Influence of Langmuir Circulations on the Intensity of Turbulent Mixing in the Near-Surface Layer of the Sea

Abstract

The purpose of the article is to describe an experimental study of the effect of Langmuir circulations (CL) on surface turbulence. A technique has been developed for determining the main dynamic characteristics of the CL using instrumental measurements of the flow velocity components with an ADCP DVS-6000 acoustic meter, background horizontal currents using the «Vostok-M» complex, and turbulent fluctuations of hydrophysical quantities recorded by the «Sigma-1» complex. The velocities of descending and ascending flows in the zones of convergence and divergence in coherent Langmuir structures are determined. Based on synchronous measurements of the velocities of currents and turbulent pulsations, the velocity of the transverse displacement of the Langmuir bands in various hydrometeorological conditions is calculated using correlation coefficients. To assess the influence of the studied CL on turbulent exchange, the dissipation rate of turbulent energy $$\varepsilon$$ was calculated. The calculation of ε was carried out from the pulsation components of the current velocity vector using the hypotheses of Kolmogorov and Taylor. The visualization of the Langmuir stripes for photographic and video filming was achieved by spreading paper markers on the sea surface. With a steady wind of 7–17 m/s, markers and algae in the water, sea foam, etc. lined up in clearly visible bands, which made it possible to determine the spatial scales of the CL. To estimate the distribution of the rate of dissipation of turbulent energy over depth, measurements of turbulent characteristics by the «Sigma-1» complex were carried out at several horizons from 1 to 7 m with an exposure time of 15–20 min. All measurements were accompanied by registration of background hydrometeorological conditions (surface wave parameters, wind speed and direction, water and air temperature, etc.). During 9 days of active observations, 74 Langmuir cells were detected. To assess the dynamics of the CL, the following parameters were chosen: the velocity of displacement of structures perpendicular to the direction of the wind, the number of observed cells for the selected observation period; width of convergence and divergence zones and vertical velocities in them. When analyzing the dynamic characteristics of the CL and comparing it with the intensity of turbulence, it was shown that the effect of the CL on turbulence in the near-surface layer depends primarily on the wind speed and direction and on their variability. The conditions under which CLs have a noticeable effect on the turbulent regime are determined. In experiments, the mode was noted when the highest intensity of turbulence from the CL occurs at a wind of about 5 m/s, while the wind direction must be stable, changing by no more than 10°–15° for 10 min. The maximum speed in downdrafts ~ 5–32 mm/s was observed with small changes in wind direction at a speed of 3.7 m/s, in the range 5º–12º. With a sharp change in wind direction by 20º–150º, the number of observed cells and the speed in the convergence zones decrease every 10–40 min. In this case, the dissipation rate becomes much lower, $$\varepsilon \sim \left( {1.7 - 8.89} \right) \times 10^{ - 6} \,{\text{m}}^{2} /{\text{s}}^{3}$$ . The developed technique for registering CLs and taking into account their influence on turbulent mixing has shown sufficient efficiency. The results obtained provide new useful information about the role of CL in near-surface turbulence.