By comparing observations from six diverse sites in the mid‐latitude thermocline, we find that, to within a factor of 2, 〈εIW〉=7×10‐10〈N2/N02〉〈S104/SGM4〉 W kg‐1 , where 〈εIW〉 is the average dissipation rate attributable to internal waves; N0 = 0.0052 s−1 is a reference buoyancy frequency; S10 is the observed shear having vertical wavelengths greater than 10 m; and SGM is the corresponding shear in the Garrett and Munk spectrum of internal waves. The functional form agrees with estimates by McComas and Müller and by Henyey, Wright, and Flatté of the rate of energy transfer within the internal wave spectrum, provided the energy density of the internal waves is treated as a variable instead of one of the constant parameters. Following Garrett and Munk, we assume that 〈S104/SGM4〉=〈EIW2/EGM2〉 , where EIW is the observed energy density and EGM is the energy density used by Garrett and Munk. The magnitude of εIW is twice that of Henyey et al. and one third that of McComas and Müller. Thus the observations agree with predictions sufficiently well to suggest that (1) a first‐order understanding of the link between internal waves and turbulence has been achieved, although Henyey et al. made some ad hoc assumptions and Garrett and Munk's model does not match important features in the internal wave spectrum reported by Pinkel, and (2) the simplest way to obtain average dissipation rates over large space and time scales is to measure 〈N2/N02〉〈S104/SGM4〉 . Even though the observations were taken at latitudes of 42°−11.5°, the variability in the Coriolis parameter ƒ was too limited for a conclusive test of the ƒ dependence also predicted for 〈εIW〉 by the wave‐wave interaction models. An exception to the scaling occurs east of Barbados in the thermohaline staircase that is apparently formed and maintained by salt fingers. Although ε in the staircase is very low compared with rates at mid‐latitude sites, it is 8 times larger than predicted for ε due only to internal waves.

Diapycnal fluxes of momentum and heat produced by three‐dimensional turbulence play important, but poorly understood, roles in the dynamics of the main thermocline and of the equatorial undercurrent. Diverse approaches—involving inferences, measurements, and process studies—are being pursued to define these fluxes and their significance. These are reviewed to improve the coordination of future mixing studies.

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities. But the magnitude and distribution of mixing across density surfaces are also important for the Earth's climate as well as the concentrations of organisms. Most of this mixing occurs where internal waves break, overturning the density stratification of the ocean and creating patches of turbulence. Predictions of the rate at which internal waves dissipate were confirmed earlier at mid-latitudes. Here we present observations of temperature and velocity fluctuations in the Pacific and Atlantic oceans between 42 degrees N and 2 degrees S to extend that result to equatorial regions. We find a strong latitude dependence of dissipation in accordance with the predictions. In our observations, dissipation rates and accompanying mixing across density surfaces near the Equator are less than 10% of those at mid-latitudes for a similar background of internal waves. Reduced mixing close to the Equator will have to be taken into account in numerical simulations of ocean dynamics--for example, in climate change experiments.

Mixing efficiency is the ratio of the net change in potential energy to the energy expended in producing the mixing. Parameterizations of efficiency and of related mixing coefficients are needed to estimate diapycnal diffusivity from measurements of the turbulent dissipation rate. Comparing diffusivities from microstructure profiling with those inferred from the thickening rate of four simultaneous tracer releases has verified, within observational accuracy, 0.2 as the mixing coefficient over a 30-fold range of diapycnal diffusivities. Although some mixing coefficients can be estimated from pycnocline measurements, at present mixing efficiency must be obtained from channel flows, laboratory experiments, and numerical simulations. Reviewing the different approaches demonstrates that estimates and parameterizations for mixing efficiency and coefficients are not converging beyond the at-sea comparisons with tracer releases, leading to recommendations for a community approach to address this important issue.

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Profiles of currents, density, and microstructure were obtained in the Pacific Ocean on and near the equator at 140øW in late 1984 as pa• of the Tropic Heat program. During a 4•-day time series on the equator, the shear zone above the core of the undercurrent had very low mean Richardson numbers, Ri, between Va and «. Average turbulent dissipation rates, e, were high in this zone, = 10 -7 Wkg -1, and varied by a factor of 100 between minima in the early afternoon and maxima at night. The signal reached to 90 m, well into the stratified zone. Eddy coefficients, K, were low at high Ri, and gradually increased as Ri decreased, until they rose dramatically near Ri =Va. The observed K (Ri) relations are specific to time, location, and temporal and spatial resolution of the data. INTRODUCTIONAs part of Tropic Heat, turbulence and shear were sampled inten- Gregg et al., 1985]. confirmed the patterns observM principal balance for the zonal pressure gradient driving the before, but found a much larger increase of z and X at low Ri than is consistent with recent numerical models. (Here, z is the rate of undercurrent [McCreary, 1981]. Consequently, turbulence levels viscous dissipation of turbulent kinetic energy, and X is the rate of in the undercurrent are at least a decade larger than in the Gulf diffusive smoothing of turbulent temperature fluctuations; Stream, where the Coriolis force balances the horizontal pressure Ri --N Z/shear 2 is the gradient Richardson number.) In addition, a gradient [Gregg and Sanford, 1980; Gargett and Osborn, 1981]. strong diurnal mixing cycle was discovered in data from the The intense equatorial turbulence produces a vertical turbulent Thompson and in measurements taken nearby on the R/V Wecoma heat flux of about 100 W m '2 in the high-shear zone above the [Mourn and Caldwell, 1985]. undercurrent core. In view of a latent heat flux of 200 W m '2 at the surface, the turbulent flux in the thermocline is an important component of the upper ocean heat budget. The equatorial heat budget is particularly important in the central Pacific, where large-scale fluctuations in the sea surface temperature are correlated with anomalous winter weather patterns over North America [Horel and Wallace, 1981]. Consequently, obtaining an accurate description and parameterizafion of the vertical turbulent transport in the undercurrent is a primary goal of Tropic Heat, a program established to study the upper ocean heat budget in the central Pacific [Eriksen, 1985]. Small-scale turbulence plays a major role in the upper equa-sively from the R/V Thompson between 3øN and 2.5øS at 140øW torial ocean; because the Coriolis force vanishes on the equator during mid-November and early December 1984. A preliminary and is weak nearby, the vertical turbulent viscosity provides the analysis [Prior to the Tropic Heat measurements in late 1984, only a few tens of microstincture profiles had been reported from the equator. Data from the Pacific and the Atlantic revealed a consistent pattern of turbulence levels varying inverse...

Observations are presented of microstructure and velocity measurements made on the outer New England shelf in the late summer of 1996 as part of the Coastal Mixing and Optics Experiment. The depth-and timeaveraged turbulent dissipation rate was 5-50 (ϫ 10 Ϫ9 W kg Ϫ1 ). The associated average diapycnal diffusivity in stratified water was 5-20 (ϫ 10 Ϫ6 m 2 s Ϫ1 ), comparable to observed open-ocean thermocline values and too low to explain the strong variability observed in local water properties. Dissipation rates and diffusivity were both highly episodic. Turbulent boundary layers grew down from the surface and up from the bottom. The dissipation rate within the bottom boundary layer had an average of 1.2 ϫ 10 Ϫ7 W kg Ϫ1 and varied in magnitude with the strength of near-bottom flow from the barotropic tide, an along-shelf flow, and low-frequency internal waves. The average dissipation rate in the peak thermocline was 5 ϫ 10 Ϫ8 W kg Ϫ1 ; one-half of the thermocline dissipation was due to the strong shear and strain within six solibores that cumulatively lasted less than a day but contained 100-fold elevated dissipation and diffusivity. Nonsolibore, midcolumn dissipation was strongly correlated with shear from low-frequency internal waves. Dissipation was not well parameterized by Gregg-Henyey-type scaling.An alternate scaling, modified to account for observed coastal internal wave properties, was in good agreement with measured dissipation rates. At the end of the observational period Hurricane Edouard passed by, producing strong dissipation rates (4 ϫ 10 Ϫ6 W kg Ϫ1 ) and consequent mixing during and for several days following the peak winds. * Current affiliation: IOD/SIO,