This paper evaluates the performance of a newly developed free-falling microstructure profiler. The instrument is equipped with standard turbulence sensors for measuring turbulent velocity shear and temperature gradient, as well as bio-optical sensors for measuring in situ chlorophyll and turbidity variations. Simultaneous measurements with this profiler and an acoustic Doppler velocimeter were carried out in a flow tank, and data from both instruments agreed well. Turbulence spectra computed from both instruments agreed with the Kolmogorov inertial subrange hypothesis over approximately two decades in wavenumber space. Data from field tests conducted with the profiler showed that turbulence spectra measured in situ agreed with the empirical Nasmyth spectrum when corrections were made for the shear probe's spatial averaging. Dissipation rates as low as 5 ϫ 10 Ϫ10 W kg Ϫ1 were resolved when certain precautions were taken to avoid spectral bias caused by instrument vibrations. By assuming a universal form of the turbulence spectrum, turbulent kinetic energy dissipation rates below 5 ϫ 10 Ϫ10 W kg Ϫ1 can be estimated. The optical sensors resolved centimeter-scale structures of in vivo fluorescence and backscatter in field measurements.

The roles of plankton behavior, stratification, and microstructure in the formation of fine-scale plankton layers were examined using a 3-dimensional video plankton recorder mounted on a remotely operated vehicle. Vertically compressed plankton patches were observed in association with a cold pool over the Southern Flank of Georges Bank, extending from the tidal mixing front to the shelf-slope break during the months of May and June, 1994, 1995, 1997. In June 1995, 3 major plankton layers were present: a 10 m thick layer above the thermocline, a 1 m thick layer within the thermocline, and a third, 2 to 5 m thick layer immediately below the thermocline. Energy dissipation rate was lowest in the central layer and increased in both top and bottom layers. Some passive organisms and particles, e.g. the colonial diatom Chaetoceros socialis and rod-shaped diatoms, were concentrated in all 3 layers, while marine snow particles were found only in transitional regions. All stages of Calanus spp. were present in high numbers on the fringes of all 3 layers, while Oithona sp. was found only in the thin, central layer. Plankton were significantly aggregated only when the motility number, Mn (i.e. ratio of plankton swimming speed/rms turbulent velocity) was greater than 3, suggesting dominance of plankton behavior over physical structure. Under both quiescent and turbulent conditions, the Lagrangian frequency spectra (f ) for swimming plankton and passive particles decreased with a slope of f -2 . However, in quiescent conditions, the magnitude of the spectrum for swimming plankton was 10-fold greater than for passive particles, illustrating a decoupling of plankton swimming from turbulent eddies. The air/water interface, the pycnocline, and multiple shear interfaces at density discontinuities act as boundaries to vertical zones where plankton behavior may succumb to or dominate background microstructure, thus providing a mechanism for formation of plankton and particulate layers.KEY WORDS: Fine-scale vertical structure · Thin layers · Plankton behavior · Turbulence Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 267: [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] 2004 communities, species-specific patterns in abundance can form as a function of fine-scale physical structure (Owen 1989, Davis et al. 1992, Gallager et al. 1996b and may persist for many days (Donaghay et al. 1992, Cowles & Desiderio 1993, reviewed in Cowles et al. 1998. If this scale is undersampled or, worse, ignored, the result of persistent fine-scale structure will be gross underestimates of production (Cowles et al. 1998).Vertical fine-structure has been observed since the study of Eckart (1948), and is usually described in terms of mixing, such as the interaction between density stratification and horizontal shear (Gargett et al. 1984). One of the net results of shear is to redistribute horizontal variance onto vertical variance, producing the typical multiple-layer effect,...

[1] Large differences between the upstream and lee side flow characteristics of an isolated island in the Kuroshio have been identified from a three-dimensional velocity survey and from vertical profiles of fine-and micro-structure. In the island wake, the relative vorticity is O(10f ), the horizontal current divergence indicates upwelling of O(0.01 m s À1 ), and the rate of dissipation of kinetic energy is O(10 À4 W kg À1 ). Isopycnal surfaces shoal by 60 m on the lee side and surface nitrate concentration increases seven-fold. Flow blockage by the island and the Izu-Ogasawara Ridge on its flanks, induces horizontal and vertical flow separation. The associated lateral and vertical shear drive the upwelling and the vertical mixing in the wake and produces a very pronounced ''island mass effect.'' INDEX TERMS: 4279

There is concern about the appropriateness of the isotropic formula for calculating the dissipation rate of turbulent kinetic energy in a stratified fluid, such as turbulence in the thermocline. An alternative approach, given that stratification acts directly on the vertical component of the motion, is the axisymmetric theory of Batchelor (1946). Under the assumption that 〈(∂u2/∂x3)2〉 ≈ 〈(∂u2/∂x1)2〉, which is consistent with our measurements, we derive an upper and lower bound for dissipation in axisymmetric turbulence. For a value of ε/νN2 > 20 the dissipation estimate is comparable to the isotropic formula (based on the gradient of a horizontal velocity component). For decreasing values of ε/νN2 the error increases but is limited to less than 35%. Interestingly enough, in an axisymmetric turbulence the correct dissipation rate may be larger or smaller than an appropriate isotropic estimation. The important result is that the isotropic formula based on the shear of a horizontal velocity component is a good estimate of the dissipation, given the uncertainty in the current oceanographic technique. Both vertical and horizontal sampling are appropriate to estimate the dissipation rate from the isotropic formula as long as one uses a horizontal component of the velocity.

[1] Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind-scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near-zero PV in the thermocline under the Kuroshio mainstream is observed at 200-300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10-100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic-Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.Citation: Nagai, T., A. Tandon, H. Yamazaki, M. J. Doubell, and S. Gallager (2012), Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front,

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Microstructure measurements were made in the Mixed Water Region of the Oyashio/Kuroshio/Tsugaru currents system where both turbulence and double diffusion are involved in mixing. While intense turbulence is observed near the front between the Oyashio and the Tsugaru Current, double diffusion occupies a noticeable fraction in both the Tsugaru Water and the Mixed Water between the Oyashio and the Kuroshio. After determining a criterion to distinguish double diffusion from turbulence, vertical diffusivities and buoyancy fluxes are estimated using microstructure data. When turbulence is weak, double diffusion is observed around temperature and salinity anomalies, partly due to interleaving, and dominates the buoyancy flux. Vertical diffusivities due to double diffusion are parameterized as a function of the 10-m-scale density ratio. The 10-m-scale diffusivity estimates are consistent with the microstructure data when an appropriate criterion to reproduce a probability density function for the Turner angle is applied. A weighted-average diffusivity model is proposed to account properly for turbulence and double diffusion simultaneously.

Enhanced turbulent dissipation, O(10−8–10−7) Wkg−1 in the thermocline on the cyclonic side of the Kuroshio Front was observed during a period of frontogenesis, using a microstructure profiler, XBT and ADCP along 143°E across the Kuroshio Front in August 2008. The eddy diffusivity corresponding to the mixing below the central jet is estimated to be O(10−4–10−3) m2 s−1. The strong turbulent mixing we observed in the Kuroshio is in sharp contrast to previous field measurements which found that small scale diapycnal mixing in western boundary currents remains at levels typically measured in the open ocean thermocline. The turbulence in the Kuroshio is attributed to frontogenesis arising out of an estimated confluence rate of O(10−5) s−1 based on satellite altimeters, suggesting the ubiquity of a forward energy cascade from mesoscale to microscale turbulence near ocean fronts as indicated by recent theoretical studies.