Generation of attached Langmuir circulations by a suspended macroalgal farm

Abstract:

In this study, a fine-scale LES model is used to explore how Langmuir turbulence in deep ocean evolves as it flows over and through a row-structured macroalgae farm. The ocean flow is driven by a constant wind stress and a geostrophic current, under the influences of surface gravity waves, planetary rotation, and stable interior stratification. The effects of Langmuir turbulence are accounted for by adding the Craik-Leibovich (CL) vortex force into the momentum equation without explicitly resolving the surface waves. Following a triple-decomposition technique widely, the turbulent transport is divided into contributions from mean flow, standing eddies, and transient fluctuations. We find out that the row structure of the macroalgal farm causes the cross-stream variation of the current that ultimately leads to the formation of coherent standing eddies via a mechanism similar to CL2 instability theory. Specifically, the vertical vortex lines associated with this cross-varying current are tilted by the Stokes drift, driving the formation of downstream vortices that are stationary in time, phase locked in space, and periodically alternating in sign across the lateral direction. Thus, we also refer to these coherent standing eddies as attached Langmuir circulations.

These standing eddies are unique to the upper OML in the presence of aquacultural farms since the cross-stream variation of the current is excited by the canopy. They are roughly oriented along the rows of canopy elements, which are aligned with the wind direction within the present numerical framework. The vertical extent of standing eddies can occupy the entire OML, with the lateral scale of the associated downwelling regions comparable to the row spacing in the farm. Because the associated upwelling motions are concentrated in regions occupied by macroalgae elements, these standing circulations are conducive to vertical mixing and could increase nutrient availability within macroalgae farm environments. The strength of standing eddies is much larger under the effect of Stokes drift associated with the surface waves compared to the pure shear-driven scenario. The findings presented here are relevant to realistic practice, and could serve as guidance for the design of large scale macroalgae systems.