Relative Dispersion of Surface Drifters in the North Sea: The Effect of Tides on Mesoscale Diffusivity. Issue 8 (31st July 2020)

• Record Type: Journal Article
• Authors: Meyerjürgens, Jens; Ricker, Marcel; Schakau, Vanessa; Badewien, Thomas H.; Stanev, Emil V.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 8(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-07-31
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015925
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: We examine the relative dispersion and the contribution of tides on the relative diffusivities of surface drifters in the North Sea. The drifters are released in two clusters, yielding 43 pairs, in the vicinity of a tidal mixing front in the German Bight, which is located in the southeastern area of the North Sea. Both clusters indicate decreasing dispersion when crossing the tidal mixing front, followed by exponentially increasing dispersion with e‐folding times of 0.5 days for Cluster 1 and 0.3 days for Cluster 2. A transition of the dispersion regimes is observed at scales of the order of the Rossby radius of deformation (10 km). After that, the relative dispersion grows with a power‐law dependency with a short period of ballistic dispersion (quadratic growth), followed by a Richardson regime (cubic growth) in the final phase. Scale‐dependent metrics such as the relative diffusivities are consistent with these findings, while the analysis of the finite‐scale Lyapunov exponents (FSLEs) shows contradictory results for the submesoscales. In summary, the analysis of various statistical Lagrangian metrics suggests that tracer stirring at the submesoscales is nonlocal and becomes local at separation scales larger than 10 km. The analysis of meridional and zonal dispersion components indicates anisotropic dispersion at the submesoscales, which changes into isotropic dispersion on the mesoscales. Spectral analysis of the relative diffusivity gives evidence that semidiurnal and shallow‐water tides influence relative diffusivity at the mesoscales, especially for drifter separations above 50 km. Plain Language Summary: The investigation of the transport and dispersal of biogeochemical substances and pollutants is of crucial importance for a broad spectrum of scientific research questions. To determine the risk of contaminants at the ocean's surface, numerical modeling frameworks are used for the prediction of tracer distribution on different spatial and temporal scales. Especially in tidally influenced shallow shelf seas, complex coastal processes hamper traditional particle modeling approaches, and observational data from surface drifters are required for calibration purposes. In this study, we estimate spreading rates from two clusters of surface drifters released in the southern North Sea, a shelf sea strongly affected by anthropogenic pressures. We provide the first estimate of spreading rates in this ecologically and industrially important area on different spatial and temporal scales. We further demonstrate from our drifter observations that tides influence diffusivity rates and provide a detailed analysis of tidal influence at different spatial scales. Key Points: Statistical Lagrangian metrics were derived from two clusters of surface drifters to analyze drifter dispersion in the North Sea Tracer stirring at the submesoscales is nonlocal and becomes local at the mesoscales Semidiurnal and shallow‐water tides influence the relative diffusivities at the mesoscales