WHOI 260Hz Sound Source - Tuning and Assembly

Sound sources are designed to provide subsea tracking and re‐location of RAFOS floats and other Lagrangian drifters listening at 260Hz. More recently sweeps have been added to support FishChip tracking at 262Hz. These sources must be tuned to the water properties where they are to be deployed as they have a fairly narrow bandwidth. The high‐Q resonator’s bandwidth is about 4Hz. This report documents the tuning, and provides an overview of the sound source assembly.

Estimating the travel time and the most likely path from Lagrangian drifters

We provide a novel methodology for computing the most likely path taken by drifters between arbitrary fixed locations in the ocean. We also provide an estimate of the travel time associated with this path. Lagrangian pathways and travel times are of practical value not just in understanding surface velocities, but also in modelling the transport of ocean-borne species such as planktonic organisms, and oating debris such as plastics. In particular, the estimated travel time can be used to compute an estimated Lagrangian distance, which is often more informative than Euclidean distance in understanding connectivity between locations. Our methodology is purely data-driven, and requires no simulations of drifter trajectories, in contrast to existing approaches. Our method scales globally and can simultaneously handle multiple locations in the ocean. Furthermore, we provide estimates of the error and uncertainty associated with both the most likely path and the associated travel time.

Investigating the Formation of Submesoscale Structures along Mesoscale Fronts and Estimating Kinematic Quantities Using Lagrangian Drifters

Much of the vertical transport near the surface of the ocean, which plays a critical role in the transport of dissolved nutrients and gases, is thought to be associated with ageostrophic submesoscale phenomena. Vertical velocities are challenging not only to model accurately, but also to measure because of how difficult they are to locate in the surface waters of the ocean. Using unique massive drifter releases during the Lagrangian Submesoscale Experiment (LASER) campaign in the Gulf of Mexico and the Coherent Lagrangian Pathways from the Surface Ocean to the Interior (CALYPSO) experiment in the Mediterranean Sea, we investigate the generation of submesoscale structures along two different mesoscale fronts. We use a novel method to project Lagrangian trajectories to Eulerian velocity fields, in order to calculate horizontal velocity gradients at the surface, which are used as a proxy for vertical transport. The velocity reconstruction uses a squared-exponential covariance function, which characterizes velocity correlations in horizontal space and time, and determines the scales of variation using the data itself. SST and towed CTD measurements support the findings revealed by the drifter data. Due to the production of a submesoscale instability eddy in the Gulf of Mexico, convergence magnitudes of up to ∼20 times the planetary vorticity, f, are observed, the value of which is almost 3 times larger than that found in the mesoscale dominated Western Mediterranean Sea.

Mesoscale Activity in the Eastern Mediterranean: Blending Altimetry with in situ observations

<p>Mesoscale to sub-mesoscale surface dynamics in the ocean is a key parameter, driving, for instance, the dispersion of pollutants emanating from heavily populated coastal areas for example. Estimating the surface velocity can be challenging especially when data is sparse. In [1], the authors developed a near real-time 3D-Var assimilation algorithm that blends in-situ Lagrangian drifters’ positions with altimetry data to improve the estimation of the surface velocity in the Eastern Levantine Mediterranean. The algorithm was tested near the Lebanese coast and in the case of an eddy between Lebanon and Cyprus. The objective of this work is to further validate the algorithm.</p><p>First, a Comparison with Ocean color satellite images shows that eddies’ shapes and location are more consistent after the assimilation of drifter data.Independent in-situ current-meter data provided from the EGYPT campaign are also used to validate the results of the algorithm in terms of velocity intensity and direction. The comparison shows an improvement of the estimated velocity, particularly in terms of direction.</p><p>We also address the question of extending the algorithm to a larger regional scale in the Eastern Levantine Mediterranean, which is subject to a high mesoscale activity but which is less densely observed than the western part.</p><p> </p><p><span>[1] L. Issa, J. Brajard, M. Fakhri, D. Hayes, L. Mortier, P-M. Poulain. </span>Modelling Surface Currents in the Eastern Levantine Mediterranean Using Surface Drifters and Satellite Altimetry. Ocean Modelling, May 2016. Doi: 10.1016/j.ocemod.2016.05.006</p>

Improving the Accuracy of Hydrodynamic Model Predictions Using Lagrangian Calibration

While significant studies have been conducted in Intermittently Closed and Open Lakes and Lagoons (ICOLLs), very few have employed Lagrangian drifters. With recent attention on the use of GPS-tracked Lagrangian drifters to study the hydrodynamics of estuaries, there is a need to assess the potential for calibrating models using Lagrangian drifter data. Here, we calibrated and validated a hydrodynamic model in Currimundi Lake, Australia using both Eulerian and Lagrangian velocity field measurements in an open entrance condition. The results showed that there was a higher level of correlation (R2 = 0.94) between model output and observed velocity data for the Eulerian calibration compared to that of Lagrangian calibration (R2 = 0.56). This lack of correlation between model and Lagrangian data is a result of apparent difficulties in the use of Lagrangian data in Eulerian (fixed-mesh) hydrodynamic models. Furthermore, Eulerian and Lagrangian devices systematically observe different spatio-temporal scales in the flow with larger variability in the Lagrangian data. Despite these, the results show that Lagrangian calibration resulted in optimum Manning coefficients (n = 0.023) equivalent to those observed through Eulerian calibration. Therefore, Lagrangian data has the potential to be used in hydrodynamic model calibration in such aquatic systems.

Lagrangian Drifter to Identify Ocean Eddy Characteristics

Deterministic–stochastic empirical mode decomposition (EMD) is used to obtain low-frequency (non-diffusive; i.e., background velocity) and high-frequency (diffusive; i.e., eddies) components from a Lagrangian drifter‘s trajectory. Eddy characteristics are determined from the time series of eddy trajectories from individual Lagrangian drifters such as eddy radius, eddy velocity, eddy Rossby number, and the eddy–current kinetic energy ratio. A long-term dataset of the Sound Fixing and Ranging (RAFOS) float time series obtained near the California coast by the Naval Postgraduate School from 1992 to 2004 at depth between 150 and 600 m is used as an example to demonstrate the capability of the deterministic–stochastic EMD.

Multi-modal sensing drifters as a tool for repeatable glacial hydrology flow path measurements

Lagrangian drifters are a practical way to measure natural flow features in surface channels. In this study, small cylindrical drifters (length 12 cm, diameter 4 cm) were deployed in a supraglacial channel. Each drifter recorded the total water pressure, linear acceleration, magnetic field strength and rate of rotation at 100 Hz. Based on an ensemble analysis of repeated field deployments (n = 55), it was found that the pressure sensors consistently delivered the most accurate data, where values remained within ±0.11 % of the total pressure time-averaged mean (95 % confidence interval). Magnetometer readings also exhibited low variability across deployments, maintaining readings within ±2.45 % of the time-averaged mean of the magnetometer magnitude. Linear acceleration measurements were found to have substantially larger 95 % confidence intervals, spanning ±34.4 % from the time-averaged mean magnitudes. Furthermore, the drifter speed along the supraglacial channel was estimated by integrating the linear acceleration, providing a 95 % confidence interval of ±24.5 % of the time averaged mean magnitude. The major contribution of this work is to provide a statistical assessment of multimodal drifters, repeatedly deployed in a 450 m long supraglacial channel reach, with a focus on developing a repeatable field measurement methodology including uncertainty. The results of this work show that multimodal drifters are capable of highly repeatable field measurements in supraglacial channels.