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Langmuir turbulence in a depth-varying coastal channel: Insights from large eddy simulations
Summary
Scientists studied how ocean waves create spinning water currents in coastal areas like bays and rivers, which help mix pollution and tiny plastic particles throughout the water. These wave-driven currents are much stronger than previously thought and significantly change how pollutants like microplastics spread through coastal waters where people swim, fish, and get drinking water. Understanding these mixing patterns is important for predicting where ocean pollution ends up and how it might affect human health.
This study investigates wave-driven Langmuir turbulence (LT) in an idealized, depth-varying coastal channel representative of an estuarine bay or tidal river. In the open ocean, LT is a key turbulent process in the surface boundary layer, controlling the transport and mixing of momentum and density. LT arises from wave-current interactions that generate wind-aligned vortices, often visible as surface windrows of aggregated buoyant material such as plankton, bubbles, oil, and microplastics. To examine how LT influences the wind-, tide-, and density-driven circulation in a coastal channel, we develop a turbulence-resolving large eddy simulation (LES) framework with terrain-following coordinates representing a deeper central channel flanked by shallower margins. LT is generated through the Craik-Leibovich (CL) vortex force, which incorporates Stokes drift from wind-driven surface gravity waves. The simulations show that LT substantially enhances turbulent mixing, reducing vertical stratification and shear. Faster tidal currents in the deeper channel differentially advect salt, producing tidally varying lateral salinity gradients. These gradients generate baroclinically driven lateral and vertical tidal currents, whose development is both accelerated and intensified by LT. Conversely, vertical stratification and vertical shear of lateral currents can inhibit LT. Additionally, lateral shear of along‑channel currents associated with the channel bathymetry produces channel‑wide pairs of vertical vorticity that are tilted by Stokes‑drift shear, forming strong and persistent lateral circulations. Overall, the results reveal complex two‑way interactions between LT and the mean circulation, demonstrating that LT significantly modifies both tidally resolved and tidally averaged channel dynamics.