Lagrangian transport in a multiple-inlet coastal system: the difference between using 3D and depth-averaged currents

Transport time scales in estuaries quantify the flushing of the system and are relevant quantities that help to monitor their local and system-wide functioning. Here, we elucidate and quantify the main differences on transport time scales and other related statistics when using passive particles advected by either 3D or depth-averaged currents in an estuarine system. We further relate these differences to the forcing, in particular, wind and freshwater discharge. The analysis is made in the Dutch Wadden Sea: an estuarine system of intertidal basins. Due to computational constraints when using high-resolution 3D currents, we consider two years with contrasting forcing conditions, which were selected after analysis of results based on depth-averaged currents over 36 years (1980-2015). During periods with strong southwesterly winds, mainly related to the stormy seasons (autumn-winter), similar values for the system-wide residence time are found when using 3D or depth-averaged currents. During periods with weak winds, mainly occurring during spring-summer, the residence time computed from depth-averaged currents overestimates the values from 3D currents up to 5-10 days. This was also the case for the anomalous winter of 1996, which is a famous period in the North Sea region due to its low temperatures and lack of strong southwesterly winds. During these weak-wind periods, the difference in the residence is positively correlated to the freshwater discharge. However, despite these observed differences, the temporal variability of the system-wide residence time in the Dutch Wadden Sea is well captured when using depth-averaged currents, i.e., larger residence time during weak wind conditions and smaller during strong wind conditions are observed when using either 3D or depth-averaged currents.