Jeffrey T. Sherman

Observations of Temperature Microstructure in NATRE

Abstract A new autonomous instrument collected 76 profiles of temperature microstructure over a ten-day period in the eastern subtropical North Atlantic as part of the North Atlantic Tracer Release Experiment. The data between 200-m and 350-m depth was used to determine the mean rate of temperature variance dissipation 〈χ〉. The estimated diapycnal diffusivity is Ky = 1.4×10−5 m2 s−1. The distribution of χ is approximately lognormal, suggesting that the 95% confidence limits on 〈χ〉 are ±4%. This uncertainty is less than that caused by the imperfectly known probe response, possible noise spikes on the probes, and variability in the degree of microstructure anisotropy; the latter two effects were estimated from a pair of closely spaced probes. Each of these uncertainties is about ±15%. Statistically significant low-frequency variability of χ is observed with 〈χ〉 decreasing by a factor of 2 between the first and second half of the observation. This low-frequency variability is likely the largest cause of erro...

Estimates of the Vertical Wavenumber–Frequency Spectra of Vertical Shear and Strain

Abstract Measurements of vertical shear and strain were acquired from the research platform FLIP during the PATCHEX experiment in October, 1986 (34°N, 127°W). Vertical sheer was shear from two separate Doppler sonar systems. A long-range sonar, with independent estimates every 18 m, sampled from 150–1200 m in depth. A short-range sonar measured fine-scale shear over 150–180 m depth, with 1.5 m vertical resolution. Vertical strain, ∂η/∂z, was estimated from two repeatedly profiling CTDs. These sampled to 560 m once every three minutes. The time variation of the strain field is monitored in both Eulerian (fixed-depth) and semi-Lagrangian (isopycnal-following) reference frames, from 150–406 m depth. Eulerian vertical wavenumber-frequency (m, ω) spectra of vertical shear and strain exhibit a frequency dependency which is a strong function of wavenumber (ω−2–ω0 for m = 0.01–0.3 cpm). In contrast the semi-Lagrangian strain spectrum is more nearly separable in frequency and wavenumber, in closer agreement with t...

Strain: Observations of the Vertical Gradient of Isopycnal Vertical Displacement

Abstract In this work a profiling CTD, operated from the research platform FLIP, is used to monitor the fine-scale density field as a function of both depth and time. A sequence of 10 000 CTD profiles from the surface to 560 m is examined. The data were obtained off the Southern California coast in the 1986 PATCHEX experiment. The vertical separation between successive isopycynal surfaces is tracked. The separation is related to the vertical derivative of vertical displacement, and is here referred to as the strain. The purpose of this work is to present a simple picture of the fine scale strain field as it evolves in time as well as depth. When viewed in isopycnal following coordinates, the qualitative nature of the strain field depends on the characteristic vertical scale over which it is estimated. The “20 m strain” field has a wavelike character, dominated by inertial and semidiurnal tidal motion. Wavelike fluctuations are seen in the 20 m strain field even at subinertial frequencies. This suggests th...

Spray Underwater Glider Operations

AbstractOperational statistics for the Spray underwater glider are presented to demonstrate capabilities for sustained observations. An underwater glider is an autonomous device that profiles vertically by changing buoyancy and flies horizontally on wings. The focus has been on sustained observations of boundary currents to take advantage of the glider’s small size, which allows it to be deployed and recovered from small vessels close to land, and the fine horizontal resolution delivered by the glider, which is scientifically desirable in boundary regions. Since 2004, Spray underwater gliders have been deployed for over 28 000 days, traveling over 560 000 km, and delivering over 190 000 profiles. More than 10 gliders, on average, have been in the water since 2012. Statistics are given in the form of histograms for 297 completed glider missions of longer than 5 days. The statistics include mission duration, number of dives, distance over ground, and horizontal and vertical distance through water. A discuss...

Absolute Velocity Estimates from Autonomous Underwater Gliders Equipped with Doppler Current Profilers

AbstractDoppler current profilers on autonomous underwater gliders measure water velocity relative to the moving glider over vertical ranges of O(10) m. Measurements obtained with 1-MHz Nortek acoustic Doppler dual current profilers (AD2CPs) on Spray gliders deployed off Southern California, west of the Galapagos Archipelago, and in the Gulf Stream are used to demonstrate methods of estimating absolute horizontal velocities in the upper 1000 m of the ocean. Relative velocity measurements nearest to a glider are used to infer dive-dependent flight parameters, which are then used to correct estimates of absolute vertically averaged currents to account for the accumulation of biofouling during months-long glider missions. The inverse method for combining Doppler profiler measurements of relative velocity with absolute references to estimate profiles of absolute horizontal velocity is reviewed and expanded to include additional constraints on the velocity solutions. Errors arising from both instrumental bias ...

Depth-average velocity from Spray underwater gliders

AbstractThe depth-average velocity is routinely calculated using data from underwater gliders. The calculation is a dead reckoning, where the difference between the glider’s velocity over ground an...

Acoustic Doppler measurement of internal waves: New techniques and observations

In the mid 1970s, acoustic Doppler techniques were first applied to the internal wave measurement problem. Since then, both pulse‐to‐pulse coherent (short‐range) and incoherent (long‐range) sonars have been developed. Recently, a 161‐kHz device that can operate in either mode, as well as a hybrid coded mode, has been created. In the hybrid mode, broadband pulses are transmitted. Doppler shift is estimated from the phase progression within individual returns. Incoherent averaging from pulse to pulse further improves the precision of the velocity estimate. Velocity error variance is reduced by a factor of 10 relative to uncoded pulses. The hybrid sonar was first operated in the Arctic, north of Svalbard, in conjunction with the 1989 ONR experiment, Cearex. While the under ice boundary layer was examined using a fully coherent operating mode, the large‐scale internal wave field was sampled to depths of 500 m using the hybrid transmission scheme. Strong baroclinic subinertial motions were seen at diurnal tida...

Evaluating a Lithium-Seawater Battery on Gliders

AbstractNeutral-buoyancy vehicles demand high-density energy sources and lithium is light with high oxidation energy. PolyPlus Battery Company has developed a prototype lithium-seawater battery that is attractive for powering long-duration autonomous oceanographic vehicles (floats and underwater gliders). These batteries were tested in the laboratory and at sea.PolyPlus batteries use “Protected Lithium Electrodes” with proprietary “windows” protecting the volatile lithium anode from water while passing lithium ions. The cathode reduces oxygen dissolved in seawater, or hydrolyzes seawater to produce hydrogen. Not requiring additional electrolyte, fuel, or pressure cases, these cells have impressive weight advantages. Good electrode–seawater mass transfer is required but can increase drag and be impeded by biofouling.Tests assessing robustness of the PolyPlus batteries in oceanographic use, evaluating mass transfer issues, and observing biofouling impacts are reported. In sea trials, two cells were tested f...

Observations of acoustic scattering associated with ocean microstructure in the Arctic

Observations from an Arctic ice camp on the Northwest flank of the Yermak Plateau indicate that under some conditions ocean microstructure may be detectable with ‘‘standard’’ acoustic instrumentation (i.e., acoustic Doppler current profilers) of moderately high frequency (150–300 kHz). The data set, collected during the Cooperative Eastern Arctic Experiment (CEAREX), includes simultaneous observations of kinetic energy dissipation rate, temperature dissipation rate, and acoustic backscatter from both 160‐ and 300‐kHz Doppler profilers. The turbulence levels observed during CEAREX were particularly strong (e≳10−7 W kg−1) and occurred in well‐defined patches. Patches in the mixed layer were not associated with backscattered intensity anomalies. However, intensity anomalies of 2 to 6 dB were found to be coincident in time and space with the patches of strong turbulence in the thermocline. The acoustic intensity anomalies were intermittent, presumably because they were detectable only above a threshold that r...

Simultaneous observations of acoustic scattering and ocean microstructure in the arctic.

Intriguing new observations from an ice camp, manned during the Cooperative Eastern Arctic Experiment (CEAREX) on the Northwest flank of the Yermak Plateau, indicate that ocean microstructure may be detectable with ‘‘standard’’ acoustic instrumentation (i.e., acoustic Doppler current profilers) of moderately high frequency (150–300 kHz). The strength of the CEAREX data set is the simultaneous observation of kinetic energy dissipation rate, temperature dissipation rate, and acoustic backscatter from both 160‐ to 300‐kHz Doppler profilers. The turbulence levels observed during CEAREX were particularly strong (e≳10−7 W kg−1) and occurred in well‐defined patches. Backscattered intensity anomalies of 2 to 6 dB were found to be coincident in time and space with patches of strong turbulence in the thermocline. It is shown that the acoustic intensity anomalies co‐vary with the strength of the turbulent dissipation above a threshold that presumably represents the background particulate scattering level. Theoretica...