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.

A four-transducer, 600-kHz, broadband acoustic Dopple current profiler (ADCP) was rigidly mounted to the bottom of a fully turbulent tidal channel with peak flows of 1 m s Ϫ1. Rapid samples of velocity data are used to estimate various parameters of turbulence with the covariance technique. The questions of bias and error sources, statistical uncertainty, and spectra are addressed. Estimates of the Reynolds stress are biased by the misalignment of the instrument axis with respect to vertical. This bias can be eliminated by a fifth transducer directed along the instrument axis. The estimates of turbulent kinetic energy (TKE) density have a systematic bias of 5 ϫ 10 Ϫ4 m 2 s Ϫ2 due to Doppler noise, and the relative statistical uncertainty of the 20-min averages is usually less than 20%-95% confidence. The bias in the Reynolds stress due to Doppler noise is less than Ϯ4 ϫ 10 Ϫ5 m 2. The band of zero significance is never less than 1.5 ϫ 10 Ϫ5 m 2 s Ϫ2 due to Doppler noise, and this band increases with increasing TKE density. Velocity fluctuations with periods longer than 20 min contribute little to either the stress or the TKE density. The rate of production of TKE density and the vertical eddy viscosity are derived and in agreement with expectations for a tidal channel.

19861, pp 445-489 Sci. Lett 109 I1 (I 9921. 22. We thank J. Cater, H. Adams, and D Schell~ng for 4 M. M. Sarn e t a / . , Geochim. Cosmochim. Acta 53, d~scuss~ons, samples, and unpubshed sedmento-997 (1989); K Pande et a1 Chem. Geol 11 6, 245 o g c a data from the Panir secton LASMO 0 1 Pak-(I 994) stan Ltd and the Drector General of Petroleum Con-5 M. R Palmer and J M. Edmond, Geochim Cosmochim Acta 56. 2099 11 992) cessons for provsion of samples and permisson to p u b s h data from the Panr secton, the Department of Soil Conservat~on n Babar Mahal, Kathmandu; and C. France-Lanord and L. Deriy for discussons and access to unpublished data. The early stages of this research were supported and encouraged by M McCulloch and A. Chivas at Australan Natona Unlverslty Th~s project was p r~m a r y funded by NSF grant EAR-941 8207.

The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

The terms of the steady-state, homogeneous turbulent kinetic energy budgets are obtained from measurements of turbulence and fine structure from the small autonomous underwater vehicle (AUV) Remote Environmental Measuring Units (REMUS). The transverse component of Reynolds stress and the vertical flux of heat are obtained from the correlation of vertical and transverse horizontal velocity, and the correlation of vertical velocity and temperature fluctuations, respectively. The data were obtained using a turbulence package, with two shear probes, a fast-response thermistor, and three accelerometers. To obtain the vector horizontal Reynolds stress, a generalized eddy viscosity formulation is invoked. This allows the downstream component of the Reynolds stress to be related to the transverse component by the direction of the finescale vector vertical shear. The Reynolds stress and the vector vertical shear then allow an estimate of the rate of production of turbulent kinetic energy (TKE). Heat flux is obtained by correlating the vertical velocity with temperature fluctuations obtained from the FP-07 thermistor. The buoyancy flux term is estimated from the vertical flux of heat with the assumption of a constant temperature-salinity (T-S) relationship. Turbulent dissipation is obtained directly from the usage of shear probes.A multivariate correction procedure is developed to remove vehicle motion and vibration contamination from the estimates of the TKE terms. A technique is also developed to estimate the statistical uncertainty of using this estimation technique for the TKE budget terms. Within the statistical uncertainty of the estimates herein, the TKE budget on average closes for measurements taken in the weakly stratified waters at the entrance to Long Island Sound. In the strongly stratified waters of Narragansett Bay, the TKE budget closes when the buoyancy Reynolds number exceeds 20, an indicator and threshold for the initiation of turbulence in stratified conditions. A discussion is made regarding the role of the turbulent kinetic energy length scale relative to the length of the AUV in obtaining these estimates, and in the TKE budget closure.

No abstract

[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

The diffusive regime of double‐diffusive convection generates staircases consisting of thin high‐gradient interfaces sandwiched between convectively mixed layers. Simultaneous microstructure measurements of both temperature and conductivity from the staircases in Lake Kivu are used to test flux laws and theoretical models for double diffusion. Density ratios in Lake Kivu are between one and ten and mixed layer thicknesses on average 0.7 m. The larger interface thickness of temperature (average 9 cm) compared to dissolved substances (6 cm) confirms the boundary‐layer structure of the interface. Our observations suggest that the boundary‐layer break‐off cannot be characterized by a single critical boundary‐layer Rayleigh number, but occurs within a range of O(102) to O(104). Heat flux parameterizations which assume that the Nusselt number follows a power law increase with the Rayleigh number Ra are tested for their exponent η. In contrast to the standard estimate η = 1/3, we found η = 0.20 ± 0.03 for density ratios between two and six. Therefore, we suggest a correction of heat flux estimates which are based on η = 1/3. The magnitude of the correction depends on Ra in the system of interest. For Lake Kivu (average heat flux 0.10 W m−2) with Ra = O(108), corrections are marginal. In the Arctic Ocean with Ra = O(108) to O(1012), however, heat fluxes can be overestimated by a factor of four.