Lagrangian Methods for Visualizing and Assessing Frontal Dynamics of Floating Marine Litter with a Focus on Tidal Basins

Abstract

Lagrangian methods are a common tool in physical oceanography. Due to the quasi-Lagrangian characteristics of floating marine litter (FML) and the chemical substances released from it, Lagrangian methods can be used to study this environmental threat. Most of the existing investigations of this topic have been carried out in the deep ocean, where baroclinic dynamics dominate. In contrast, studies of tidally dominated, shallow regions are much fewer in number. Compared to the deep ocean, shallow shelves are more strongly influenced by freshwater inputs, bottom stress, complex coastlines, and wind, which imply higher diffusion rates, especially in the presence of tides. Furthermore, they steer the transport of FML from rivers to the deep ocean with fronts as an important driver. The present chapter reviews Lagrangian methods for visualizing and assessing frontal dynamics in tidal basins with data obtained from numerical modeling and satellite-tracked drifters. The specific requirements for the two data sources are described and discussed. Some of these methods are applied in the North Sea, located on the European northwest shelf, where tidal mixing fronts and fronts due to freshwater runoff exist. It is demonstrated how surface convergence and gradients in temperature, salinity, and density are connected with the accumulation of virtual and satellite-tracked drifters. The effect of tides on the propagation of Lagrangian particles is shown to be significant and demonstrates the importance of tidal forces and vertical dynamics in Lagrangian simulations in tidal basins. The chapter ends with the future outlook, illuminating the numerous knowledge gaps remaining and proposing areas for future research.