Surface Buoy Research Articles

An acoustic observatory for sperm whales in the Eastern Mediterranean Sea

A deep-water acoustic observatory for real-time detection and 3D localization of sperm whales has been developed, deployed and operated for 3-month periods in summer 2020 and 2021 off SW Crete in the Eastern Mediterranean Sea. Detection and localization rely on receptions of regular clicks at 3 hydrophones suspended from surface buoys at depths of ∼100 m and ∼1 km apart. Travel times of significant arrivals extracted onboard the buoys are transmitted, together with other supporting data, via mobile broadband to a land-based analysis center where the data from all buoys are combined to enable detection and 3D localization of vocalizing animals exploiting direct and surface-reflected arrivals and using a Bayesian approach. The large separations between hydrophones result in small localization uncertainties for ranges up to ∼6 km, whereas, on the other hand, they pose significant challenges related, e.g., to peak association and synchronization between the buoys. The design and implementation, as well as results from the operation of the system will be presented. [Work supported by Ocean Care – SAvEWhales project.]

OOIPy: A Python toolbox for accessing and analyzing sata from the Ocean Observatories Initiative

The recent advent of long-term, large-scale, and publicly available underwater acoustic datasets can unlock new discoveries and fuel a deeper understanding of the ocean environment. One such dataset is provided by the Ocean Observatories Initiative (OOI), who maintain multiple research arrays distributed across the Pacific and Atlantic Ocean that feature a variety of instruments and platforms from hydrophones to water column profilers and surface buoys. However, analyzing the OOI data can be challenging, as tools for accessing the data are either completely lacking, or require a significant amount of additional programming effort to prepare data for analysis. With the development of the Python package OOIPy we aim to address these problems for the OOI data products. OOIPy combines low level functions for accessing OOI data in Python with higher level functions for data processing, analysis, and visualization. Currently, data products from low-frequency and broadband hydrophones, which are only available through the OOI raw data server, as well as conductivity, temperature, depths sensors in the northeast Pacific Ocean are supported. Our vision for the future is to expand OOIPy and allow scientists to focus on the analysis, rather than the accessing and preprocessing, of OOI data products. [Work supported by ONR.]

The Impacts of Gustiness on Air–Sea Momentum Flux

The exchange of momentum across the air–sea boundary is an integral component of the earth system and its parametrization is essential for climate and weather models. This study focuses on the impact of gustiness on the momentum flux using three months of direct flux observations from a moored surface buoy. Gustiness, which quantifies the fluctuations of wind speed and direction, is shown to impact air–sea momentum fluxes. First, we put forward a new gustiness formula that simultaneously evaluates the impact of fluctuations in wind direction and speed. A critical threshold is established using a cumulative density function to classify runs as either gusty or non-gusty. We find that, during runs classified as gusty, the aerodynamic drag coefficient is increased up to 57% when compared to their non-gusty counterparts. This is caused by a correlated increase in vertical fluctuations during gusty conditions and explains variability in the drag coefficient for wind speeds up to 20 m/s. This increase in energy is connected with horizontal fluctuations through turbulent interactions between peaks in the turbulent spectra coincident with peaks in the wave spectra. We discus two potential mechanistic explanations. The results of this study will help improve the representation of gustiness in momentum flux parameterizations leading to more accurate ocean models.

Modeling and Numerical Simulation of a Buoyancy Controlled Ocean Current Turbine

Increased global renewable power demands and the high energy density of ocean currents have motivated the development of ocean current turbines (OCTs). These compliantly mooring systems will maintain desired near-surface operating depths using variable buoyancy, lifting surface, sub-sea winches, and/or surface buoys. This paper presents a complete numerical simulation of a 700 kW variable buoyancy controlled OCT that includes detailed turbine system, inflow, actuator (i.e., generator and variable buoyancy), sensor, and fault models. Simulation predictions of OCT performance are made for normal, hurricane, and fault scenarios. Results suggest this OCT can operate between depths of 38 m to 329 m for all homogeneous flow speeds between 1.0-2.5 m/s. Fault scenarios show that rotor braking results in a rapid vertical OCT system assent and that blade pitch faults create power fluctuations apparent in the frequency domain. Finally, simulated OCT operations in measured ocean currents (i.e., normal and hurricane conditions) quantify power statistics and system behavior typical and extreme conditions.

A Predictive Model of Chlorophyll a in Western Lake Erie Based on Artificial Neural Network

The reoccurrence of algal blooms in western Lake Erie (WLE) since the mid-1990s, under increased system stress from climate change and excessive nutrients, has shown the need for developing management tools to predict water quality. In this study, process-based model GLM-AED (General Lake Model-Aquatic Ecosystem Dynamics) and statistical model ANN (artificial neural network) were developed with meteorological forcing derived from surface buoys, airports, and land-based stations and historical monitoring nutrients, to predict water quality in WLE from 2002 to 2015. GLM-AED was calibrated with observed water temperature and chlorophyll a (Chl-a) from 2002 to 2015. For ANN, during the training period (2002–2010), the inputs included meteorological forcing and nutrient concentrations, and the target was Chl-a simulated by calibrated GLM-AED due to the lack of continuously daily measured Chl-a concentrations. During the testing period (2011–2015), the predicted Chl-a concentrations were compared with the observations. The results showed that the ANN model has higher accuracy with lower Chl-a RMSE and MAE values than GLM-AED during 2011 and 2015. Lastly, we applied the established ANN model to predict the future 10-year water quality of WLE, which showed that the probability of adverse health effects would be moderate, so more intense water resources management should be implemented.

Multi-Year Simulation of Western Lake Erie Hydrodynamics and Biogeochemistry to Evaluate Nutrient Management Scenarios

During the 1970s, harmful cyanobacteria (HFCB) were common occurrences in western Lake Erie. Remediation strategies reduced total P loads and bloom frequency; however, HFCB have reoccurred since the mid-1990s under increased system stress from climate change. Given these concurrent changes in nutrient loading and climate forcing, there is a need to develop management tools to investigate historical changes in the lake and predict future water quality. Herein, we applied coupled one-dimensional hydrodynamic and biogeochemical models (GLM–AED) to reproduce water quality conditions of western Lake Erie from 1979 through 2015, thereby removing the obstacle of setting and scaling initial conditions in management scenarios. The physical forcing was derived from surface buoys, airports, and land-based stations. Nutrient loads were reconstructed from historical monitoring data. The root-mean-square errors between simulations and observations for water levels (0.36 m), surface water temperature (2.5 °C), and concentrations of total P (0.01 mg L−1), PO4 (0.01 mg L−1), NH4 (0.03 mg L−1), NO3 (0.68 mg L−1), total chlorophyll a (18.74 μg L−1), chlorophytes (3.94 μg L−1), cyanobacteria (12.44 μg L−1), diatoms (3.17 μg L−1), and cryptophytes (3.18 μg L−1) were minimized using model-independent parameter estimation, and were within literature ranges from single year three-dimensional simulations. A sensitivity analysis shows that 40% reductions of total P and dissolved reactive P loads would have been necessary to bring blooms under the mild threshold (9600 MTA cyanobacteria biomass) during recent years (2005–2015), consistent with the Annex 4 recommendation. However, these would not likely be achieved by applying best management practices in the Maumee River watershed.

Seasonality in Saharan Dust Across the Atlantic Ocean: From Atmospheric Transport to Seafloor Deposition

AbstractSaharan dust is transported in great quantities from the North African continent every year, most of which is deposited across the North Atlantic Ocean. This dust impacts regional and global climate by affecting the atmospheric radiation balance and altering ocean carbon budgets. However, little research has been carried out on time series of Saharan dust collected in situ across the open Atlantic. Here, we present a unique three‐year time series of Saharan dust along a trans‐Atlantic transect, sampled by moored surface buoys and subsurface sediment traps. Results show a good correlation between the particle‐size distributions of atmospheric dust and the lithogenic particles settling to the deep ocean floor, confirming the aeolian origin of the lithogenic particles intercepted by the subsurface sediment traps, even in the distal western part of the Atlantic Ocean. Dust from both dry and wet deposition as collected by the sediment traps, shows increased deposition fluxes and coarser grain size in summer and/or autumn that coincides with increased precipitation at the sampling sites as derived from satellite data. In contrast, both buoys that sampled dust during transport at sea level show little seasonal variation in both concentration and particle size, as the large amounts of dust transported in summer and early autumn at high altitudes are far above their sampling range. This implies that wet deposition in summer and autumn defines the typical seasonal trends of both the dust deposition flux and its particle‐size distribution observed in the sediment traps.

Characterizing underwater noise during rain at the northeast Pacific continental margin.

Large scale studies of underwater noise during rain are important for assessing the ocean environment and enabling remote sensing of rain rates over the open ocean. In this study, approximately 3.5 yrs of acoustical and meteorological data recorded at the northeast Pacific continental margin are evaluated. The acoustic data are recorded at a sampling rate of 64 kHz and depths of 81 and 581 m at the continental shelf and slope, respectively. Rain rates and wind speeds are provided by surface buoys located in the vicinity of each hydrophone. Average power spectra have been computed for different rain rates and wind speeds, and linear and nonlinear regression have been performed. The main findings are (1) the linear regression slopes highly depends on the frequency range, rain rate, wind speed, and measurement depth; (2) noise levels during rain between 200 Hz and 10 kHz significantly increase with increasing wind speed; and (3) the highest correlation between the spectral level and rain rate occurs at 13 kHz, thus, coinciding with the spectral peak due to small raindrops. The results of this study indicate that previously proposed algorithms for estimating rain rates from acoustic data are not universally applicable but rather have to be adapted for different locations.

Improved Current Estimates from Spar Buoy-Mounted ADCP Measurement Station: A Case Study in the Ligurian Sea

Current measurements in the open sea are generally acquired by Acoustic Doppler Current Profilers (ADCPs). In the case of ADCPs mounted on spar buoy, current profiles require to be post-processed, to properly take into account the buoy influence: in fact, ADCP compass may reflect alterations induced by the metal structure of the buoy and apparent currents can occur due to the large displacement of the platform. Uncertainty analysis is finally required to properly consider both these effects and to compute robust velocity estimates. A new methodology is tested for a measurement station in the Ligurian Sea, where an ADCP was mounted on the surface buoy of the W1-M3A (Western 1 Mediterranean Moored Multisensor Array) oceanographic observatory, facing upwards at the depth of about 40 m. Marine current numerical models and historical data in the area have been used as a basis for comparison to test the consistency of the proposed method. A very good agreement is obtained. Only minor discrepancies are reported (e.g., monthly averages from the reference model slightly underestimate the west-east current component along the entire profile), but, in general, the application of the proposed methodology ensures that the spar buoy-mounted ADCP system is able to provide reliable measurements for oceanographic studies and validation of 3D hydrodynamic models.

Coastal Observation Using a Vertical Profiling System at the Southern Coast of Korea

At a coastal station near the southern coast of Korea, the vertical profiles of temperature salinity dissolved oxygen and velocity were obtained using a vertical profiler, Aqualog, every summer from 2016 to 2020. At the site, fishing activity was not allowed, and it was possible to maintain the profiler continuously and stably. It was set to travel every one or 2 h for two to 4 months. Thus, we were able to observe the variations of the water properties from hourly to monthly scales. The sensors were contaminated much less than we expected, and the data could be used without correction at least for our coastal applications. The main phenomena we observed are tides, coastal warming, fresh water, and responses to typhoons. On the daily time scale, the most prominent phenomenon is semi-diurnal tides, with which the thickness and temperature of coastal warm waters changed. The warm water also showed fluctuations between 10 and 15 days. The data also revealed that the tide showed strong seasonality. In summer, when the water is strongly stratified, the tidal current is baroclinic, while in winter, when the water is well mixed, the current is barotropic. Responses to typhoon induced winds were rather complicated. In one case, increase in the upper mixed layer was observed. The thick mixed layer disappeared in about a day due to advection. In another case the upper mixed layer became thinner, while the wind became stronger due the advection of the offshore water. Hydrographic observations conducted every 2 months, of course, or point measurement at a surface buoy could not show such continuous changes. More and more local fishermen are showing interest in oceanographic information, and data from the profiler could be of much use to them.

Long-term analysis of ocean noise floor in the northeast Pacific Ocean

Characterizing the ocean noise floor is important for tracking long-term acoustic changes and creating environmental regulations to mitigate the effects of human activities on the ocean soundscape. Underwater noise levels have been measured at two sites in the northeast Pacific off the coast of Oregon between 2015 and 2019. The acoustic data were recorded continuously at a sample rate of 64 kHz at 81 m depth and 581 m depth at the continental shelf and slope, respectively. Sequential one-minute power spectral density estimates are computed and used to calculate spectral probability density functions (SPDFs) for every month of the measurement period. The ocean noise floor is then defined as the 5th percentile of the SPDF. Multi-year data are used to compute time series of the ocean noise floor at various frequencies, which are then examined for long-term trends and seasonal patterns. The frequency dependence of the noise floor is analyzed and the results are compared between two measurement sites. Environmental data from surface buoys and Conductivity, Temperature, Depth (CTD) instruments collocated with the hydrophones are used to study how the ocean noise floor changes with wind, temperature, and sound speed. [Work supported by ONR.]

An Autonomous Platform for Near Real-Time Surveillance of Harmful Algae and Their Toxins in Dynamic Coastal Shelf Environments

Efforts to identify in situ the mechanisms underpinning the response of harmful algae to climate change demand frequent observations in dynamic and often difficult to access marine and freshwater environments. Increasingly, resource managers and researchers are looking to fill this data gap using unmanned systems. In this study we integrated the Environmental Sample Processor (ESP) into an autonomous platform to provide near real-time surveillance of harmful algae and the toxin domoic acid on the Washington State continental shelf over a three-year period (2016–2018). The ESP mooring design accommodated the necessary subsystems to sustain ESP operations, supporting deployment durations of up to 7.5 weeks. The combination of ESP observations and a suite of contextual measurements from the ESP mooring and a nearby surface buoy permitted an investigation into toxic Pseudo-nitzschia spp. bloom dynamics. Preliminary findings suggest a connection between bloom formation and nutrient availability that is modulated by wind-forced coastal-trapped waves. In addition, high concentrations of Pseudo-nitzschia spp. and elevated levels of domoic acid observed at the ESP mooring location were not necessarily associated with the advection of water from known bloom initiation sites. Such insights, made possible by this autonomous technology, enable the formulation of testable hypotheses on climate-driven changes in HAB dynamics that can be investigated during future deployments.

E&P Notes (December 2020)

E&P Notes (December 2020)

Measured Rogue Waves and Their Environment

We conduct a comprehensive analysis of over 350 years of accumulated ocean surface elevation time series and examine evidence for the effects of nonlinear frequency modulation, wave directional spread, surface current shear, and wind forcing on the likelihood of rogue wave development. Hourly sections of positional time series from 34 surface buoys are examined to identify over 8000 rogue wave events, recording wave sizes, times of occurrence, and geographic locations. The initial dataset is subjected to a quality control process to identify and remove false positives. We investigate the correlation of rogue events with the specified marine environmental factors in an attempt to validate the predictions of earlier theoretical and modeling analyses. The rogue event dataset is contrasted with a control, non-rogue dataset containing a total of nearly 510,000 hourly data segments of surface wave data. The analysis combines the wave records with surface current and wind data from state-of-the-art global coupled models. Statistics of the identified rogue events are summarized, and results of the environmental factor analysis are presented and discussed. This study finds some support for a causal connection between each of the environmental factors and the development of rogue waves. Results also suggest that localized environmental conditions at specific sites, such as seasonal variations in directional spread and/or high surface current vorticity, may provide useful signals of greater rogue wave threat.

Numerical Analysis for Predicting the In-situ Stability and Performance of Ocean Surface Buoy: Static Analysis

It is very important to predict the in-situ stability and performance of ocean surface buoys prior to the their installations in order to prevent their damages and to achieve high quality observation data. In the present study, a numerical method was set up to predict the static behaviors of the single point moored observation buoys subject to stationary loads by wind and/or currents. The numerical analysis was applied to the condition of an existing hydraulic experiment, and it appeared that both numerical and experimental results agreed well within an appropriate range of the drag coefficient of buoy. In addition, through the numerical analysis, the possibility of total submersion of buoy at very high current velocities was demonstrated together with a suggestion of its counter-action.

Multi-Agent-Based Acoustic Sensor Node Deployment in Underwater Acoustic Wireless Sensor Networks

The deployment of acoustic sensor nodes in 3-D underwater acoustic wireless sensor networks (UAWSN) is a difficult task due to various aquatic conditions and physical obstacles. This work proposes multi-agent-based acoustic sensor node deployment (MASD) to deploy the acoustic nodes at ideal positions to enhance coverage and seamless connectivity. The proposed scheme works is threefold: 1) AUV initiates random walk in the network to gather the information and prospective common reference points; 2) the base station gets this information through surface buoys and computes the routing path, feasible locations for deploying new nodes; and 3) AUV collects this information and follows the path to deploy nodes with the help of agents. The multi-agent-enabled deployment framework (MADF) is proposed to support the deployment process at each level of the proposed MASD scheme. The performance of propagation loss, coverage, and overhead tradeoffs are analyzed to validate the proposed scheme. Mobility issues can be further re-investigated in shallow water as a future direction to the MASD scheme.

Design and Application of Buoy Single Point Mooring System with Electro-Optical-Mechanical (EOM) Cable

This paper presents the design of a hybrid system named the Mooring Buoys Observation System for Benthic with Electro Optical Mechanical Cable (MBOSBC) for long-term mooring buoy observation in deep oceans. MBOSBC is comprised of three main modules: a surface buoy, a benthic node, and an Electro-Optical-Mechanical (EOM) cable. The Surface buoy provides energy for the entire system, the Benthic node is used to observe scientific phenomena on the seabed, and the Electro-Optical-Mechanical (EOM) cable connects the sea surface buoy and the benthic observation node, serving as the information and power transmission link. This paper provides design criteria for a single point mooring buoy using an EOM cable. The environmental load of the buoy under varying wind, wave, and current conditions is analyzed and the mooring hydrodynamic force of EOM is calculated. After calculating the load of the EOM cable under extreme marine conditions, the hydrodynamic force needed for the system is analyzed. After physically testing the strength of the designed EOM cable, a near-shore test of MBOSBC was carried out, in order to verify that the system has the expected function.

Building the integrated observational network of “Transparent Ocean”

We live in an exciting era for ocean science, particularly when compared to three decades ago when the ocean was vastly underexplored. Progress can only be made through discovery, observation, understanding of the ocean from the surface to the deep sea, from global to local processes, and from physics, biogeochemistry to ecology. Ocean observation is the basis of oceanic scientific research and is a key to global competition in marine science and technology. Breakthroughs depend on innovation in marine observation and exploration technology. Based on the current frontier issues of ocean science, we report ways to build a three-dimensional observation network of “Transparent Ocean”. The network will effectively promote China’s construction of an international collaborative innovation network for marine science. The three-dimensional observation network “Transparent Ocean” focuses on building an intelligent mobile platform and integrates satellites, fixed platforms, vessel network, and submarine observation systems to make the oceans truly transparent. This network includes four sub-programs. (1) An Ocean Star Cluster Program, which resolves ocean variability at sub-meso and sub-week scales with a centimeter-level accuracy, to partially reveal the marine life system and ecosystem with a 10-m vertical interval in the euphotic layer, moving a significant step forward towards a “Transparent Ocean” down to the vicinity of thermocline both dynamically and bio-optically. (2) An Air-Sea Interface Program, which aims to build an observational network that integrates fixed and mobile platforms to achieve a three-dimensional view of key elements/variables of the air-sea interface. (3) A Starry Deep Sea Program, which builds a deep sea observation system with deep Argo, deep gliders, deep-sea autonomous underwater vehicles (AUVs), intelligent buoys, and other facilities. The system will provide support for information procurement through deep sea navigation and positioning technology. (4) An Undersea Perspective Program, which integrates seabed observation network, communication, navigation and positioning, with intelligent charging technologies to build an intelligent exploration network from water to the seabed. In addition, there is a Deep Blue Brain Project, which will build a marine internet of things with the goal to enhance the ability of intelligent services from data to customers. To date, breakthroughs have been made in research and development of cutting-edge marine technologies, providing new means for ocean observation and exploration. With respect to the ocean observation network, China has built a pilot mooring network with over one hundred sub-surface moorings and almost ten surface buoys in the western Pacific, South China Sea and the Indian Ocean over the past decade. In the next decade, it is essential to integrate observations, process studies and prediction/projection, so as to meet multiple demands of end users.

Argo floats in the southern Baltic Sea

Summary This work aims to familiarize the reader with issues related to modern oceanographic measurement techniques performed by Argo autonomous profiling floats. The opportunity for this is the three years of innovative activity on the part of Argo-Poland in the Baltic Sea. Based on the experience and results acquired by the Institute of Oceanology of the Polish Academy of Sciences (IO PAN), we can say that a revolution in the Baltic Sea monitoring is underway. During three years of activity, the floats launched by IO PAN provided more than 1600 CTD profiles, including 600 O2 profiles. Together with synoptic data from ships, data from moorings and surface buoys, the Argo float measurements are an important part of the southern Baltic monitoring system. Two Argo floats launched by IO PAN collected enough data to determine the dynamics of the oxygen content in various layers, the extent of hypoxic and anoxic zones, and to detect small baroclinic inflows to the Gotland and Gdansk Deeps.

AUV-Aided Localization for Underwater Acoustic Sensor Networks With Current Field Estimation

Accurate sensor localization is a crucial requirement for the deployment of underwater acoustic sensor networks (UASNs) in a large variety of applications. However, the asynchronous clock, stratification effect and mobility characteristics of underwater environment make it challenging to realize accurate node localization for UASNs. This paper develops an autonomous underwater vehicle (AUV) aided localization solution for UASNs, subjected to asynchronous clock, stratification effect and mobility constraints in cyber channels. A hybrid architecture including surface buoys, AUVs, active and passive sensor nodes, is first presented to construct a cooperative location-aware network. Then, an iterative least squares estimator is developed for AUVs to capture the unknown water current parameters, through which the relationship between propagation delay and location estimation can be established. With the assistance of AUVs, two asynchronous localization algorithms are designed to estimate the locations of active and passive sensor nodes. Particularly, motion and ray compensation strategies are jointly employed to improve the localization accuracy. It is worth noticing that, the proposed localization algorithms incorporate the current field estimation into the localization process of UASNs, and more importantly, they can eliminate the influences of asynchronous clock, stratification effect and node mobility together. Moreover, performance analyses for the proposed localization solution are also presented. Finally, simulation and experimental results reveal that the node localization accuracy in this paper can be significantly improved as compared with the other works.