Shelf-wide circulation impacts the flushing time of coastal bays

Once exiting an estuary into the shelf, transport and retention of materials are subject to the shelf dynamics, and sometimes the deep ocean dynamics. However, the impact of the deep ocean is rarely considered in previous coastal modeling studies, as coastal models typically have a fine resolution only for the coastal region and the domain rarely extends beyond the shelf (depth <200 m). This study demonstrates the role of deep and shelf ocean circulation on the flushing of estuarine bays. With a cross-scale and well-calibrated ocean model for the northwestern Gulf of Mexico (Coarse Small Model) and another one for the entire Gulf of Mexico (Refined Large Model), we examine the flushing time for Galveston Bay through Lagrangian particle-tracking simulations. Both models have similar results regarding salinity and currents near the coast, but Coarse Small Model persistently overestimates/underestimates the flushing time during winter/summer, respectively, compared to Refined Large Model. Analysis of sea surface height and geostrophic currents suggests that Coarse Small Model’s inability to capture the deep ocean synoptic circulations leads to the overestimations of estuarine materials’ retention on the inner shelf and unrealistic flushing time for coastal bays during winter. By increasing the resolution in the deep Gulf from 10 to 5 km, Refined Small Model produces results similar to Refined Large Model. This study highlights the role of shelf and deep ocean dynamics on exchange between estuarine bays and coastal ocean and emphasizes the importance of resolving the shelf-wide dynamics in models focusing on estuarine and coastal waters.