Variability of advection and dispersion due to tides and wind forcing in a multiple-inlet coastal system

We present results on the Lagrangian transport of water parcels in a multiple-inlet coastal system: the Dutch Wadden Sea. We performed realistic hydrodynamic numerical simulations spanning 36 years, which are coupled to a Lagrangian module. The aim is to relate the characteristics of Lagrangian transport to the forcing. For this, the displacement of clouds of passive Lagrangian particles representing water columns is split into its advective and dispersive components, with the latter mainly due to chaotic tidal stirring. Advection is defined as the displacement vector of the center of mass of patches of passive particles, and the dispersion is quantified by the covariance matrix of their positions. Both quantities are determined for each tidal cycle spanning the duration of the simulations. Maps of advection reveal mostly large-scale features (basin-scale), whereas the spatial distribution of dispersion shows the predominance of localized structures covering mostly the regions around the inlets and strong bathymetric gradients. A strong correlation is found between the wind and the system-wide advection, showing a marked annual periodicity attributed to the seasonality of the wind forcing, which is stronger during winter and autumn. On the other hand, the magnitude of the tides (especially, due to the spring-neap cycle) governs the magnitude of the dispersion, i.e. mixing in the system. However, an enhancement in dispersion is observed during a few storms.