Turbulence Reduction and the Langmuir Circulation

Yalin Fan, Naval Research Laboratory, Stennis Space Center, MS, United States, Zhitao Yu, US Naval Research Laboratory, Washington, DC, United States, Ivan Savelyev, U. S. Naval Research Laboratory, Remote Sensing Division, Washington, DC, United States, Peter P Sullivan, National Center for Atmospheric Research, Mesoscale Microscale Meteorology, Boulder, CO, United States, Junhong Liang, Louisiana State University, Baton Rouge, LA, United States, Tracy Haack, Naval Research Laboratory, Marine Meteorology Division, Monterey, CA, United States, Eric Terrill, University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, Tony de Paolo, Scripps Institution of Oceanography, La Jolla, CA, United States and Robert Kipp Shearman, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States
Abstract:
Langmuir turbulence (LT) is believed to be one of the leading causes of turbulent mixing in the upper ocean. Large eddy simulation (LES) models that solve the Craik–Leibovich equations are used to study LT in the upper ocean, yielding new insights that could not be obtained from field observations or turbulent closure models, most of these studies were conducted under idealized conditions with simplified oceanic and wind conditions. Idealizing and isolating individual processes makes it easier to study their effects, but can also unrealistically magnify their impact, due to the lack of complex and nonlinear interactions of multiple dynamical processes taking place in the real ocean.

In this study, we expand previous large eddy simulation (LES) modeling investigations of Langmuir turbulence (LT) to real ocean conditions using field observations collected under the multi-platform field campaign “Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research (CASPER-East)”. The measurement site has strong local variabilities of temperature and salinity and experienced large variations in wind forcing and several cooling events.

While LT enhances the turbulence in the water column and deepens the mixed layer during most of the simulation period, being consistent with previous studies, significant reduction in turbulent intensity is observed in the simulation with Stokes drift compared to that without Stokes drift during a one-day period, in contradiction to previous findings. Two main reasons are found to contribute to reducing the turbulence: the large misalignment between the wind and surface gravity waves and the interaction of LT with deep convection. The large wind-wave misalignment not only reduces the turbulence in the water column and traps it in a shallower surface layer, but also traps the momentum in shallower surface layer, thus producing large surface mean currents and consequently leads to further reduction of the turbulent intensity. During the cooling event, strong upwelling induced by LT at the base of the mixed layer counteracts on the downwelling associated with the deep convection and reduces the total turbulence level in the water column.