Observed growth of Langmuir circulation

Abstract

Surface velocity patterns and upper ocean density profiles are presented from a period following a sudden increase in wind. A prior wind speed of 8 m/s failed to produce visible signs of Langmuir circulation. After the wind speed increased to 13 m/s, Langmuir circulation developed within 15 min. The initial scale observed was about 16 m, streak to streak, and may have been restricted by the depth scale of the measurements. This spacing is about two thirds of the dominant wavelength (4-s-period waves). The streak spacing (two cell widths) increased at roughly 40 m/h for the next hour. The density measurements indicate a depth of mixing which increased over the same period at a rate of 20 m/h; thus the vertical to horizontal aspect ratio was about 1:1. Prior stratification was weak (buoyancy frequency of about 1.5 cph), and probably did not affect the circulation initially.

Observations and Theories of Langmuir Circulation: A Story of Mixing

“…In MBLEX, a newer system (PADS) was used to image a continuous area 35° in bearing by 450 m in range (figure 4). Details concerning the former are found in Smith (1992) and concerning the latter in Smith (1998). To estimate time series of the velocity associated with Langmuir circulation, data averaged over 1 to 3 minutes were employed.…”

“…It was soon realized that velocities could be estimated from the Doppler shift of the backscattered sound signal, producing quantitative estimates of the near-surface velocity field associated with wind-driven convection (Smith et al 1987). Surface-scattering acoustic intensity and Doppler data both show evidence of spacing proportional to 2 to 3 times the mixed layer depth, particularly as the mixed layer deepens (Smith 1992). It is also observed that the convergences, as delineated by lines of bubble clouds, can merge into "Y-junctions (Thorpe 1992b, Thorpe 1992a.…”

Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

“…The instrument uses conventional ADCP electronics but has a modified transducer head that creates four narrow-azimuth beams (38) spaced 308 apart in the horizontal plane. These beams are broad in elevation (248), intersect the sea surface at a shallow angle, and have an intensity-weighted return that is dominated by scattering in the upper 1-3 m when bubbles injected by breaking waves are sufficiently strong (Crawford and Farmer 1987;Smith 1992). Standard range gating produces successive sampling cells along the sea surface with dimensions of about 2.5 m (along beam) 3 5 m (cross beam).…”

Observations of coherent bands of algae in a surface shear layer

“…Since then, many theories have been advanced to explain the formation of the helical vortices now known as Langmuir circulations (reviewed by Pollard 1977;Leibovich 1983). Although it has been > 50 yrs since Langmuir's original paper, the direct observational picture of natural Langmuir circulations remains incomplete (Kenney 1977;Thorpe and Hall 1982;Weller and Price 1988;Smith 1992).…”