Measuring Turbulent Dissipation Using a Tethered ADCP

The structure function method for estimating the dissipation rate of turbulent kinetic energy, previously validated for measurements from seabed fixed mounts, is applied to data from 1.2-MHz acoustic Doppler current profiler (ADCP) instruments operating in pulse�pulse coherent mode and mounted in midwater below a tethered buoy. Movements of the buoy introduce additional relative velocity components, but it is hypothesized that these flow components should not seriously interfere with the turbulence information because (i) horizontal or vertical translation induces the same flow component in all cells of an ADCP beam and (ii) any rotation of the instrument about its center induces flow components that are normal to the beam direction, and thus neither affect the structure function. This hypothesis is tested by comparing a series of dissipation measurements from a moored ADCP with those from a free-falling Vertical Microstructure Profiler (VMP) shear probe deployed from a nearby research vessel. The results indicate generally good conformity in both mean and variability over almost two decades of dissipation rates. The noise level of the structure function estimates with the pulse�pulse coherent ADCP is close to that of the VMP at ~3 � 10�10 W kg�1. This approach offers the prospect of long time series measurements of dissipation rate from moorings, albeit with restricted vertical range of a few meters

Observations of a diapycnal shortcut to adiabatic upwelling of Antarctic Circumpolar Deep Water

In the Southern Ocean, small-scale turbulence causes diapycnal mixing which influences important water mass transformations, in turn impacting large-scale ocean transports such as the Meridional Overturning Circulation (MOC), a key controller of Earth's climate. We present direct observations of mixing over the Antarctic continental slope between water masses that are part of the Southern Ocean MOC. A 12 h time series of microstructure turbulence measurements, hydrography, and velocity observations off Elephant Island, north of the Antarctic Peninsula, reveals two concurrent bursts of elevated dissipation of O(10�6)�W�kg�1, resulting in heat fluxes �10 times higher than basin-integrated Drake Passage estimates. This occurs across the boundary between adjacent adiabatic upwelling and downwelling overturning cells. Ray tracing to nearby topography shows mixing between 300 and 400 m is consistent with the breaking of locally generated internal tidal waves. Since similar conditions extend to much of the Antarctic continental slope where these water masses outcrop, diapycnal mixing may contribute significantly to upwelling

Impact of vertical mixing on sea surface pCO2 in temperate seasonally stratified shelf seas

A key parameter in determining the exchange of CO2 across the ocean-atmosphere interface is the sea surface partial pressure of carbon dioxide (pCO2). Temperate seasonally stratified shelf seas represent a significant sink for atmospheric CO2. Here an analytical model is used to quantify the impact of vertical mixing across the seasonal thermocline on pCO2. The model includes the impacts of the resultant dissolved inorganic carbon, heat, salt, and alkalinity fluxes on the solubility of CO2 and the effect of the inorganic carbon sink created by the primary production fuelled by the flux of limiting nutrient. The results indicate that diapycnal mixing drives a modest but continuous change in pCO2 of order 1–10 µatm d−1. In quantifying the individual impacts of the fluxes of the different parameters, we find that the impact of the fluxes of DIC and nitrate fluxes dominate. In consequence, both the direction and magnitude of the change in pCO2 are strongly dependent on the C:N uptake ratio in primary production. While the smaller impacts of the heat and salt fluxes tend to compensate for each other at midshelf locations, the heat flux dominates close to the shelf break. The analysis highlights the importance of the accurate parameterization of the C:N uptake ratio, the surface-mixed layer depth, and the TKE dissipation rate within the seasonal thermocline in models to be used to predict the air-sea exchange of carbon dioxide in these regimes. The results implicate storms as key periods of pCO2 perturbation

Turbulent mixing in the seasonally-stratified western Irish Sea: a Thorpe Scale perspective

The seasonal thermocline in shelf-seas represents an important biogeophysical barrier to the vertical flux of nutrients into the photic zone. Episodic weakening of this barrier plays an important role in sustaining the sub-surface chlorophyll maximum in summer and hence impacts the carbon draw-down in the seasonally-stratified zones of the shelf seas. Here we present estimates of the rate of turbulent kinetic energy dissipation inferred from microstructure shear probes and compare them with dissipation rates inferred from a standard conductivity-temperature-depth instrument and from a fast thermistor (Thorpe Scale methodology) at a site in the seasonally-stratified Irish Sea. All methods show strong dissipation rates in response to tidal stresses near the bed (order 10−2 Wm−3) with qualitatively similar temporal and spatial patterns. In the interior of the water column, however, only the microstructure shear probe estimates resolve the mixing in the region of the thermocline

Internal mixing processes in a seasonally stratified shelf sea

A key process in the maintenance of enhanced levels of primary production in the summer stratified regions of continental shelf seas is thermocline mixing. Current vertical exchange models however fail to accurately simulate turbulent processes in stratified regions and are heavily dependent on the inclusion of unjustified levels of background diffusion. We therefore seek to better understand internal mixing processes and to improve current turbulence models. Using measurements of current velocity, vertical structure and turbulent dissipation rate made at a site in the Celtic Sea interior the three likely candidate processes responsible for internal mixing are investigated, they are: Internal tide/internal waves generated at the shelf break and propagating on to the shelf. Internal waves or lee waves generated locally by hydraulic control over shelf sea banks and bumps. Inertial oscillations, most likely generated by changes in wind forcing. Internal waves generated at the shelf break were found to make no significant contribution to the energy available for mixing at our site. The thermocline is shown to be held in a marginally stable state by a strong and persistent thermocline shear layer, dominated for much of the time by low frequency, inertial shear. It is suggested that sporadically enhanced levels of shear from either inertial waves or a weak internal wave field add to an already tenuous background state, potentially fuelling turbulence via instability. Using our high quality dataset, the dependence of the turbulent dissipation rate, ε, on local shear and buoyancy frequency is examined and compared to current turbulence parametisations. Traditional models based on local stability fail to replicate any of our observations, however, a simple turbulence model suggested by MacKinnon and Gregg (2003) which scales dissipation rates to low frequency N2 and S2 successfully simulates much of the characteristics of thermocline ε

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Acoustic observations of zooplankton in lakes using a Doppler current profiler

In this paper we test the usefulness of acoustic backscatter measurements from a 614 kHz Acoustic Doppler Current Profiler (ADCP) for the qualitative and quantitative characterisation of zooplankton distributions in lakes. ADCP-based backscatter estimates were compared with frequent net hauls obtained during a calibration experiment in which the acoustic backscatter was strongly dominated by vertical migrating Chaoborus flavicans larvae.2. The correlation between backscatter estimates and the C. flavicans concentration was very good. Vertical swimming speed of larvae, measured directly by the ADCP, was up to a maximum of 5 mm s−1 and agreed very well with the observed vertical movement of the backscatter contour lines. Although the strong backscatter from C. flavicans overwhelmed the signal from the remaining zooplankton, a good correlation between backscatter strength and the total remaining zooplankton concentration, dominated by Cyclops spp., was found for the depth and time intervals where no C. flavicans were present.3. In addition to the calibration experiment, longer-term ADCP measurements from different lakes revealed a strong temporal correlation between the onset of the up- and downward migration of zooplankton and the local sunset and sunrise.4. We conclude that ADCPs can be used to monitor plankton distributions both temporally and spatially. It also seems possible to estimate plankton densities after appropriate calibration