Thomas M. Dillon

Spatial scales of current speed and phytoplankton biomass fluctuations in lake tahoe.

Spectral analysis of current speed and chlorophyll a measurements in Lake Tahoe, California and Nevada, indicates that considerably more variance exists at longer length scales in chlorophyll than in the current speeds. Increasingly, above scales of approximately 100 meters, chlorophyll does not behave as a simple passive contaminant distributed by turbulence, which indicates that biological processes contribute significantly to the observed variance at these large length scales.

Turbulent mixing near the Yermak Plateau during the Coordinated Eastern Arctic Experiment

During the ice camp component of the Coordinated Eastern Arctic Experiment (CEAREX) in March–April 1989, 1500 profiles of temperature, conductivity, and velocity shear microstructure were obtained near the Yermak plateau. The dominant signal in isopycnal displacements was the diurnal variability near the plateau, consistent with the enhanced diurnal currents found in this region previously from the Fram III and Fram IV ice camps. The dissipation rate of turbulent kinetic energy e reached maximum values of about 10−6 W kg−1 in two distinct regions, the surface mixing layer and the pycnocline between 120 and 220 m. There was a significant component of diurnal variability in e in each region. In the surface layer, e was proportional to the cube of the ice-relative current speed at 30 m below the ice, and it decayed approximately exponentially with increasing depth. Major mixing events in the pycnocline were correlated with large-amplitude, short-duration, isopycnal displacements. The lowest dissipation rates were recorded over the deep Nansen basin, north of the plateau, and in the subsurface core of a submesoscale baroclinic eddy in northern Fram Strait. The time-averaged vertical eddy diffusivity near the slope of the plateau was about 2.5×10−4 m2 s−1 in the pycnocline above the Atlantic layer, implying an upward heat flux of 25 W m−2, although only a small fraction of this heat reached the base of the mixed layer. The results confirm that current interactions with steep topography are critical to the modification of Atlantic Water in the eastern Arctic Ocean.

Internal waves and mixing in the upper equatorial Pacific Ocean

Microstructure measurements in the equatorial Pacific at 140°W in late 1984 show a pronounced diurnal variation in both high-frequency internal wave energy and kinetic energy dissipation rate. Observations indicated that after sunset, internal waves (presumably generated by convective overturns in the mixed layer) propagate downward and increase turbulence levels in the pycnocline. It is proposed that large mixed layer eddies in the South Equatorial Current interact with the large shear caused by the Equatorial Undercurrent to generate a westward going anisotropic wave field. The momentum transport in the radiated wave field results in a drag force on the equatorial mean flow field. The observed mean wind stress at 140°W during Tropic Heat I (which is twice as large as the annual mean wind) is closer to the estimated radiation stress (∼−10−4 m2 s−2) at the base of the mixed layer (≈30 m) than to the estimated turbulent stress (∼−10−5 m2 s−2). A wave dissipation model based on the observed turbulent kinetic energy dissipation rate is introduced in order to estimate the wave momentum flux divergence in the stratified region above the undercurrent core. The model predicts that most of the downward wave momentum flux penetrates through the undercurrent core. It is hypothesized that when the wind stress is strong, the equatorial Pacific Ocean responds by generating a westward traveling internal wave field which transports much of the surface wind stress below the actively mixing surface layer.

Numerical models of wind-driven circulation in lakes

Abstract The state-of-the-art of numerical modelling of large-scale wind-driven circulation in lakes is presented. The governing equations which describe this motion are discussed along with the appropriate numerical techniques necessary to solve them in lakes. The numerical models are categorized into three large primary groups: the layered models, the Ekman-type models, and the other three-dimensional models. Discussions and comparison of models are given and future research directions are suggested.

On the Horizontal Extent of the Canada Basin Thermohaline Steps

Abstract Microstructure profiles of temperatures through the diffusive thermohaline staircase above the Atlantic layer core in the Canada Basin of the Arctic Ocean are used to investigate the horizontal scales of layers. Daily profiles during two periods, 23 March–3 April, and 17–26 April 1985, show that diffusive steps are present throughout the 200 km drift track. A 20 hour series on 22 April, sampled at about 5 profiles per hour, indicates that particular diffusive layers can be traced for at least 600 m. The temperature of some layers varies by up to 0.01°C h−1 (30 m lateral motion); therefore, we cannot reliably trace steps in this location if the sampling distance is larger than about 15 m. Analysis of a longer time series with variable spacing from 0.4 to 5 km indicates that layers can rarely be traced between profiles more than 1 km apart.

Shannon entropy as an indicator of age for turbulent overturns in the oceanic thermocline

The Shannon entropy is a measure of the degree of intricacy contained in any graphable n-dimensional realization of an observable quantity. We use the Shannon entropy to measure the intricacy of density overturns in the oceanic thermocline and find that the Shannon entropy is related to the Thorpe scale LT [Thorpe, 1977] and the Ozmidov scale LO [Ozmidov, 1965]. We find that (1) small Shannon entropy corresponds to small values of ROT (≡LO/LT), while large Shannon entropy is associated with large values of ROT; (2) density spectra are typically more steep than inertial subrange spectra when both Shannon entropy and ROT are small, whereas the spectral slope tends to be flatter than inertial subrange spectra when both Shannon entropy and ROT are large; (3) the Grashof number is very large (O(1010)) when Shannon entropy is small, indicating that these patches are extremely density unstable; (4) spectral bandwidth is much larger for patches with small Shannon entropy than for those with large entropy, indicating that large-scale, or “bulk” Reynolds number is large when entropy is small. We discuss the hypothesis that the degree of intricacy, and hence the Shannon entropy, increases with increasing time in a turbulent overturn and is observed to decrease only when the resolution limits of the measuring system are exceeded. On the basis of these arguments we suggest that some classes of overturns are created with Thorpe scale larger than the Ozmidov scale. In these overturns the kinetic energy dissipation rate (e) is small during the initial growth of the overturn. Later, a small-scale structure develops, and a more complex, higher-order flow evolves. This behavior is discussed and compared with gridgenerated laboratory turbulence, in which initially small, energetic, rapidly growing boundary layers detached from the grid and advect downstream, forcing ROT to be largest adjacent to the grid and thereafter decrease as a result of entrainment.

Some statistical and dynamical properties of turbulence in the oceanic pycnocline

Statistics of turbulence length scales in individual mixing patches are used to describe the nature of mixing in the oceanic pycnocline near steep bottom topography. The majority of the observed mixing events can be explained with the conventional ideas of shear-driven turbulence. The statistics of some large patches, however, deviate substantially from expected values, suggesting that there is at least one other significant source of turbulent kinetic energy besides small-scale shear production. On the basis of inertial subrange energy arguments, it is proposed that the overturning seen in these events is a release of potential energy to kinetic energy which is consistent with advective instability in a finite-amplitude internal wave field. We find that within large turbulent patches the turbulent kinetic energy dissipation rate er averaged over a region of height r has a lognormal distribution consistent with Kolmogorovs third hypothesis, σ2ln(er) = A + μ, ln (LP/r), where σ2ln(er) is the variance of ln (er); r satisfies LP ≫ r ≫ η; LP is the size of the mixing patch; η is the Kolmogorov scale; A depends on the large-scale flow field; and μ is the intermittency coefficient, which is found to be approximately 0.4 in large patches. The variance of ln (er) also depends on the Reynolds number and the characteristic length scales of the mixing patches.

Observations of a surface mixed layer

Abstract The turbulent energy budget is empirically evaluated for an actively mixing surface layer, and Niller s ( Journal of Marine Research , 33 , 405–422, 1975) mixed layer model is shown to be consistent with observation during both wind stress driven and convection driven periods. The entrainment rate is best predicted when the turbulent energy production from shear stress at the mixed layer interface is assumed to be mostly dissipated and unavailable for entrainment processes. Other observations include the remarkable persistence of a non-turbulent shear flow on a relic mixed layer interface and the rapid decay of turbulent fluctuations when the driving force stops.