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S. A. Thorpe, M. J. Ulloa, D. Baldwin, and A. J. Hall

1. Introduction The use of upward-pointing sonar as a means to investigate the upper ocean has developed rapidly in the last 15 years. It has proved to be a useful means of collecting information about surface waves, particularly their breaking and the bubble clouds they create, surface currents, internal waves, and Langmuir circulation (see, e.g., Thorpe and Hall 1983 ; Zedel and Farmer 1991 ). Many of the observations, however, are restricted by the lack of a system with independent power

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Jerome Smith, Robert Pinkel, and Robert A. Weller

suggest Langmuir circulation. Six Doppler sonars were mounted on FL1P's hull. One was directed such that the beam grazed the sea surface.Surface velocities were measured in the crosswind direction over 600 to 1400 m from FLIP. During a periodof strong Langmuir circulation, the sonar detected crosswind surface convergences at scales up to about 3 timesthe mixed layer depth. This ratio of maximum spacing to mixed layer depth stayed roughly constant as themixed layer depth varied from 40 to 60 m. Peak

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S. A. Thorpe

exist. Turbulence in boundary layers often contains coherent structures that persist for short periods of time. These are frequently of a vortical nature; examples are Kelvin–Helmholtz billows, sometimes associated with temperature ramps, and the pattern of alternately counterrotating circulating vortices with horizontal axes known as Langmuir circulation (see Thorpe 1995 ). Evidence of the coexistence of internal waves and Langmuir circulation during a period of strong winds is provided by Smith

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Øyvind Breivik, Peter A. E. M. Janssen, and Jean-Raymond Bidlot

), although it is somewhat unclear whether this effect will be strong enough to explain the observed Langmuir circulation. The turbulent kinetic energy (TKE) equation with a Stokes drift shear term can be written as Here is the TKE per unit mass (with q the turbulent velocity), ρ w is the water density, and ϵ is the dissipation [see, e.g., Stull 1988 , p. 152]. The term involving the Reynolds stresses multiplied by the gradient in Stokes drift velocity υ i represents the production of Langmuir

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Wu-ting Tsai, Guan-hung Lu, Jheng-rong Chen, Albert Dai, and William R. C. Phillips

finescale filaments, apparently associated with turbulence production by wind shear. We view these vortices as akin to Langmuir circulation ( Langmuir 1938 ), which are wind-aligned, counterrotating rolls beneath wind-driven surface waves. Fig . 1. Comparison between (a) wind-driven ( Tsai et al. 2013 ) and (b) free-propagating surface waves ( Tsai et al. 2015 ). (top) Numerical simulations showing the prospective views of surface elevations and temperature distributions and (bottom) streamwise

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Ramsey R. Harcourt and Eric A. D’Asaro

. 1994 ). These models ignore any surface wave effects and set the level of TKE to that found in solid-wall boundary layers. In contrast, observations ( D’Asaro 2001 ; Tseng and D’Asaro 2004 ) show that the vertical turbulent kinetic energy (VKE) magnitude is higher in the ocean boundary layer than in solid-wall boundary layers with the same applied stress. We hypothesize that Langmuir circulations, generated through the Craik–Leibovich (CL) mechanism ( Craik and Leibovich 1976 ), can quantitatively

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Martín S. Hoecker-Martínez, William D. Smyth, and Eric D. Skyllingstad

et al. 1997 ; Wang and Müller 2002 ; Harcourt and D’Asaro 2008 ; Grant and Belcher 2009 ; Noh et al. 2011 ) and empirical ( Kukulka et al. 2009 , 2010 ) forcing have contributed to the general understanding of upper-ocean physics. LES of shallow mixed layers driven by strong wind forcing show that turbulence near the surface is driven mainly by shear associated with the Stokes drift of the surface waves (Langmuir turbulence; McWilliams et al. 1997 ). Langmuir circulation transports momentum

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S. A. Thorpe, P. Bowyer, and D. K. Woolf

bubbles are investigated using numerical models. The input of bubbles, either at aconstant rate in a "steady-state" model or in an initial injection where the development of a bubble "plume"is followed, is kept constant. So too are the model representations of Langmuir circulation and turbulence. Anincrease in temperature results in a reduction of bubble numbers, a halving at 4-m depth for a 10-C rise intemperature, while an increase in saturation level of 10% increases the bubble concentrations by

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Tomas Chor, James C. McWilliams, and Marcelo Chamecki

a shows the normalized mean turbulent flux profile for an SFT from a convective regime, which will be detailed in section 3 . SFTs are known to be effectively transported by large convective plumes ( Kaimal et al. 1976 ; Wyngaard and Brost 1984 ; Moeng and Sullivan 1994 ), which is sketched in Fig. 1a as red arrows. In the case of wavy oceanic regimes SFTs are also thought to be efficiently transported vertically by Langmuir circulations. Fig . 1. Mean turbulent flux profiles for (a) an

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Jan Erik Weber

value of the undulating Stokes drift has a minimum at these nodal points if the anglebetween the crossing waves is less than 76.4% If this angle is larger than 76.4-, the Stokes drift at the surfacehas a maximum here. The roll motion described in the present paper is discussed in connection with thebasis for the recent theoretical development of Langmuir circulations. Finally, a solution for the steady,horizontally averaged drift current in a rotating ocean is presented.1. Introduction Streak

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