dominated by the semi-diurnal tides, whereas at the frontier tide only explain 10 % of the total variance, dominated by low-frequency and diurnal tides. Furthermore, we can relate sub-inertial frequency oscillations with periods of 5 and 9-13 days to Coastal Trapped Waves which intensify the flow speed and enable AW inflows into the NT in winter. Monthly variability in the velocity field is forced by large scale wind systems in both timeseries. However, we observe that the flows in the middle of the NT are more meteorologically driven, than at the Atlantic frontier. Especially over winter time when westerly winds impact the North Sea circulation, we capture an intensification of flows and high suspended matter concentrations from backscatter. We observe that the variability of the deep currents is much larger than previously expected on different timescales, and show that the forcing mechanisms of the cross-shelf exchange vary spatially.