Lagrangian surface drifter observations in the North Sea: An overview on high resolution tidal dynamics and submesoscale surface currents

Abstract. A dataset of 85 Lagrangian surface drifter trajectories covering the central North Sea area and the Skagerrak from 2017–2021 of 17 deployments is presented. The data have been quality controlled, uniformly structured and assimilated in a standard NetCDF format. Using appropriate methods presented in detail here, surface currents were calculated from the drifter position data and gridded surface current maps with 0.125° spatial resolution were derived. The maps present for the first time mean currents of the south-eastern North Sea and the Skagerrak from Lagrangian observations. Tidal energy spectra were analyzed separately for the southern and northern areas of the North Sea, and tidal ellipses were calculated to determine the tidal impact on surface currents. Significant differences between the shallow shelf and the deeper areas of the North Sea are evident. While the shallow nearshore areas are dominated by tidal currents, deeper areas such as the Skagerrak register a high mean residual circulation driven by high density gradients.

Measurements using Eulerian approaches and remote sensing methods are restricted in temporal and spatial coverage, in particular, to capture submesoscale dynamics. For this reason, Lagrangian measurements, to a large extent, provide new insights in the complex submesoscale dynamics of the North Sea. Exemplarily, the Skagerrak region is used to illustrate the ability of reconstructing mesoscale and submesoscale current patterns using drifter observations.

This unique dataset covering the entire south-eastern North Sea and the Skagerrak offers further analysis possibilities and can be used for the investigation of various hydrodynamic and environmental issues, e.g., the analysis of submesoscale current dynamics at ocean fronts, the determination of the kinetic eddy energy and the propagation of pollutants in the North Sea.