Rapid mixed layer depening by the combination of Langmuir and shear instabilities : a case study
Plueddemann, Albert J.
Trowbridge, John H.
Sullivan, Peter P.
MetadataShow full item record
KeywordMixed layer; Shear structure/flows; Wind effects; Turbulence; Thermocline; Internal waves; Advection
Langmuir circulation (LC) is a turbulent upper-ocean process driven by wind and surface waves that contributes significantly to the transport of momentum, heat, and mass in the oceanic surface layer. The authors have previously performed a direct comparison of large-eddy simulations and observations of the upper-ocean response to a wind event with rapid mixed layer deepening. The evolution of simulated crosswind velocity variance and spatial scales, as well as mixed layer deepening, was only consistent with observations if LC effects are included in the model. Based on an analysis of these validated simulations, in this study the fundamental differences in mixing between purely shear-driven turbulence and turbulence with LC are identified. In the former case, turbulent kinetic energy (TKE) production due to shear instabilities is largest near the surface, gradually decreasing to zero near the base of the mixed layer. This stands in contrast to the LC case in which at middepth range TKE production can be dominated by Stokes drift shear. Furthermore, the Eulerian mean vertical shear peaks near the base of the mixed layer so that TKE production by mean shear flow is elevated there. LC transports horizontal momentum efficiently downward leading to an along-wind velocity jet below LC downwelling regions at the base of the mixed layer. Locally enhanced vertical shear instabilities as a result of this jet efficiently erode the thermocline. In turn, enhanced breaking internal waves inject cold deep water into the mixed layer, where LC currents transport temperature perturbation advectively. Thus, LC and locally generated shear instabilities work intimately together to facilitate strongly the mixed layer deepening process.
Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 2381-2400, doi:10.1175/2010JPO4403.1.
Showing items related by title, author, creator and subject.
Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons Majumder, Sudip; Tandon, Amit; Rudnick, Daniel L.; Farrar, J. Thomas (John Wiley & Sons, 2015-09-26)We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26° N in the Arabian Sea during ...
Montesi, Laurent G. J. (American Geophysical Union, 2007-04-27)The microstructure of ductile shear zones differs from that of surrounding wall rocks. In particular, compositional layering is a hallmark of shear zones. As layered rocks are weaker than their isotropic protolith when ...
White, Brian L.; Nepf, Heidi M. (Cambridge University Press, 2007-11-23)Results are presented from an experimental study of shallow flow in a channel partially obstructed by an array of circular cylinders. The cylinder array is a model for emergent vegetation in an open channel, but also ...