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Passive localization and tracking of a mobile emitter, and joint learning of its reverberant three-dimensional (3D) acoustic environment, where critical structural features are unknown, is a key open problem. Unaccounted-for occluders are potentially present, so that the emitter can lose line-of-sight to the receivers, and can only be observed through its reflected raypaths. The locations of reflective boundaries must therefore be jointly estimated with the emitter's position. A multistage global optimization and tracking architecture is developed to solve this problem with a relatively unconstrained model. Each stage of this architecture establishes domain knowledge such as synchronization and initial environment estimation, which are inputs for the following stages of more refined algorithms. This approach is generalizable to different physical scales and modalities and improves on methods that do not exploit the motion of the emitter. In one stage of this architecture, particle swarm optimization is used to simultaneously estimate the environment and the emitter location. In another stage, a Hough transform-inspired boundary localization algorithm is extended to 3D settings, to establish an initial estimate of the environment. The performance of this holistic approach is analyzed and its reliability is demonstrated in a reverberant watertank testbed, which models the shallow-water underwater acoustic setting.

Glacier ice loss impacts sound propagation within Arctic fjords. Regular calving events contribute to a collection of floating ice fragments, known as brash ice, at the ocean surface that obstruct the natural and anthropogenic acoustic signals, yet are difficult to characterize. Transmission loss measurements using a maximum length sequence ( m-sequence) signal were conducted in September 2017 near Hansbreen glacier in Hornsund Fjord, Svalbard with dense brash ice present at the water surface. An acoustic model of the brash ice surface was inferred through consideration of the experimental geometry, arrival amplitude, and travel time difference between the direct and surface reflected arrivals from the source to two receivers. The inferred surface was then incorporated into a forward simulation of the environment using sound speed profiles measured during the experiment. BELLHOP ([Porter and Bucker (1987). J. Acoust. Soc. Am. 82(4), 1349-1359],), a ray tracing code available in the Acoustics Toolbox (HLS Inc., San Diego, CA), was used to track the time difference of arrivals and amplitudes of the modeled direct and surface reflected rays. Comparisons between the measured and simulated results provide insight into the geometric shape and reflection characteristics of the brash ice surface within this and similar environments.

Climate shifts are particularly conspicuous in the Arctic. Satellite and terrestrial observations show significant increases in the melting and breakup of Arctic tidewater glaciers and their influence on sea level rise. Increasing melt rates are creating an urgency to better understand the link between atmospheric and oceanic conditions and glacier frontal ablation through iceberg calving and melting. Elucidating this link requires a combination of short and long-time scale measurements of terminus activity. Recent work has demonstrated the potential of using underwater sound to quantify the time and scale of calving events to yield integrated estimates of ice mass loss (Glowacki and Deane, 2020). Here, we present estimates of subaerial calving flux using underwater sound recorded at Hansbreen, Svalbard in September 2013 combined with an algorithm for the automatic detection of calving events. The method is compared with ice calving volumes estimated from geodetic measurements of the movement of the glacier terminus and an analysis of satellite images. The total volume of above-water calving during the 26 days of acoustical observation is estimated to be 1.7 ± 0.7 × 107 m3, whereas the subaerial calving flux estimated by traditional methods is 7 ± 2 × 106 m3. The results suggest that passive cryoacoustics is a viable technique for long-term monitoring of mass loss from marine-terminating glaciers.

The impact of bubbles on underwater acoustic communication has mostly been studied in environments with strong winds ( $>$ 12 m/s), and it has been found that the main impact is the strong attenuation due to dense bubble clouds. Without continuous replenishment during high winds, these dense bubble clouds dissipate rapidly in a few minutes, along with the strong attenuation. We find that after the dense bubble clouds dissipate, they leave behind a diffused bubble cloud with very small bubbles. The lifetime of this diffused bubble cloud is much longer than that of the dense bubble cloud. The attenuation due to these residual bubbles is small, but they result in an increase in channel variability due to their random motion. Furthermore, because they can be transported by currents to distant locations, we argue that these bubbles have a more persistent impact in many environments, including ones where winds are not very strong, but there are other sources of bubbles (e.g., shipping channels). We give a statistical characterization of the propagation through these bubbles, and show the impact on acoustic communications with experimental data.

Elucidating the influence of oceanic biological activity on the organic composition of sea spray aerosol (SSA) is crucial to understanding marine cloud properties relevant to climate. Numerous mari...

We present a framework for estimating concentrations of episodically elevated high-temperature marine ice nucleating particles (INPs) in the sea surface microlayer and their subsequent emission into the atmospheric boundary layer. These episodic INPs have been observed in multiple ship-based and coastal field campaigns, but the processes controlling their ocean concentrations and transfer to the atmosphere are not yet fully understood. We use a combination of empirical constraints and simulation outputs from an Earth system model to explore different hypotheses for explaining the variability of INP concentrations, and the occurrence of episodic INPs, in the marine atmosphere. In our calculations, we examine the following two proposed oceanic sources of high-temperature INPs: heterotrophic bacteria and marine biopolymer aggregates (MBPAs). Furthermore, we assume that the emission of these INPs is determined by the production of supermicron sea spray aerosol formed from jet drops, with an entrainment probability that is described by Poisson statistics. The concentration of jet drops is derived from the number concentration of supermicron sea spray aerosol calculated from model runs. We then derive the resulting number concentrations of marine high-temperature INPs (at 253 K) in the atmospheric boundary layer and compare their variability to atmospheric observations of INP variability. Specifically, we compare against concentrations of episodically occurring high-temperature INPs observed during field campaigns in the Southern Ocean, the Equatorial Pacific, and the North Atlantic. In this case study, we evaluate our framework at 253 K because reliable observational data at this temperature are available across three different ocean regions, but suitable data are sparse at higher temperatures. We find that heterotrophic bacteria and MBPAs acting as INPs provide only a partial explanation for the observed high INP concentrations. We note, however, that there are still substantial knowledge gaps, particularly concerning the identity of the oceanic INPs contributing most frequently to episodic high-temperature INPs, their specific ice nucleation activity, and the enrichment of their concentrations during the sea–air transfer process. Therefore, targeted measurements investigating the composition of these marine INPs and drivers for their emissions are needed, ideally in combination with modeling studies focused on the potential cloud impacts of these high-temperature INPs.

Studies over the last two decades have shown that submicron particulates (SMPs) can be transferred from the seawater into sea spray aerosol (SSA), potentially impacting SSA cloud seeding ability. This work reports the first concurrent bulk and sea surface microlayer (SSML) SMP (0.4–1.0 μm) measurements, made during two mesocosm phytoplankton blooms in a region devoid of active wave breaking and bubble formation, providing insight into how biological and physicochemical processes influence seawater SMP distributions. Modal analyses of the SMP size distributions revealed contributions from multiple, biologically related particulate populations that were controlled by the microbial loop. With negligible bubble scavenging occurring, SSML enrichment of SMPs remained low throughout both experiments, suggesting scavenging is vital for SMP enrichment in the SSML. Our findings are discussed in the context of SMP transfer into SSA and its potential importance for SSA cloud seeding ability.

Passive localization and tracking of a mobile emitter, and joint learning of its reverberant 3D environment, is a challenging task in various application domains. In underwater acoustic monitoring with a receiver array, for example, a submarine may need to be tracked in a setting with natural and man-made obstacles, such as seamounts or piers. If such obstacles occlude the line of sight from this vessel to the receivers, then the non-line of sight reflected arrivals from the reverberant environment must be leveraged for localization. Hence, we need to precisely map these reflective features in order to deliver robust performance. We propose a multi-stage global optimization and tracking architecture to approach this problem. Each stage of this architecture establishes domain knowledge such as synchronization and occluder mapping, which are inputs for the following stages of more refined algorithms. This approach is generalizable to different physical scales, and improves on methods that do not exploit emitter motion. We further introduce a robust neural network-based reflector estimation method that outperforms its alternatives in realistic application settings. The performance of this holistic approach is analyzed and its reliability is demonstrated both in simulation and in a real-life reverberant watertank, which models shallow-water acoustic environments.

High reverberation, depth-dependent speed of sound and intricate environmental structures are a few of the challenges that make the development of reliable underwater acoustic localization (UAL) methods hard. Since the nontrivial physics of underwater acoustics and the inherent random effects amount to a highly complex statistical model of the measured data, it is natural to consider deep neural networks (NNs) as a central tool in the development of UAL methods. Indeed, recent studies have shown that some properly designed NNs can lead to unprecedented localization capabilities and enhanced accuracy. However, out of the already myriad available possibilities, it is not immediately clear how to choose a proper NN architecture—input structure, layer types, training procedure, loss function(s), etc.—that will lead to successful operation. In this talk, we will present an analytically informed architecture that can learn (for now, in simulations) to localize in complex underwater environments. We will also provide analytical arguments that can explain the robustness of such architectures to certain types of propagation/environment model mismatches. The operation of the discussed architecture will be demonstrated via simulation results in a rich, reverberant channel. The results suggest that deep NNs could become a viable solution to the UAL problem.

Submarine-melting of ice at the glacier-ocean interface accounts for a large portion of the ice-loss at tidewater glaciers and produces sound via bubble-release. The sound production is dominant in the sub-surface region near the glacier-ocean interface. This depth-dependence of the sound is studied by melting ice blocks in a glacial bay at various depths up to 20 m and recording their acoustics over a large frequency range. The acoustic energy decreases with depth in line with expectations from the physics of the phenomenon and is fit to an exponentially decaying curve. The estimated variation will be useful for interpreting the sound in marine-terminating glaciers bays in terms of the submarine-melting activity.

Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. The effects of gravity, surface tension and surface curvature on cylinder breakup are explored. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature and gravity are all important for cases between these two extremes. Predicted unstable mode wavenumber and bubble size show good agreement with direct numerical simulations of plunging breaking waves and air cylinders.

Underwater acoustic localization has traditionally been challenging due to the presence of unknown environmental structure and dynamic conditions. The problem is richer still when such structure includes occlusion, which causes the loss of line-of-sight (LOS) between the acoustic source and the receivers, on which many of the existing localization algorithms rely. We develop a semi-blind passive localization method capable of accurately estimating the source's position even in the possible absence of LOS between the source and all receivers. Based on typically-available prior knowledge of the water surface and bottom, we derive a closed-form expression for the optimal estimator under a multi-ray propagation model, which is suitable for shallow-water environments and high-frequency signals. By exploiting a computationally efficient form of this estimator, our methodology makes comparatively high-resolution localization feasible. We also derive the Cram\'er-Rao bound for this model, which can be used to guide the placement of collections of receivers so as to optimize localization accuracy. The method improves a balance of accuracy and robustness to environmental model mismatch, relative to existing localization methods that are useful in similar settings. The method is validated with simulations and water tank experiments.

We present analysis of a dataset collected at an underwater communications network testbed in Singapore for a contiguous 24-day period in 2018. The communication performance showed strong diurnal variability, mainly due to changes in arrival timings of various ray paths. The observed changes are believed to be caused primarily by sound speed changes due to bubbles advected from the nearby shipping channel. Diurnal noise variability also contributed to performance variability. This work demonstrates the importance of longterm communication datasets in improving our understanding of the acoustic communication channel.

We present a framework for estimating concentrations of episodically elevated high-temperature marine ice nucleating particles (INPs) in the sea surface microlayer and their subsequent emission into the atmospheric boundary layer. These episodic INPs have been observed in multiple ship-based and coastal field campaigns, but the processes controlling their ocean concentrations and transfer to the atmosphere are not yet fully understood. We use a combination of empirical constraints and simulation outputs from an Earth System Model to explore different hypotheses for explaining the variability of INP concentrations, and the occurrence of episodic INPs, in the marine atmosphere. In our calculations, we examine two proposed oceanic sources of high-temperature INPs: heterotrophic bacteria and marine biopolymer aggregates (MBPAs). Furthermore, we assume that the emission of these INPs is determined by the production of supermicron sea spray aerosol formed from jet drops, with an entrainment probability that is described by Poisson statistics. The concentration of jet drops is derived from the number concentration of supermicron sea spray aerosol calculated from model runs. We then derive the resulting number concentrations of marine high-temperature INPs (≥ 253 K) in the atmospheric boundary layer and compare their variability to atmospheric observations of INP variability. Specifically, we compare against concentrations of episodically occurring high-temperature INPs observed during field campaigns in the Southern Ocean, the Equatorial Pacific, and the North Atlantic. We find that heterotrophic bacteria and MBPAs acting as INPs provide only a partial explanation for the observed high INP concentrations. We note, however, that there are still substantial knowledge gaps, particularly concerning the identity of the oceanic INPs contributing most frequently to episodic high-temperature INPs, their specific ice nucleation activity, and the enrichment of their concentrations during the sea-air transfer process. Therefore, targeted measurements investigating the composition of these marine INPs as well as drivers for their emission are needed, ideally in combination with modeling studies focused on the potential cloud impacts of these high-temperature INPs.

Climate-change induced melting is leading to accelerating ice loss at tidewater glaciers worldwide. A significant component of the freshwater flux from these glaciers arises from submarine melting at the glacier-ocean interface. This melting causes a distinct acoustic signature due to the release of pressurized bubbles underwater, opening up the possibility of monitoring this phenomenon on a large scale using passive acoustic systems. To evaluate the use of sound in monitoring submarine glacier melting, we made acoustic measurements using vertical hydrophone arrays in four glacial bays in Svalbard in 2019. As the recording array was moved away from the glacier, the variation in the recorded acoustic level due to melting ice did not follow a uniformly decreasing trend as one might expect. Moreover, the acoustic intensities at different glaciers were clustered at different levels. These observations indicate that the geometry of the glacier-ocean interface, thermohaline structure of the underwater channel and presence of floating ice in the bay played a role in determining the acoustic field. Most of this variation can be explained through propagation modeling. Moving forward, this model-based interpretation of the field will play an integral part in inverting the sound to estimate the submarine melt rate.

Sea spray emissions are an important source for ice nucleating particles (INPs) over remote ocean regions. Over the past years, our understanding of marine organic surfactants acting as INPs has advanced a lot. However, there are still significant knowledge gaps regarding the role of larger marine biogenic particles (e.g. polymers, diatom fragments, protists and bacteria) which are potentially the drivers of episodically observed high INP concentrations.In this study, we use a combination of ARM (Atmospheric Radiation Measurement) observations and output from E3SM (Energy Exascale Earth System Model) simulation runs to investigate the impact of larger marine biogenic particles acting as INPs. We use heterotrophic bacteria and nanogels (polymeric particles) as two hypothesized classes of marine INPs which can get transported across the sea-air interface. Based on the offline-calculated concentrations of these ice nucleating entities in the ocean surface layer, we conduct sensitivity studies to estimate INP concentration ranges, relying on current knowledge of enrichment factors and ice nucleation activities (e.g., ns values from McCluskey et al. (2018)). In comparison to observations of episodic high INP concentrations, our estimated concentrations are consistently lower. However, one of the main conclusions of our study is that large uncertainties regarding the links between ocean biology, organic matter in sea spray and ice nucleation properties, remain. Therefore, comprehensive observational datasets, including sea spray size distributions, aerosol and INP compositions, and ice nucleation efficiencies of individual marine species, are needed.

Arctic glacial bays are among the loudest natural environments in the ocean, owing to heavy submarine melting, calving, freshwater discharge, and ice–wave interactions. Understanding the coherence and vertical directionality of the ambient sound there can provide insights about the mechanisms behind the ice loss in these regions. It can also provide key information for operating technologies such as sonar, communication, and navigation systems. To study the unexplored sound coherence and vertical directionality in glacial bays, a vertical hydrophone array was deployed, and acoustic measurements were made at four glacier termini in Hornsund Fjord, Spitsbergen, in June and July 2019. The measurements show that the sound generated by melting glacier ice is more dominant in the upper portion of the water column near the glacier terminus. The melt water from the submarine melting and the freshwater discharge from the glacier create a glacially modified water duct near the sea surface. This disrupts the inter-sensor vertical coherence in the channel. However, some coherence across the duct is preserved for sound arising from spatially localized events at low frequencies. Overall, the observations in this study can help improve the understanding of the submarine melting phenomenon in glacial bays.

A simulation framework for bubbly flow and the sound radiated by breaking waves is presented. It consists of a two-phase flow solver, an algorithm to track bubbles and bubble creation rates, and a module to compute the sound generated by newly-formed bubbles. The sounds from breaking, third-order Stokes waves of 0.25 m wavelength and two slopes are calculated. The results show encouraging agreement with existing laboratory observations and identify the importance of air cylinder breakup in bubble creation. Remaining problems include modeling boundary effects that inhibit bubble coalescence in seawater and the generation of sound by the breakup of air cylinders.

Dense clouds of bubbles—ranging in scale from 10s of micrometers to cm—are generated by breaking waves on wind-driven seas. Wave-induced bubbles increase ocean albedo and play an important role in air-sea exchange processes by enhancing the transfer of gases and generating sea spray aerosol. Because bubbles both scatter sound efficiently and generate pulses of sound on formation, they create important acoustical effects, including the generation of ambient sound, the attenuation and scattering of sound forward scattered from the sea surface, and enhanced surface backscattering at low grazing angles. Examples of these effects will be illustrated along with the treatment of a bubble cloud acoustical scattering regime characterized by low absorption but high phase distortion relevant to high-frequency, coherent acoustic communications. [Work supported by ONR Ocean Acoustics.]

Ice sheets are expected to be major contributors to sea level rise in the coming decades and centuries. Ocean-forced melting at marine terminating glaciers has the potential to be an important driver of mass loss from these ice sheets, but is difficult to measure directly using conventional techniques, and thus remains poorly understood. The air pressure in bubbles trapped in glacier ice is typically greater than the hydrostatic pressure near the surface of the water. This pressure difference causes the bubbles to expand rapidly and radiate sound when they are released as the ice melts. This sound presents a promising signal which could be used to make passive acoustic measurements of submarine melting at glacier termini, if it can be characterized sufficiently. Prior work has demonstrated that there is considerable variability in the acoustic energy radiated by the release of individual bubbles. Here we show that the total acoustic energy radiated by small blocks of glacier ice as they melt is correlated with the total potential energy stored by the compressed air bubbles in the ice. We find that the conversion efficiency is highest for the most energetic blocks of ice, and present some modelling efforts to explain this trend.

Transmission loss measurements were conducted in the meltwater-modified surface layer near Hansbreen Glacier in Hornsund Fjord in southwestern Svalbard in September 2017 [Deane and Glowacki, JASA 143, 1711 (2018)]. An ITC-1007 (International Transducer Corporation) source emitting an m-sequence signal (149 dB re 1 µPa, 11 kHz carrier frequency) was tethered at 6.7 m depth to a boat allowed to drift 20 m to 200 m from a pair of stationary receivers. These two receivers, HTI-96-Min hydrophones (High Tech, Inc.), were tethered at 8 and 17 m to a buoy anchored 500 m from the glacier terminus. Within this environment, and typical for a glacierized fjord, regular calving events contributed to a collection of icebergs, or brash ice, at the water surface obstructing the transmission path, particularly the surface reflected path. A ray-based approach using Bellhop, a code available in the Acoustics Toolbox, was used to simulate the environment as a means of comparison to the measured data. The variability of the brash ice surface was included in the model through incorporating a simulated surface at the ocean surface. Comparisons between outcomes from increasingly complex iterations of this model with the collected data are discussed.

A technique for Lagrangian tracking of bubbles and detecting their time-evolution behaviors is presented. Five possible behaviors are considered: formation, extinction, continuity, binary fragmentation, and binary coalescence. The technique is based on establishing a network of mappings between bubbles identified at adjacent time instants. The mappings are determined by selecting the minimum from a set of pseudo-distance errors, which are themselves based on constraints imposed on bubble position, velocity, and volume between adjacent time instants. The technique is validated through numerical inspection of the pseudo-distance errors and visual verification of over 16,000 bubble events identified in a simulated breaking wave. The accuracies for continuity, binary fragmentation, and binary coalescence are estimated to be 99.5 % , 90 % , and 95 % , respectively, when the analysis is limited to bubbles of sizes of at least two grid lengths. The effects of varying pseudo-distance error parameters and time resolution are also investigated. The technique robustly tracks bubbles and the occurrence of binary fragmentation and binary coalescence in a breaking wave when these processes occur away from the complex air–water interface structures and bubbles are of comparable scale. Detecting the fragmentation and coalescence of large-scale and complex air–water interfaces remains an outstanding problem.

Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10 and 20 meters in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially-modified waters relative to deeper waters, indicating deeper zones as most appropriate for inter-seasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys. This article is protected by copyright. All rights reserved.

Breaking ocean waves influence wave dynamics, momentum transfer, air–sea exchange, ocean albedo, and ambient noise generation, all of which are impacted by the transient, two-phase flow in a whitecap. Lasting O(1s) or so, actively breaking whitecaps contain air fractions up to 0.6, bubbles ranging in size O(10–1000) $\mu\hbox{m}$ and turbulent dissipation rates O(1) W $\cdot \hbox{kg}^{ - 1}$ . Strong fluid turbulence, high air fractions, large bubbles, and short duration make active whitecaps a challenging process to study. This paper presents a model for the performance of high-frequency Doppler sonar (0.5–2 MHz) when used to probe the interior of actively breaking whitecaps. The results suggest that the ability of high-frequency sonars to penetrate the interior of bubble plumes in whitecaps becomes limited for air fractions greater than 0.03–0.06 and plumes become completely impenetrable for air fractions greater than 0.08–0.17. This severely limits their usefulness as a tool to probe the interior of breaking waves. Moreover, the bias introduced by the terminal rise velocity of large bubbles interacting with fluid turbulence within the wave crest will need to be accounted for when interpreting any backscatter signals that are returned from the plume interior. At this time, in situ methods such as optical fiber probes, conductivity cells, and cameras remain the best option for field studies of the interior of breaking oceanic waves.

Measurements of energy dissipated in breaking laboratory waves, averaged over time and space and directly visualized with a bioluminescent technique, are presented. These data show that the energy dissipated in the crest of the breaking waves is constrained: average turbulence intensity within the crest saturates at around 0.5–1.2 W kg−1, whereas breaking crest volume scales with wave energy lost. These results are consistent with laboratory and field observations of the Hinze scale, which is the radius of the largest bubble entrained within a breaking crest that is stabilized against turbulent fragmentation. The Hinze scale depends on turbulence intensity but lies in the restricted range 0.7–1.7 mm over more than two orders of magnitude variation in underlying unbroken wave energy. The results have important implications for understanding the energetics of breaking waves in the field, the injection of turbulence into the upper ocean, and air–sea exchange processes in wind-driven seas.

Marine-terminating glaciers worldwide are melting rapidly in response to climate shifts, resulting in the delivery of freshwater into the oceans. Submarine melting at the glacier-water interface accounts for a significant component of the freshwater delivery from the glacier. This melting produces a distinct acoustic signature, providing a potentially viable modality to monitor glacial ice melting on a large scale via acoustic sensing. In order to evaluate the utility of ambient noise oceanography as a tool to quantify glacial ice-melt, in June 2019 we deployed a vertical hydrophone array and made acoustic measurements at some glacier termini in Hornsund Fjord, Spitsbergen. Quantification via array processing proves to be challenging due to (1) the space- and time-varying sound-speed profile in the underwater channel, and the way it refracts sound in an unknown manner, (2) limited vertical resolution of the array due to its aperture, and (3) interference from other noise sources such as melting icebergs, for example, contaminating the melt sound recordings. We present preliminary results from the processing which reveal different acoustic levels arising from submarine melting at different glaciers. The sound from the melt seems to be more dominant in the upper layers of the water at the glacier-sea interface.

Melting glaciers and ice sheets are important contributors to sea level rise, but the rates at which they are losing mass are difficult to measure precisely. Using the acoustic signals produced by the rapid release of pressurized air from bubbles contained in the ice is a promising avenue for obtaining quantitative estimates of the rate of submarine melting at the termini of tidewater glaciers. The amplitude and character of the observed average signal generated by the bubble release events have been found to depend strongly upon the hydrostatic pressure of the water the bubbles are released into, and therefore on the depth at which said events occur; this dependence must be better understood before the goal of measuring melting rates can be achieved. Data from field experiments performed at the Hornsund fjord in Svalbard, Norway, in which the acoustic melting signal from blocks of glacier ice was recorded at varying depths, will be presented, and compared to the output of an idealized model for the acoustic signal produced by the release of bubbles from melting ice.

Although it is well known that bubbles have a strong influence on underwater acoustic propagation, typical acoustic communication channel models ignore the effect of bubbles. The rationale is that bubbles are only persistently present in the upper water column in special environments such as the surf zone. In other environments, one expects that the bubbles injected by episodic events such as passing ships quickly rise to the surface or dissolve and do not have a long term effect on the communication channel. We believe that bubbles have a large influence on acoustic communications in many environments with seemingly low bubble injection rates, as bubbles affecting typical communication frequency bands can persist in the water for long periods of time. Such environments include waters near shipping lanes, coral reefs, or places with strong winds. In this paper, we present a model for the persistence of bubbles, and validate it with controlled measurements in a wind-wave channel. We show that bubbles affecting medium-range communication systems can persist in the water column for many tens of minutes after they are injected. This can result in a significant impact on underwater acoustic communications.

Field data is often expensive to collect, time-consuming to prepare to collect, and even more time-consuming to process after the experiment has concluded. However, it is often the practice that such data are used for the research activity that was concomitant with the experiment, and then for little else after the funded research activity is completed. This paper discusses useful steps that can be taken to 1) collect sufficient environmental statistics such that subsequent research can be accomplished long after the experiment has completed, and that results from a given experiment may be reasonably compared with those of another, and 2) prepare signals for transmission and subsequent recording such that research trades for different modulation and coding schemes may be undertaken post-experiment, without the need for retransmission of additional waveforms, and 3) creation of an sufficiently meaningful model and collection of sufficient data to enable post-experimental replay of the environment.

Glacierized fjords are dynamic regions, with variable oceanographic conditions and complex ice-ocean interactions, which are still poorly understood. Recent studies have shown that passive underwater acoustics offers new promising tools in this branch of polar research. Here, we present results from two field campaigns, conducted in summer 2013 and spring 2014. Several recordings with a bespoke two-hydrophone acoustic buoy were made in different parts of Hornsund Fjord, Spitsbergen in the vicinity of tidewater glaciers to study the directionality of underwater ambient noise. Representative segments of the data are used to illustrate the analyses, and determine the directions of sound sources by using the time differences of arrivals between two horizontally aligned, broadband hydrophones. The results reveal that low frequency noise (< 3 kHz) is radiated mostly from the ice cliffs, while high-frequency (> 3 kHz) noise directionality strongly depends on the distribution of floating glacial ice throughout the fjord. Changing rates of iceberg production as seen for example in field photographs and logs are, in turn, most likely linked to signal amplitudes for relevant directions. These findings demonstrate the potential offered by passive acoustics to study the dynamics of individual tidewater glaciers.

Sea spray aerosol (SSA) particles represent one of the most abundant surfaces available for heterogeneous reactions to occur upon and thus profoundly alter the composition of the troposphere. In an effort to better understand tropospheric heterogeneous reaction processes, fundamental laboratory studies must be able to accurately reproduce the chemical complexity of SSA. Here we describe a new approach that uses microbial processes to control the composition of seawater and SSA particle composition. By inducing a phytoplankton bloom, we are able to create dynamic ecosystem interactions between marine microorganisms, which serve to alter the organic mixtures present in seawater. Using this controlled approach, changes in seawater composition become reflected in the chemical composition of SSA particles 4 to 10 d after the peak in chlorophyll-a. This approach for producing and varying the chemical complexity of a dominant tropospheric aerosol provides the foundation for further investigations of the physical and chemical properties of realistic SSA particles under controlled conditions.

Climate-driven ice-water interactions in the contact zone between marine-terminating glaciers and the ocean surface show a dynamic and complex nature. Tidewater glaciers lose volume through the poorly-understood process of calving. A detailed description of the mechanisms controlling the course of calving is essential for the reliable estimation and prediction of mass loss from glaciers. Here we present the potential of hydroacoustic methods to investigate different modes of ice detachments. High-frequency underwater ambient noise recordings are combined with synchronized, high-resolution, time-lapse photography of the Hans Glacier cliff in Hornsund Fjord, Spitsbergen to identify three types of calving events: typical subaerial, sliding subaerial and submarine. A quantitative analysis of the data reveals a robust correlation between ice impact energy and acoustic emission at frequencies below 200 Hz for subaerial calving. We suggest that relatively inexpensive acoustic methods can be successfully used to provide quantitative descriptions of the various calving types.

The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate.

Surfactants are ubiquitous in the global oceans: they help form the materially‐distinct sea surface microlayer (SML) across which global ocean‐atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under‐characterized during conditions of vigorous wave breaking when in‐situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area‐time‐integral of foam evolution is used to develop a time‐varying stabilization function, urn:x-wiley:21699275:media:jgrc22369:jgrc22369-math-0001 and a stabilization factor, urn:x-wiley:21699275:media:jgrc22369:jgrc22369-math-0002, which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind‐driven oceanic whitecaps, and the laboratory and ocean urn:x-wiley:21699275:media:jgrc22369:jgrc22369-math-0003 distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in‐situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10 m/s. The analysis procedure also provides a framework to assess surfactant‐driven variability within and between whitecap coverage data sets.

Air-entraining breaking waves form oceanic whitecaps and play a key role in climate regulation through air-sea bubble-mediated gas transfer, and sea spray aerosol production. The effect of varying sea surface temperature on air entrainment, subsurface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied. By using a breaking wave analog in the laboratory over a range of water temperatures (Tw = 5°C to Tw = 30°C) and different source waters, we have examined changes in air entrainment, subsurface bubble plumes, and surface foam evolution over the course of a breaking event. For filtered seawater, air entrainment was estimated to increase by 6% between Tw = 6°C and Tw = 30°C, driven by increases of about 43% in the measured surface roughness of the plunging water sheet. After active air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. Thereafter, the trend reversed and bubbles degassed more quickly in warmer water. The largest observed temperature-dependent differences in subsurface bubble distributions occurred at radii greater than about 700 μm. Temperature-dependent trends observed in the subsurface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the subsurface bubble plume. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined.

Mike Buckingham’s contributions to underwater acoustics and acoustical oceanography span a broad range of topics, from field expeditions to measure Arctic ambient noise in the marginal ice zone to a complete model for propagation and attenuation in marine sediments. The hallmark of Mike’s work is the formulation of elegant analytical solutions to canonical problems, providing physical insight. I will attempt to illustrate Mike’s approach with examples from his work and illustrations of how it has motivated and influenced future approaches.

Transmission loss measurements were conducted in the meltwater-modified surface layer near Hansbreen Glacier in Hornsund Fjord in southwestern Svalbard in September 2017 [Deane and Glowacki, JASA 143, 1711 (2018)]. An m-sequence source signal (149 dB re 1 μPa, 11 kHz carrier frequency) was tethered at 7 m depth to a boat drifting from 0 to 200 m. This signal was received by two Hitech HTI-96 hydrophones at 8 and 17 m depth deployed from a stationary boat anchored 500 m from the glacier. Within this environment, and typical for a glacierized fjord, regular calving events contributed to an ice melange top boundary layer with larger icebergs occasionally obstructing the signal transmission path. The propagation environment was upward refracting, causing propagation sound to repeatedly reflect from the surface layer. A ray-based approach was applied to model the measured data. The variability of the top boundary was included in the model by incorporating surface scattering and inserting icebergs. Comparisons between several increasingly complex iterations of this model with the collected data will be presented. [Work supported by the NDSEG Fellowship, ONR Grant Nos. N00014-17-1-2633 and N00014-14-1-0213, and the Polish National Science Centre Grant No. 2013/11/N/ST10/01729.]

We explore recent developments and future directions for ambient noise cryology: the study of ice-ocean interactions using their underwater noise signatures. The study of ice-ocean interactions is currently spurred by climatic shifts in polar regions and their implications, which include sea level rise and geopolitical stability. There are many important ice-ocean interactions, and a broad range of observational techniques are used to study them, such as satellite remote sensing, ship-based observations with in situ sensors and AUV’s, boreholes, ground-penetrating radar, photogrammetry, seismometry, and differential GPS. Despite such an extensive suite of techniques, submarine calving and ice melting—which play a critical role in the mass balance of ice shelves and marine-terminating glaciers—remain difficult to measure. Progress in quantifying these processes with their underwater noise signatures will be discussed along with future directions for the field.