Ice Canopy Research Articles

Observed transmissions and ocean-ice-acoustic coupled modelling in the Beaufort Sea

An ocean-ice-acoustic coupled model is configured for the Beaufort Sea. The model uses outputs from a data assimilating global scale ice-ocean-atmosphere forecast to drive a bimodal roughness algorithm for generating a realistic ice canopy. The resulting range-dependent ice cover obeys observed roughness, keel number density, depth, and slope, and floe size statistics. The ice is inserted into a parabolic equation acoustic propagation model as a near-zero impedance fluid layer along with a model defined range-dependent sound speed profile. Year-long observations of transmissions at 35 Hz from the Coordinated Arctic Acoustic Thermometry Experiment and 925 Hz from the Arctic Mobile Observing System source were recorded over the winter of 2019–2020 on a free-drifting, eight-element vertical line array designed to vertically span the Beaufort duct. The ocean-ice-acoustic coupled model predicts receive levels that reasonably agree with the measurements over propagation ranges of 30–800 km. At 925 Hz, seasonal and sub-seasonal ocean and ice driven variations of propagation loss are captured in the data and reproduced in the model.

Oceanographic and climatic influences on Trooz Glacier, Antarctica during the Holocene

To evaluate the significance of recent widespread glacial recession in the Antarctic Peninsula, it is imperative to extend the limited instrumented record of glacier change with long sedimentary archives of the Holocene. Reconstructing glacial histories that capture the variability of environments in the Antarctic Peninsula will enable better constraint of ocean and climate forcings on regional glacial stability. Two sediment cores are analyzed from the 2007 RV/IB N.B. Palmer cruise to Collins Bay, the embayment immediately offshore Trooz Glacier on the Graham Land coast, and provide an archive to test controls on long-term stability of outlet glaciers in an open bay setting. Radiocarbon and Lead-210 (a short-lived radioisotope) age-depth models provide a timeline for Trooz Glacier behavior and associated oceanographic changes over the last 10,000 years.Magnetic susceptibility, grain size, diatom assemblage, total organic carbon, nitrogen, and stable isotopes of organic carbon are utilized as proxies for changing glacial and ocean conditions. Outer Collins Bay deglaciated by ∼10000 cal yr B.P. Diatom abundance and organic content signal productivity increase during further glacial recession from ∼8900 to 6100 cal yr B.P., which coincided with upwelling of relatively warm Circumpolar Deep Water into the bay. High productivity, characterized by Thalassiosira antarctica-dominated diatom assemblages with prominence of Fragilariopsis kerguelensis, indicate open marine conditions with incursion of offshore currents into the bay from ∼6100 to 760 cal yr B.P. A sharp decrease in organic content, diatom abundance, and grain size indicate more sea ice and ice-tongue/ice-canopy advance from ∼760 to 240 cal yr B.P., when the floating ice canopy in outer Collins Bay retreated to near its present-day position and productivity increased. During the period of the 1950's to 1990's, Trooz Glacier was one of only two Antarctic Peninsula glaciers that advanced slightly. Over the Holocene, oceanographic forcing dominated the long-term stability of Trooz Glacier due to its open aspect, and it is unknown how Trooz Glacier will respond to continued influence of Circumpolar Deep Water in the future. Understanding ocean forcings on glacial stability, together with other local controls like drainage basin configuration and bay geometry, should help contextualize the modern retreat and improve prediction of glacial response to prevalent Circumpolar Deep Water circulation in this dynamic region.

Under-ice ambient noise in the Arctic Ocean: observations at the long-term ice station

Under-ice ambient noise in the Arctic Ocean is studied using the data recorded by autonomous hydrophones at the long-term ice station during the 9th Chinese National Arctic Research Expedition. Time-frequency analysis of two 7-s-long ice-induced noise samples shows that both ice collision and ice breaking noise have many outliers in the time-domain (impulsive characteristic) and abundant frequency components in the frequency-domain. Ice collision noise lasts for several seconds while the duration of ice breaking noise is much shorter (i.e., less than tens of milliseconds). Gaussian distribution and symmetric alpha stable (sαs) distribution are used in this paper to fit the impulsive under-ice noise. The sαs distribution can achieve better performance as it can track the heavy tails of impulsive noise while Gaussian distribution fails. This paper also analyzes the meteorological variables during the under-ice noise observation experiment and deduces that the impulsive ambient noise was caused by the combined force of high wind speed and increasing atmosphere temperature on the ice canopy. The Pearson correlation coefficients between long-term power spectral density variations of under-ice ambient noise and meteorological variables are also studied in this paper.

Three-dimensional acoustic propagation effects induced by the sea ice canopy.

This work examines three-dimensional (3D) propagation caused by interaction with the sea ice canopy using an approximate normal mode/parabolic equation hybrid model. The effects of horizontal refraction are assessed through comparison of two-dimensional (2D) and 3D solutions for the modal amplitudes and depth-averaged transmission loss. The following 3D effects are described: diffraction of sound into shadow zones behind ice keels, horizontal defocusing of sound behind ice keels, and horizontal focusing of sound that has propagated between ice keels. A statistical analysis shows that the 3D solution is characterized by 20% greater variance in the depth-averaged transmission loss than the 2D solution.

High-resolution Holocene climate record from Maxwell Bay, South Shetland Islands, Antarctica

The highest resolution Holocene sediment core from the Antarctic Peninsula to date was collected during the first SHALDRIL cruise (NBP0502). Drilling yielded a 108.2-m-long core (87% recovery; site NBP0502–1B) from Maxwell Bay, South Shetland Islands. This high-resolution sediment record comes from a region that is currently experiencing dramatic climate change and associated glacial retreat. Such records can help to constrain the nature of past climate change and causal mechanisms, and to provide a context for evaluating current climate change and its impacts. The base of the drill site sampled till and/or proximal glacimarine sediments resting directly on bedrock. Glacimarine suspension deposits composed of dark greenish gray silty mud with variable diatom abundance and scattered very fine sand laminations make up the majority of the sedimentary section. Detailed sedimentological and geochemical analyses, including magnetic susceptibility, total organic carbon (TOC) content, carbon and nitrogen isotopic composition, pebble content, and biogenic silica content, allow subdivision of the glacimarine section into nine units, and seismic facies analyses resulted in the identification of six distinct seismic units. We used 29 radiocarbon ages to construct an age model and calculate sedimentation rates that vary by two orders of magnitude, from 0.7 mm/a to ?30 mm/a. Radiocarbon ages from glacimarine sediments just above the till date back to between 14.1 and 14.8 ka. Thus, ice was grounded in the fjord during the Last Glacial Maximum and eroded older sediments from the fjord. Following initial retreat of grounded ice from Maxwell Bay, the fjord was covered by a permanent floating ice canopy, probably an ice tongue. The highest sedimentation rate corresponds to an interval that contains abundant sand laminations and gravelly mud intervals and likely represents a melt-out phase or period of rapid glacial retreat from 10.1 ka to 8.2 ka. There is no evidence for an early Holocene climatic reversal, as recorded farther south at the Palmer Deep drill site. Minimum sea-ice cover and warm water conditions occurred between 8.2 and 5.9 ka. From 5.9 to 2.6 ka, there was a gradual cooling and more extensive sea-ice cover in the bay. After 2.6 ka, the climate varied slightly, causing only subtle variation in glacier grounding lines. There is no compelling evidence for a Little Ice Age readvance in Maxwell Bay. The current warming and associated glacial response in the northern Antarctic Peninsula appears to be unprecedented in its synchroneity and widespread impact.

The decay of flexural-gravity waves in long sea ice transects

Flexural oscillations of floating sea ice sheets induced by ocean waves travelling at the boundary between the ice and the water below can propagate great distances. But, by virtue of scattering, changes of ice thickness and other properties encountered during the journey affect their passage, notwithstanding attenuation arising from several other naturally occurring agencies. We describe here a two-dimensional model that can simulate wave scattering by long (approx. 50 km) stretches of inelastic sea ice, the goal being to replicate heterogeneity accurately while also assimilating supplementary processes that lead to energy loss in sea ice at scales that are amenable to experimental validation. In work concerned with scattering from solitary or juxtaposed stylized features in the sea ice canopy, reflection and transmission coefficients are commonly used to quantify scattering, but on this occasion, we use the attenuation coefficient as we consider that it provides a more helpful description when dealing with long sequences of adjoining scatterers. Results show that scattering and viscosity both induce exponential decay and we observe three distinct regimes: (i) low period, where scattering dominates, (ii) high period, where viscosity dominates, and (iii) a transition regime. Each regime’s period range depends on the sea ice properties including viscosity, which must be included for the correct identification of decay rate.

Transport of spring floodwater from rivers under ice to the Alaskan Beaufort Sea

Spring floods carry Alaskan river water north to a frozen Beaufort Sea. A plume of water from the Sagavanirktok River (SR) was identified and traced by measuring salinity, δ18O, and dissolved silica in discrete water samples collected beneath landfast ice in the coastal Alaskan Beaufort Sea from late May to early June 2004 during high river flow. An Optimum Multiparameter analysis was used to calculate the fractions of SR water from the measured geochemical parameters. The SR plume followed the northwestward flowing local circulation and moved ∼17 km north and ∼15 km west under ice from the river mouth. The river plume was ∼1–1.5 m thick beneath the ice and flowed above a persistent halocline in the top 2.5 m of the water column. The calculated volume of SR water beneath the ice on 2 June was ∼0.5 km3, approximately 50% of the river discharge during the study period. The volume of water that was not accounted for was assumed to flow above the ice canopy or was not captured during the study. Interactions of SR water with another under‐ice plume from the Kuparuk River resulted in increased northward transport of both freshwater discharges.

Swath mapping the base of the arctic ice canopy

Satellite data have demonstrated that the areal extent of the Arctic ice canopy has decreased over the past few decades, however, whether sea ice thickness in the Arctic Ocean has substantially decreased remains controversial because of limited information available for the base of the ice canopy. Studies of sea ice thickness to date have involved analysis of ice draft data acquired by single-beam ice profiling sonars on US and British Navy submarines or single-beam moored sonars. During 1998 and 1999, a seafloor-mapping sonar mounted on the hull of the US Navy nuclear submarine USS Hawkbill serendipitously acquired sonar returns from the base of the arctic ice canopy despite design features intended to minimize such returns. Standard topographic processing of the data revealed coherent signals collected between the outgoing sonar pulse and the first returned bottom echoes. Existing processing software was modified to produce swath maps of the information collected from the beginning of ping transmission until the seafloor echoes were detected. These images demonstrate that the recorded water-column signals are different on the port and starboard sides of the system and that individual features can be traced from one side of the submarine to the other. Further software modification allowed us to produce the first wide-swath (2–6 km) sidescan images of the base of the Arctic ice canopy. Although the resultant maps cannot resolve the debate regarding changes in Arctic sea ice thickness, the approach will have far-reaching impact on the design of future systems, whether based on submarines or unmanned vehicles. Our analyses demonstrate that system performance is regulated by the ratio between sonar operating depth and height of the sonar above the seafloor. Where the ratio approaches 1, echoes from the seabed interfere with echoes from the ice canopy, preventing the creation of ice maps. When the sonar is operated below 135 m, signal attenuation results in ice maps with little useful information. However, where the sonar operational depth is less than 135 m and the total water depth ranges between 500 and 1200 m, our approach produces swaths up to 6 km wide exhibiting detailed lineations similar to those observed in higher frequency upward-looking sidescan sonar data.

Ecology of Southern Ocean pack ice

Around Antarctica the annual five-fold growth and decay of sea ice is the most prominent physical process and has a profound impact on marine life there. In winter the pack ice canopy extends to cover almost 20 million square kilometres--some 8% of the southern hemisphere and an area larger than the Antarctic continent itself (13.2 million square kilometres)--and is one of the largest, most dynamic ecosystems on earth. Biological activity is associated with all physical components of the sea-ice system: the sea-ice surface; the internal sea-ice matrix and brine channel system; the underside of sea ice and the waters in the vicinity of sea ice that are modified by the presence of sea ice. Microbial and microalgal communities proliferate on and within sea ice and are grazed by a wide range of proto- and macrozooplankton that inhabit the sea ice in large concentrations. Grazing organisms also exploit biogenic material released from the sea ice at ice break-up or melt. Although rates of primary production in the underlying water column are often low because of shading by sea-ice cover, sea ice itself forms a substratum that provides standing stocks of bacteria, algae and grazers significantly higher than those in ice-free areas. Decay of sea ice in summer releases particulate and dissolved organic matter to the water column, playing a major role in biogeochemical cycling as well as seeding water column phytoplankton blooms. Numerous zooplankton species graze sea-ice algae, benefiting additionally because the overlying sea-ice ceiling provides a refuge from surface predators. Sea ice is an important nursery habitat for Antarctic krill, the pivotal species in the Southern Ocean marine ecosystem. Some deep-water fish migrate to shallow depths beneath sea ice to exploit the elevated concentrations of some zooplankton there. The increased secondary production associated with pack ice and the sea-ice edge is exploited by many higher predators, with seals, seabirds and whales aggregating there. As a result, much of the Southern Ocean pelagic whaling was concentrated at the edge of the marginal ice zone. The extent and duration of sea ice fluctuate periodically under the influence of global climatic phenomena including the El Niño Southern Oscillation. Life cycles of some associated species may reflect this periodicity. With evidence for climatic warming in some regions of Antarctica, there is concern that ecosystem change may be induced by changes in sea-ice extent. The relative abundance of krill and salps appears to change interannually with sea-ice extent, and in warm years, when salps proliferate, krill are scarce and dependent predators suffer severely. Further research on the Southern Ocean sea-ice system is required, not only to further our basic understanding of the ecology, but also to provide ecosystem managers with the information necessary for the development of strategies in response to short- and medium-term environmental changes in Antarctica. Technological advances are delivering new sampling platforms such as autonomous underwater vehicles that are improving vastly our ability to sample the Antarctic under sea-ice environment. Data from such platforms will enhance greatly our understanding of the globally important Southern Ocean sea-ice ecosystem.

Low-frequency long-range propagation and reverberation in the central Arctic: Analysis of experimental results

BBN and the ASW Environmental Acoustic Support program office of ONR conducted a low-frequency reverberation and transmission loss experiment in the central Arctic during the spring of 1992. In this article we report analysis of these data which extracts the controlling propagation and reverberation characteristics. The recorded source levels are combined with the received levels recorded at range to estimate the dependence of the scattering strength of the ice canopy on incident and backscattered grazing angle in frequency bands between 10 and 70 Hz. Results show that the monostatic backscattering strength is roughly proportional to frequency and angle squared, with a nominal value of −47 dB at 40 Hz and 8.5° grazing.

Evidence of recent volcanic activity on the ultraslow-spreading Gakkel ridge.

Seafloor spreading is accommodated by volcanic and tectonic processes along the global mid-ocean ridge system. As spreading rate decreases the influence of volcanism also decreases, and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates (<1.5 cm yr(-1)). Here we present three-dimensional sonar maps of the Gakkel ridge, Earth's slowest-spreading mid-ocean ridge, located in the Arctic basin under the Arctic Ocean ice canopy. We acquired this data using hull-mounted sonars attached to a nuclear-powered submarine, the USS Hawkbill. Sidescan data for the ultraslow-spreading (approximately 1.0 cm yr(-1)) eastern Gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley. The western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999, suggesting that an axial eruption was imaged shortly after its occurrence. These findings demonstrate that eruptions along the ultraslow-spreading Gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought.

Matched-field evaluation of acoustic scattering from Arctic ice

This paper investigates the three-dimensional acoustic scatter from large-scale features under the Arctic ice cover using field data collected during the CEAREX experiments in April 1989. The analysis involves the identification and isolation of the features under the ice canopy, and the simultaneous evaluation of their spatial scattering characteristics. A two-step matched-field processing (MFP) algorithm is used to solve this complex multiparameter estimation problem. Using one large (∼1200 m) and four short (∼300 m) vertical line arrays, and a crossed horizontal hydrophone array in a matched-field processing scheme, beamforming under the ice sheet was performed to obtain an approximate map of the rough interface between ice and water. This analysis yields estimates for scattering strengths for frequencies ranging between 25 and 105 Hz. It is shown that scatter from the elastic ice surface is a frequency selective phenomenon. This beamformed map is then used to isolate a single discrete scatterer, and subsequently, its scattering characteristics are evaluated. Simultaneously, an idealized environmental model is developed to compute scatter from a canonical protuberance. Matching the theoretical results from the analytical model with the experimental data, which consists of contributions from multiple scatterers, yields the statistics of the distribution of ice protuberances.

Wind‐induced ice floe rotation and icequakes north of the Alaskan Beaufort Sea Coast

Linear sea ice canopy fractures (leads) and ice floe rotation appear to be interrelated. This study focused on a highly persistent storm wind event. As the wind stress increased over the ice pack, a separate rotation threshold for four floe‐attached buoys was observed. The northwesternmost buoy recorded the start of clockwise ice‐floe rotation just as hydrophones recorded increased oceanic ambient noise. Several days later, when the surface wind speeds and computed stress at all four buoys dropped to ∼60% and e−1 of their respective peak storm values, a counterclockwise rotation began at each buoy with a concomitant impulsive ambient noise peak. The rotationally‐induced “noise bursts” are called icequakes in an analogy with earthquakes.

Analysis of spatial reverberation statistics in the central Arctic

The horizontal covariance structure of long-range acoustic transmissions measured during the CEAREX 89 experiment is evaluated. Air-gun signatures received at a two-dimensional, horizontal array are processed for signal covariance at bubble pulse frequencies. The modal spectrum of the received signals is compact due to mode stripping, and the signals received on the array have low signal to noise ratio since the source level of the air gun is insufficient to overcome the spreading and scattering losses to the receiving camp. To increase SNR the received waveforms are stacked over 27 shots. The resulting waveforms exhibit the extremely high coherence characteristic of Arctic propagation. The average signal is removed from the stacked waveforms and the spatial covariance of the residual scattered field is estimated at the air-gun bubble pulse frequencies. The resulting statistics show the anisotropy characteristic of low grazing angle rough surface scattering. Based on these observations, an inversion for the spatial statistics of the ice canopy roughness is obtained using an elastic perturbation theory parametrization.

High-frequency forward scattering from the arctic canopy: Experiment and high-frequency modeling

A high-frequency forward scattering experiment carried out under multiyear ice is outlined. The experiment employed an omnidirectional transmitter emitting short tone bursts centered at either 25 or 45 kHz. The receiver was a four quadrant array, and the quadrants were recorded separately. Each quadrant had a broadbeam pattern, greater than 40 (70) deg at the 3-dB down points in the horizontal (vertical) direction at 45 kHz. A large number of transmitter/receiver geometries were obtained by varying both the transmitter depth and horizontal range. Horizontal transmitter/receiver separations ranged from 400 to 2800 m. Model/experiment comparisons are shown for horizontal ranges less than 500 m, since these ranges can be most readily compared with our high-frequency scattering models developed to date, which ignore refraction in the water column. Both individual realizations and ensemble experimental results are shown and compared with models. Specular point theory [D. E. Funk and K. L. Williams, J. Acoust. Soc. Am. 91, 2606 (1992)] is used as a starting point, and then catastrophe theory [P. L. Marston, in Physical Acoustics, Vol. XXI, edited by R. N. Thurston and A. D. Pierce (Academic, 1992); K. L. Williams et al., J. Acoust. Soc. Am. 94, 1890(A) (1993)] is used where inclusion of diffracted effects is essential. The models capture many of the attributes of the data. Much of the difference between models and experiment can be traced to limited knowledge of the ice canopy topography. Although the experiment was carried out along paths where the one-dimensional profile of the under-ice surface had been previously determined with upward looking sonar, and these profiles were used in the models, out of plane scattering could not be included. That would require full two-dimensional knowledge of the under-ice surface.

Modeling of low-frequency transmission loss in the central Arctic

The magnitude of low-frequency (10−100 Hz) propagation loss in the central Arctic is known to be larger than predicted by most free surface scattering theories. This high loss is introduced primarily by scattering at the ice canopy, with which the acoustic path interfers regularly due to the presence of a strongly upward refracting surface duct. While distinct feature scattering theories such as boss or elastic keel models predict higher losses, these results are typically too frequency dependent to agree with the data over the entire band of interest. Perturbation theory gives close to the correct frequency dependence, but historically predicts too little loss when the dynamics of the ice canopy is neglected or the ice is assumed to be locally reacting. In this paper it is proposed that when perturbation theory is extended to include scattering of incident acoustic energy into propagating elastic modes in the ice, the resulting expressions correctly predict low-frequency losses measured in the Arctic.

The Beaufort Sea continental shelf as a seasonal source of atmospheric methane

Methane concentrations in the Beaufort Sea under the winter ice canopy offshore from northern Alaska are 3 to 28 times greater than they are in late summer when the ice is absent in a similar region offshore from northern Canada where methane is in approximate equilibrium with the atmosphere. These observations suggest that methane concentrates in the water under the sea‐ice cover during winter and ventilates rapidly in late summer as the ice melts and retreats. Conditions similar to those on the Beaufort Sea shelf likely exist on the much larger Siberian shelf, making the Arctic Ocean margin a possible seasonal, high‐latitude, marine source of about 0.1 Tg yr−1 atmospheric methane. The small addition of methane likely contributes to the late‐summer increase in atmospheric methane that is observed each year particularly in the northern hemisphere.

Measurement of large scale deformation of the Arctic ice pack using acoustic backscatter

The ONR AEAS Program sponsored a low‐frequency reverberation study program in the Central Arctic as part of the AREA‐92 Exercise in March–April of 1992. The ZIRCON ice camp was the site of the monostatic and bistatic active sonar experiment with acoustic sources, a 256‐channel billboard receive array, and real‐time processing and display system. The reverberation data collected have been coherently processed between sequential pings to estimate the bearing, range, and radial velocity of ice scattering footprints of various range and bearing extents. Due to the highly repeatable nature of reverberation from the ice canopy, high source strength, and the gain of the receiver array, this processing is shown to be capable of unambiguously estimating the movement of these footprints over a 250‐km‐radius area to better than 10‐m accuracy for transmissions as far apart as 4 h. Strain rate estimates using the technique agree with sparse GPS data in the area covered, and are shown to be well modeled by a simple three‐parameter strain rate field. The details of the output from the processing also shed light on the fundamental acoustic propagation and backscatter processes themselves. [Work supported by ONR.]

Modeling and analysis of low‐angle ice canopy reverberation in the Arctic

Recent advances in modeling coherent loss in the Arctic have made it possible to attempt a constrained inversion of monostatic Arctic reverberation for the ice scattering matrix. An analysis is presented of low‐frequency active reverberation data collected in the Central Arctic in 1992 where such an inversion yields meaningful results and compare them to theoretical predictions. Historical and measured ice roughness statistics collected in tandem with the acoustic portion of the experiment were used to obtain elastic perturbation theory predictions of scattering loss and backscatter. These losses were integrated into the Kraken normal mode model to yield highly accurate predictions of the coherent signal structure to very long ranges in the Arctic waveguide, as verified by TL measurements. Based on these predictions, a significant portion of the time/frequency structure of the backscattered return received on a 256‐element hydrophone array was modeled. To construct a well‐conditioned inverse problem, prop...

An Alternative Representation of the Ice Canopy for Calculating Microwave Brightness Temperatures over a Thunderstorm

Passive microwave brightness temperatures (TB's) at 92 and 183 GHz from an aircraft thunderstorm overflight are compared with values calculated from radar-derived hydrometer profiles and a modified proximity sounding. Two methods for modeling particles in the ice canopy are contrasted. The fist is a “traditional” approach employing Marshall–Palmer ice spheres. The second, or “alternative,” method partitions 20% of the ice water content into a Marshall–Palmer component for graupel and hail, and 80% into a modified gamma spherical particle size distribution function representing ice crystals. Results from the alternative approach are superior to those from the traditional method in the anvil and mature convective core. In the decaying convective region, the traditional approach yields better agreement with observed magnitude. Neither method, however, matches the geometry of the observed TB depression associated with the decaying convective core. This is likely due to the presence of graupel, which is not detected as a special signature in radar reflectivity, but does diminish TB's through scattering. Brightness temperatures at the relatively high microwave frequencies considered are shown to be very sensitive to the ice-particle size distribution.