## 1. Introduction

[2] By its very nature, turbulence in geophysical flows is highly intermittent in space and time. Turbulence characteristics such as the kinetic energy *e*_{tr}, its dissipation ɛ, eddy diffusivities *K*, scalar dissipation *χ*, turbulent scales *L*_{tr} are subjected to sharp variations with typical spatial scales of tens/hundreds meters vertically/horizontally and temporal scales ranging from minutes to hours. Such mesoscale inhomogeneity of hydrophysical fields is called “external or outer intermittency,” and is associated with variations of mean fields, patchiness of turbulent regions and presence of interfaces that separate turbulent and nonturbulent regions [*Sreenivasan*, 2004]. Conversely, the small-scale, fine-scale intermittency of turbulence or internal intermittency occurs at spatial scales from meters to millimeters, and is usually confined within turbulent regions (layers, patches, wakes, plumes, etc.). It is attributed to random inhomogeneous spatial distribution of vortex filaments within turbulent regions, where they stretch and dissipate energy in isolation [*Kuo and Corrsin*, 1971].

[3] Mesoscale intermittency is a phenomenon characteristic of turbulent mixing in oceans, seas, large lakes and reservoirs, which may not be observed in other turbulent flows. The measured vertical and horizontal sizes of turbulent zones in the upper ocean are subjected to specific statistical regularities. Their probability distributions appear to be approximately lognormal while the distances between turbulent regions follow a double exponential distribution [*Lozovatsky et al.*, 1993; *Pozdinin*, 2002]. Internal intermittency, however, is inherent to any high Reynolds number turbulent flow [*Monin and Yaglom*, 1975] due to its inhomogeneous microstructure.

[4] Internal intermittency of turbulence in oceans and lakes affects processes at the scales of inertial-convective and diffusive subranges [*Tennekes and Lumley*, 1972]. Among them are the viscous dissipation of energy, biochemical processes (planktonic mating, predator-prey contacts, chemical reactions [*Seuront and Schmitt*, 2005]), thermal convection and redistribution of salinity concentration or multi/double diffusive convective fluxes [*Sánchez and Roget*, 2007]. The influence of small-scale turbulent fluctuations on the propagation of light and sound in the ocean is an important problem for various applications of ocean optics and acoustics [*Tyson*, 1991; *Colosi et al.*, 1999].

[5] In application to aquatic ecosystems, turbulent oscillations of various scales influence aggregation, incubation and foraging processes of small-scale planktonic organisms [*Druet*, 2003]. Internal intermittency can affect phyto and zooplankton species less than several millimeters in size [*Peters and Marrasé*, 2000], specifically, floating microscopic algae that are responsible for photosynthesis in coastal oceans [*Margalef*, 1985, 1997]. Zooplankton larger than ∼1 cm usually do not react to small-scale intermittency of turbulence [*Squires and Yamazaki*, 1995, 1996].

[6] Intermittency of biochemical (plankton, nitrites) and scalar (fluorescence concentration, temperature) variables have been studied by Seuront and coauthors in a series of papers [e.g., *Seuront et al.*, 1999, 2001, 2002; *Seuront and Schmitt*, 2005]. In particular, it was found that phytoplankton patchiness substantially increased the predator-prey encounter rates, but the encounter was much less influenced by turbulence when ɛ was considered as an intermittent variable rather than a mean value [*Seuront et al.*, 2001]. The patchiness of small-scale phytoplankton distribution in a tidal current (the Eastern English Channel) increased with decreasing turbulence intensity [*Seuront and Schmitt*, 2005] and it varied depending on the phase of tidal cycle. This finding is directly consistent with the present results.

[7] The first results on intermittency of ocean turbulence at scales of inertial-convective subrange, with application to scalar dissipation *χ*, were presented by *Fernando and Lozovatsky* [2001] and for the velocity field and ɛ by *Seuront and Schmitt* [2001] and *Yamazaki et al.* [2006]. *Seuront and Schmitt* [2001] concluded that fluorescence is more intermittent than the velocity, but less intermittent than the conductivity fields in the Neko Seto Sea, offshore the Japanese coast. The distributions of ɛ and *χ* in deep ocean and shallow waters, at the scales from tens of centimeters to several meters that are affected by internal as well as external intermittencies, were found to be approximately lognormal [e.g., *Baker and Gibson*, 1987; *Gibson*, 1991; *Gregg et al.*, 1993; *Rehmann and Duda*, 2000; *Lozovatsky and Fernando*, 2002; *Lozovatsky et al.*, 2006; *Yamazaki and Lueck*, 1990; *Davis*, 1996], but they disputed the applicability of lognormal approximation to the distribution of ɛ in the ocean.

[8] Most theoretical studies on internal intermittency (see reviews of *Lesieur* [1990], *Frisch* [1995], and *Seuront et al.* [2005]) employed a suite of scaling models, either of the fluctuations of ɛ_{r} or *q*th-order statistical moments of velocity increments 〈Δ*V _{r}^{q}*〉, which are also called the

*q*th-order structure functions (SF). The angle brackets indicate ensemble averaging over a specific volume in the inertial subrange with a characteristic radius

*r*. Laboratory experiments, DNS, and atmospheric measurements have produced voluminous literature on internal intermittency (see reviews of

*Sreenivasan and Antonia*[1997],

*Anselmet et al.*[2001],

*Tsinober*[2001],

*Vassilicos*[2001],

*Seuront et al.*[2005], and

*Lovejoy and Schertzer*[2007]). Specific findings of previous theoretical and laboratory studies will be given in sections 5 and 6 in relation to our results.

[9] In all, despite recent progress, small-scale intermittency within turbulent patches of the pycnocline or in the surface and bottom boundary layers has not been extensively studied and remains a relatively unexplored area in physical oceanography, though its oceanic applications abound. The goal of this paper is to investigate internal intermittency of marine turbulence near the seabed during different phases of a nonstratified reversing tidal flow and determine whether the intermittency parameters depend on the boundary layer and microscale Reynolds numbers. The analysis is based on measurements of vertical velocity *w* using a bottom mounted Acoustic Doppler Velocimeter (ADV). An overview of the scaling concepts in relation to structure functions analysis is given in section 2. Section 3 contains a brief summary of the measurement site and its hydrography as well as averaged turbulence parameters. The data have already been analyzed for mean flow and tidally induced temporal variations of averaged dissipation rate and friction velocity [*Lozovatsky et al.*, 2008a, 2008b]. The methodology of the SF analysis and calculation of the scaling exponents of the transverse structure functions (TSF) as well as the dissipation rate are presented in section 4, followed by the results in section 5. This includes the evolution of basic turbulence parameters (section 5.1) and transverse structure function exponents (TSFE) during the tidal cycle (section 5.2), a comparison of scaling exponents with log-Levy and lognormal intermittency models (section 5.3) and a discussion of dynamical relevance of model parameters (section 5.4). The dependence of the second-order TSFE on microscale turbulent Reynolds number is presented (section 5.5). The possible influence of Taylor hypothesis on evaluating TSFE is addressed in section 6 as well as other sources of uncertainty that may affect results. Conclusions are given in section 7.