Atmospherically Driven Seasonal and Interannual Variability in the Lagrangian Transport Time Scales of a Multiple-Inlet Coastal System

Intense short-term wind events can flush multiple-inlet systems and even renew the water entirely. Nonetheless, little is known about the effect of wind variations at seasonal and interannual scales on the flushing of such systems. Here, we computed two Lagrangian transport time scales (LTTS), the residence and exposure times, for a multiple-inlet system (the Dutch Wadden Sea) over 36 years using a realistic numerical model simulation. Our results reveal pronounced seasonal and interannual variability in both system-wide LTTS. The seasonality of the LTTS is strongly anti-correlated to the wind energy from the prevailing directions, which are from the southwesterly quadrant and coincidentally aligned with the geographical orientation of the system. This wind energy, which is stronger in autumn-winter than in spring-summer, triggers strong flushing (and hence low values of the LTTS) during autumn-winter. The North Atlantic Oscillation (NAO) and the Scandinavia Pattern (SCAN) are shown to be the main drivers of interannual variability in the local wind and, ultimately, in both LTTS. However, this coupling is much more efficient during autumn-winter when these patterns show larger values and variations. During these seasons, a positive NAO and a negative SCAN induce stronger winds in the prevailing directions, enhancing the flushing efficiency of the system. The opposite happens during positive SCAN and negative NAO, when weaker flushing during autumn-winter is observed. Thus, large-scale atmospheric patterns strongly affect the interannual variability in flushing and are potential drivers of the long-term ecology and functioning of multiple-inlet systems.Plain Language Summary In multiple-inlet coastal systems, strong wind events efficiently renew the water in these systems. In this paper, we investigate if the flushing of such systems has also a marked response to wind variability at longer time scales. To quantify the flushing, we compute the time that particles spend in the system before leaving it (known as the residence time), and the total time they spend within it considering future returns (known as the exposure time). Our 36-year simulation of the hydrodynamics of the Dutch Wadden Sea (DWS) shows that the wind induces seasonal and interannual variations in both spatially-averaged quantities. The seasonality is related to the wind energy from the dominant directions, which is much larger during autumn-winter than during spring-summer. This variation leads to a reduction of both time scales by, on average, a factor of 1.7 from spring-summer to autumn-winter. Two well-known North Atlantic large-scale atmospheric patterns, primarily active during autumn-winter, induce interannual variations in the wind and consequently in both time scales. Thus, future changes in these patterns could strongly affect water transport and the ecology of the DWS. Similar situations are likely to occur in other multiple-inlet systems.