Langmuir Supercells Under the Influence of Tidal Forcing, Surface Buoyancy, and Misaligned Net Current, Winds, and Waves
Abstract
Langmuir circulation (LC) consists of pairs of parallel counter-rotating vortices (or cells) oriented approximately in the downwind direction, characterizing the wave- and wind-driven Langmuir turbulence within the upper ocean. In neutrally stratified inner shelf regions, the largest of the LC engulf the water column interacting with the bottom boundary layer and serving as a dominant mixing process heavily influencing sediment re-suspension and its lateral transport. Gargett et al. (2004) denoted observed full-depth cells as Langmuir supercells (LSC) because of their important role as vectors for the transport of sediment and bioactive material on shallow shelves. The quasi-organized nature of LSC makes them more effective than bottom boundary layer shear turbulence at moving material out of the low-speed layer near bottom and into the bulk flow for lateral transport. In the coastal ocean, LSC has been observed under tidal and geostrophic flow components and surface heat fluxes which serve to influence LSC strength and coherency, as measured through bottom-mounted acoustic Doppler current profilers (ADCP). Large-eddy simulations (LES) of LSC flows are proposed closely following ADCP measurements of LSC at two different locations on inner shelf regions along the eastern coast of the U.S. in order to elucidate the dynamics by which tidal and geostrophic flows, surface buoyancy and misaligned wind and waves combine to affect LSC and associated vertical transport. Preliminary simulations under idealized settings have shown that the crosswind component of the tidal and geostrophic flows generate bottom boundary layer turbulence disrupting the intensity and structure of the LSC. Furthermore, idealized LES has shown that surface cooling and surface heating can strengthen or weaken LSC, respectively, while misaligned wind and waves are expected to weaken the cells. The preliminary simulations have been performed by imposing a pressure gradient to drive the tidal flow. The planned LES for this project will be based on a new methodology recently developed by the PI. In the new approach, the LES will be forced via direct imposition of the tidal flow component measured in the field ensuring that the vertical shear of the tidal current generating turbulence in the LES is in close agreement with the tidal current vertical shear generating turbulence in the field. This approach provides a level platform to assess the LES-resolved turbulence via comparison with the turbulence measured in the field. The proposed LES will be used to obtain a scaling of vertical velocity fluctuations representative of the strength of non-local vertical mixing induced by LSC under the various processes affecting its strength and coherency. The scaling of vertical velocity fluctuations will be used to inform a KPP (K-profile parameterization) developed by the PI accounting for the local transport of the overall Langmuir turbulence and the non-local transport induced by the LSC. The updated LSC KPP will be implemented in the one-dimensional vertical water column General Ocean Turbulence Model (GOTM) and in the Regional Ocean Model System (ROMS) for validation via comparisons with the LES and the field measurements. Specifically, the LSC KPP within ROMS will be evaluated through the following two categories: a) the destruction of vertical stratification and b) the evolution of cross-shelf transport and resulting coastal setup/setdown, which balances the observed geostrophic alongshelf flow. Both of these processes depend on the 2 distribution of momentum through the vertical, as represented in the models, and as actually realized in the ocean.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 04, 2018
- Source ID
- N000141812837
Entities
People
- Andres Tejada-martinez
Organizations
- Office of Naval Research
- United States Navy
- University of South Florida