Impact of Islands on Tidally Dominated River Plumes: A High‐Resolution Modeling Study.

When flow passes over topographic features such as headlands and islands, island wakes can arise at the lee side of the flow. Island wakes are associated with enhanced biological productivity, increased mixing, and water mass transformation. While previous studies have mainly focused on the dynamical and biological effects of island wakes in the open ocean, here we focus on a large tidally‐dominated estuary with numerous islands, aiming to investigate the impact of such wakes on the offshore transport of river plumes. To this end, we use numerical simulations with unprecedented grid resolution in the plume region and around the islands. Our study area is the Pearl River Estuary, a region where satellite images indicate that oscillating wakes occur in the lee and far downstream of the islands. We show that submesoscale island wakes are ubiquitous in the plume‐influenced region and can affect a large area around the islands as the tidal flow reverses. These strong vorticity tails correspond well with the horizontal patterns of salinity gradients and salinity mixing. Sensitivity experiments show that these flow disturbances will largely decrease after the hypothetical "removal" of the islands. Analysis based on an isohaline coordinate framework shows that the isohaline surface area is limited by the presence of islands. It is proven that this "limiting" effect of islands on the plume extension is related to the salinity mixing and the associated diahaline water exchange. Plain Language Summary: It matters how far the fresh water from the river can reach because it usually carries nutrients and sediments into the ocean. When this thin layer of fresh water encounters islands in the coastal area, water exchange between fresher and saltier water will occur. Here, we use a numerical model in the Pearl River Estuary to test the effects of islands on the rate of water exchange and the extent of freshwater coverage. We found that the extent of freshwater coverage is limited due to the presence of islands. This is because the islands accelerate the water exchange not only in the neighboring water around the islands but also in the island wakes, which extend several island diameters away from the islands. In an alternative numerical experiment in which all the small islands are "removed," the flow disturbances will be largely decreased. Since the total estuarine mixing is controlled by the upstream runoff, the area covered by the isohaline surface needs to be increased to maintain the same amount of the total mixing in the "No Island" case. Key Points: High‐resolution simulations indicate that mixing around islands in river plumes is limiting offshore freshwater transportA large part of the mixing is related to submesoscale island wakesIsohaline analysis suggests that after the removal of the islands, the same amount of mixing will be redistributed on the isohaline surface [ABSTRACT FROM AUTHOR]