Taming turbulence closure in tidally driven simulations of coastal oceans and estuaries

A widespread prevalence of subsurface acoustic ducts impacting mid-frequency sound propagation was observed over the outer shelf and the continental slope during a field experiment in July-August 2022 in the Pacific Northwest coastal ocean of North America. Simulations of the coastal shelf ocean using LiveOcean, a tidally driven operational model ( MacCready et al. , 2021 ), based upon a widely used variant of the Regional Ocean Modeling System (ROMS), were compared with observations of the thermohaline stratification layers responsible for the ducts, and found to have a nearly complete absence of these acoustic features due to excessive parametrized mixing. After implementing additional realistic constraints in the ‘k-ε‘ second moment closure (SMC) to control instabilities in the turbulence mixing model with low background mixing, the source of instabilities was identified in a coding error for the default, third-order upstream advection of the turbulence parameters for TKE k and its dissipation ε, a longstanding and significant bug impacting mixing parametrization, and one also found in the older SMC ‘Mellor-Yamada 2.5’ mixing parametrization option in ROMS. With code improvements, LiveOcean was able to successfully simulate the production of observed subsurface acoustic ducts. The primary process for generating the ducts along the outer shelf involves the southward transport of low sound speed water during upwelling, combined with the cross-shelf displacement of higher sound speed water from offshore beneath this layer in bottom-driven Ekman transport. • Coastal ocean features impacting acoustics measured intensively • Regional ocean model code improved by correction of longstanding errors • Simulation of observed subsurface acoustic ducts elucidates their formation