Langmuir turbulence in shallow water. Part 1. Observations

Abstract

During extended deployment at an ocean observatory off the coast of New Jersey, a bottom-mounted five-beam acoustic Doppler current profiler measured large-scale velocity structures that we interpret as Langmuir circulations filling the entire water column. These circulations are the large-eddy structures of wind-wave-driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing. Many observational characteristics agree with former descriptions of Langmuir circulations in deep water. The three-dimensional velocity field reveals quasi-organized structures consisting of pairs of surface-intensified counter-rotating vortices, aligned approximately downwind. Maximum downward velocities are stronger than upward velocities, and the downwelling region of each cell, defined as a pair of vortices, is narrower than the upwelling region. Maximum downward vertical velocity occurs at or above mid-depth, and scales approximately with wind speed. The estimated crosswind scale of cells is roughly 3–6 times their vertical scale, set under these conditions by water depth. The long axis of the cells appears to lie at an angle ∼10°–20° to the right of the wind. A major difference from deep-water observations is strong near-bottom intensification of the downwind ‘jets’ found typically centred over downwelling regions. Accessible observational features such as cell morphology and profiles of mean velocities, turbulent velocity variances, and shear stress components are compared with the results of associated large-eddy simulations (reported in Part 2) of shallow water flows driven by surface stress and the Craik–Leibovich vortex forcing generally used to represent generation of Langmuir cells. A particularly sensitive diagnostic for identification of Langmuir circulations as the energy-containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor, plotted in the Lumley ‘triangle’ corresponding to realizable turbulent flows. When Langmuir structures are present in the observations, the Lumley map is distinctly different from that of surface-stress-driven Couette flow, again in agreement with the large-eddy simulations (LES). Unlike the LES, observed velocity fields contain two distinct and significant scales of variability, documented by wavelet analysis of observational records of vertical velocity. Variability with periods of many minutes is that expected from Langmuir cells drifting past the instrument at the slowly time-varying crosswind velocity. Shorter period variability, of the order of 1–2 min, has roughly the observed periodicity of surface wave groups, suggesting a connection with the wave groups themselves and/or the wave breaking associated with them in high wind conditions.

Surface Wave Effects on High-Frequency Currents over a Shelf Edge Bank

“…The bank is located at the edge of the Texas-Louisiana continental shelf and rises from depths of 100-150 m to about 20-m below the surface. When compared with previous observations from shallow (e.g., Gargett and Wells 2007) and deep water (e.g., Smith 1998), our measurements represent both shallow-and deep-water conditions. The vertical velocity fluctuations described here show strong correlations with winds and surface wave properties.…”

“…Coherent vortices in the mixed layer have been studied using Doppler sonar techniques (e.g., Zedel and Farmer 1991;Plueddemann et al 1996;Smith 1992Smith , 1998 and these investigators report the consistency of wind/ wave forcing of Langmuir circulation. In the existing literature on Langmuir circulation studies, the majority of the observations is from deep water environments and there are only a few references related to shallow-water environments (e.g., Hunter and Hill 1980;Marmorino et al 2005;Gargett and Wells 2007). In a shallow-water environment, the physical setup is different since top and bottom boundary layers can merge under wind and buoyancy forcing, and thus bottom frictional effects can become a significant factor in mixed layer dynamics.…”

“…In a shallow-water environment, the physical setup is different since top and bottom boundary layers can merge under wind and buoyancy forcing, and thus bottom frictional effects can become a significant factor in mixed layer dynamics. Gargett and Wells (2007) report that Langmuir circulation plays an important role in sediment transport in the shallow-water environment.…”

Transfer Across the Air-Sea Interface

“…The interaction between the mean particle drift of surface waves (Stokes drift) and wind-driven surface shear current generates Langmuir circulation consisting of counter-rotating vortices roughly parallel to the wind direction (Langmuir 1938). The Langmuir circulation can be seen at the surface by the collection of surface foam in meandering lines in the along-wind direction or by sub-surface observations, following bubbles trapped in downwelling regions between vortices and current profiles (Smith 1998;Thorpe 2004;Gargett and Wells 2007). McWilliams et al (1997) defined a turbulent Langmuir number describing the relative influence of directly wind-driven shear and Stokes drift:…”

A comparison of acoustic turbulence profiling techniques in the presence of waves

“…Wave contamination has either been avoided (e.g., Lohrmann et al 1990;Stacey et al 1999) or wave separation has been performed in an ad hoc basis depending on deployment conditions. Wave-turbulence separation can be performed in frequency space if conditions allow, e.g., Gargett and Wells (2007). Spectral separation can also be accomplished by identifying and interpolating beneath the wave peak.…”