Ice Underside Research Articles

Risk Analysis of Autonomous Underwater Vehicle Operation in a Polar Environment Based on Fuzzy Fault Tree Analysis

Autonomous underwater vehicles have long been used in marine explorations, and their application in recent polar expeditions is particularly noteworthy. However, the complexity and extreme conditions of the polar environment pose risks to the stable operation of autonomous underwater vehicles. This study adopted the methodology of fuzzy fault tree analysis to deeply analyze the operational risks of autonomous underwater vehicles in polar environments. While traditional fault tree analysis maps the causal relationships and probabilities between basic and intermediate events, fuzzy fault tree analysis models the uncertainty of data and determines the failure probability by integrating expert opinions. This study revealed that polar environment-induced failures play a more substantial role in autonomous underwater vehicle loss in polar regions than inherent system failures. The study identified ‘recovery failure’ and ‘poor communication’ as the major risk factors facing autonomous underwater vehicles in polar environments, exhibiting the highest failure probabilities. Specifically, among various polar environmental factors, ‘large ice concentration’, ‘ice thickness’, and ‘roughness of ice underside’ under ‘bad’ conditions were found to have a significant impact on the autonomous underwater vehicle’s failure probability. The fuzzy fault tree analysis method in this study successfully filled the gap created by the absence of historical data by effectively incorporating expert opinions, enabling a quantitative presentation of the impact of polar environments, which has been previously difficult to convey in qualitative terms.

Ocean turbulent boundary-layer influence on ice crystal behaviour beneath fast ice in an Antarctic ice shelf water plume: The “dirty ice”

The oceanic connection between ice shelf cavities and sea ice influences sea ice development and persistence. One unique feature in regions near ice shelves is the potential for sea ice growth due to crystal accretion on its underside. Here we present observations of ocean boundary-layer processes and ice crystal behaviour in an Ice Shelf Water outflow region from the Ross/McMurdo Ice Shelves. From a fast ice field camp during the Spring of 2015, we captured the kinematics of free-floating relatively large (in some cases 10s of mm in scale) ice crystals that were advecting and then settling upwards in a depositional layer on the sea ice underside (SIPL, sub-ice platelet layer). Simultaneously, we measured the background oceanic temperature, salinity, currents and turbulence structure. At the camp location the total water depth was 536 m, with the uppermost 50 m of the water column being in-situ super-cooled. Tidal flow speeds had an amplitude of around 0.1 m s-1 with dissipation rates in the under-ice boundary layer measured to be up to ε=10-6 W kg-1. Acoustic sampling (200 kHz) identified backscatter from large, individually identifiable suspended crystals associated with crystal sizes larger than normally described as frazil. Measurement of crystals in the SIPL found dimensions of the range 5-200 mm with an average of 93-101 mm depending on the year. The existence and settlement of crystals has implications for understanding SIPL evolution, the structure of sea ice, as well as the fate of Ice Shelf Water.

Roughness and storage capacity of freshwater ice in the Straits of Mackinac

In the event of a maritime oil spill in winter conditions, the fate of oil released under ice presents considerable difficulties with respect to detection, containment and recovery compared to spills that form slicks in open water. This study presents the first quantification of roughness and associated storage capacity of the ice-water interface for freshwater ice in the Laurentian Great Lakes in North America. Ground Penetrating Radar (GPR) observations are used to retrieve ice thicknesses every 2.5 cm on a grid over ice ranging between 0.26 and 0.48 m in the Straits of Mackinac to derive a digital elevation model of the ice bottom. The root mean square of height deviations of the ice underside is observed to be 0.03 m (over a correlation length of approximately 20 m) with a storage capacity of 0.03 m3/m2. The observed physical characteristics relevant to oil entrainment at the ice underside are similar to those reported for first year sea ice and may store similar quantities of oil during the initial spill. However, the difference in the physical structure between first year ice (with brine channels) and freshwater ice (consolidated ice cap) may result in a disparity in the timing of oil appearance, and require considerably different remediation treatment.

Seasonal Evolution of Light Transmission Distributions Through Arctic Sea Ice

AbstractLight transmission through sea ice is a critical process for energy partitioning at the polar atmosphere‐ice‐ocean boundary. Transmission of sunlight strongly impacts sea ice melting by absorption, as well as heat deposition, and primary productivity in the upper ocean. While earlier observations relied on a limited number of point observations, the recent years have seen an increase in spatially distributed light measurements underneath sea ice using remotely operated vehicles covering a wide range of ice conditions. These measurements allow us to reconstruct the seasonal evolution of the spatial variability in light transmission. Here we present measurements of sea ice light transmittance distributions from 6 years of Arctic under‐ice remotely operated vehicle operations. The data set covers the entire melt period of Central Arctic sea ice. Data are combined into a pseudo time series describing the seasonal evolution of the spatial variability of sea ice optical properties from spring to autumn freezeup. In spring, snowmelt increases light transmission continuously, until a secondary mode originating from translucent melt ponds appears in the histograms of light transmittance. This secondary mode persists long into autumn, before snowfall reduces overall light levels again. Comparison to several autonomous time series measurements from single locations confirms the detected general patterns of the seasonal evolution of light transmittance variability. This also includes characteristic spectral features caused by biological processes at the ice underside. The results allow for the evaluation of three different light transmittance parameterizations, implying that light transmission in current ice‐ocean models may not be accurately represented on large scales throughout all seasons while ice thickness alone is a poor predictor of light transmittance.

Observations of amplified roughness from crystal accretion in the sub‐ice ocean boundary layer

AbstractIce crystal accretion on the underside of sea ice and ice shelves, a signature of pressure‐induced supercooling, has the potential to alter the energy balance in the ocean boundary layer through enhanced hydrodynamic roughness. Here we present estimates of crystal‐driven ocean boundary layer roughness in supercooled water beneath sea ice adjacent to the McMurdo/Ross Ice Shelf. Data were collected from four sites in McMurdo Sound, Antarctica, between 2007 and 2015, and represent a range of ice shelf‐affected conditions. The results show that drag of the rough ice underside in the presence of platelets is 6–30 times larger than typical levels homogeneously applied in ice‐ocean interaction models. The crystal‐enhanced drag promotes increased entrainment into the boundary layer from the upper ocean, which has the potential to affect ice shelf evolution and sea ice growth through enhanced turbulent exchange of heat and momentum.

Distribution of algal aggregates under summer sea ice in the Central Arctic.

The sea ice cover of the Arctic Ocean has changed dramatically in the last decades, and the resulting consequences for the sea-ice-associated ecosystem remain difficult to assess. Algal aggregates underneath sea ice are of great importance for the ice-associated ecosystem and the pelagic-benthic coupling. However, the frequency and distribution of their occurrence is not well quantified. During the IceArc expedition (ARK-27/3) of RV Polarstern in late summer 2012, we observed different types of algal aggregates floating underneath various ice types in the Central Arctic basins. We investigated the spatial distribution of ice algal aggregates and quantified their biomass, using under-ice image surveys obtained by an upward-looking camera on a remotely operated vehicle. On basin scale, filamentous aggregates of Melosira arctica are more frequently found in the inner part of the Central Arctic pack ice, while rounded aggregates mainly formed by pennate diatoms are found closer to the ice edge, under melting sea ice. On the scale of an ice floe, the distribution of algal aggregates in late summer is mainly regulated by the topography of the ice underside, with aggregates accumulating in dome-shaped structures and at the edges of pressure ridges. The average biomass of the aggregates from our sites and season was 0.1–6.0 mg C m−2. However, depending on the approach used, differences in orders of magnitude for biomass estimates may occur. This highlights the difficulties of upscaling observations and comparing results from surveys conducted using different methods or on different spatial scales.Electronic supplementary materialThe online version of this article (doi:10.1007/s00300-014-1634-3) contains supplementary material, which is available to authorized users.

Antarctic sea ice thickness affects algae populations

In the waters off Antarctica, algae grow and live in the sea ice that surrounds the southern continent—a floating habitat sure to change as the planet warms. As with most aquatic ecosystems, microscopic algae form the base of the Southern Ocean food web. Distinct algae populations reside in the sea ice surface layers, on the ice's underside, and within the floating ice itself. The algae that reside on the floating ice's underside are particularly important for the region's krill population, while those on the interior or surface layers are less accessible. Understanding how changing sea ice properties will affect the regional biology, then, depends on understanding how algae populations interact with the ice.

Changing characteristics of arctic pressure ridges

The advent of multibeam sonar permits us to obtain full three-dimensional maps of the underside of sea ice. In particular this enables us to distinguish the morphological characteristics of first-year (FY) and multi-year (MY) pressure ridges in a statistically valid way, whereas in the past only a small number of ridges could be mapped laboriously by drilling. In this study pressure ridge distributions from two parts of the Arctic Ocean are compared, in both the cases using mainly data collected by the submarine “Tireless” in March 2007 during two specific grid surveys, in the Beaufort Sea at about 75° N, 140° W (N of Prudhoe Bay), and north of Ellesmere Island at about 83° 20′ N, 64° W. In the Beaufort Sea the ice was mainly FY, and later melted or broke up as this area became ice-free during the subsequent summer. N of Ellesmere Island the ice was mainly MY. Ridge depth and spacing distributions were derived for each region using the boat's upward looking sonar, combined with distributions of shapes of the ridges encountered, using the Kongsberg EM3002 multibeam sonar. The differing shapes of FY and MY ridges are consistent with two later high-resolution multibeam studies of specific ridges by AUV. FY ridges are found to fit the normal triangular shape template in cross-section (with a range of slope angles averaging 27°) with a relatively constant along-crest depth, and often a structure of small ice blocks can be distinguished. MY ridges, however, are often split into a number of independent solid, smooth blocks of large size, giving an irregular ridge profile which may be seemingly without linearity.Our hypothesis for this difference is that during its long lifetime an MY ridge is subjected to several episodes of crack opening; new cracks in the Arctic pack often run in straight lines across the ridges and undeformed ice alike. Such a crack will open somewhat before refreezing, interpolating a stretch of thin ice into the structure, and breaking up the continuity and linearity of the ridge crest. Many such episodes over a number of years can cause the ridge to become simply a series of blocks. This has implications for ridge strength and for permeability to spilled oil. As the percentage of MY ice in the Arctic diminishes, Arctic ridging will be more and more dominated by FY ridges, and we discuss the implications of this change of character of the ice underside in the light of the statistics that we have generated for the two types of ridge.

The use of autonomous underwater vehicles to map the variability of under-ice topography

We review the development of autonomous underwater vehicle (AUV) use under sea ice to map the three-dimensional (3-D) structure of the ice underside. The author, after extensive experience in under-ice profiling from submarines using single-beam sonar, carried out the first under-ice sidescan sonar profiling from an AUV in 2002 in the Greenland Sea. This was followed in August 2004 by the first full multibeam sonar experiment, using Kongsberg EM2000 sonar aboard the Autosub-II vehicle off NE Greenland. Two experiments using a small Gavia vehicle deployed through holes in the ice followed in 2007 and 2008, in the Beaufort Sea and off Ellesmere Island. Examples of the 3-D imagery are shown, and the two approaches of using a large vehicle deployed from a ship and a small through-ice vehicle are compared and found to be complementary. The imagery has shown that although first-year (FY) ridges have the familiar shape of a triangular prism made of small ice blocks, multi-year (MY) ridges are found to be broken up by lead formation into a chain of individual large ice blocks rather than a coherent linear feature. New work and future plans are described.

Living conditions, abundance and composition of the metazoan fauna in surface and sub-ice layers in pack ice of the western Weddell Sea during late spring

Abstract The surface and sub-ice layer habitats and their metazoan fauna were studied on a drifting pack-ice floe in the western Weddell Sea from 29 November 2004 to 1 January 2005 during the “Ice Station POLarstern” (ISPOL). Flooding of the floe occurred at some places, and the establishment of surface layers with a brownish colour due to growing algae was observed at several sampling sites. The average surface-layer temperature, brine salinity and brine volume were −1.4 °C, 25.3 and 54%, respectively. The temperature–salinity relationship in the surface layer was seldom at equilibrium conditions. Chlorophyll a (Chl a) concentrations in the brine varied between 1.0 and 53.5 μg L−1. Surface-layer thickness, salinity, Chl a concentration and copepod abundances were generally higher at the edge of the floe than in the inner part. The sympagic copepod species Drescheriella glacialis/racovitzai and Stephos longipes, with abundances ranging between 0 and 3830 ind. L−1 (median: 2 ind. L−1) and 0 and 1293 ind. L−1 (median: 4 ind. L−1), respectively, were the dominant members of the surface-layer meiofauna. Their populations consisted mainly of adults and early naupliar stages, which points to an active reproduction of these species within the surface layer. Other taxa found in the surface layer were undetermined turbellarians, the gastropod Tergipes antarcticus, and, for the first time, the ctenophore Callianira antarctica, and the amphipods Eusirus antarcticus and Eusirus tridentatus. During the course of our study, slight melting at the ice underside took place, releasing sympagic organisms to the water column. Chl a concentrations in the sub-ice water layer were very low (0.1–0.5 μg L−1), except for 25 December when the Chl a concentration at 0 m depth increased to 2.3 μg L−1. The most dominant sympagic copepod species found in the sub-ice layer was Ectinosoma sp., with abundances ranging between 1 and 599 ind. m−3 (median: 25 ind. m−3). Other sympagic copepod species occurring regularly in this habitat were D. glacialis/racovitzai, Diarthrodes cf. lilacinus, Idomene antarctica and S. longipes. All of these sympagic species were generally found in higher abundances at 0 m depth underneath the ice than at 5 m depth, in contrast to pelagic copepod species that occurred more frequently at 5 m depth. Niche separation and probable life-cycle strategies of dominant sympagic metazoans are discussed.

Living conditions, abundance and biomass of under-ice fauna in the Storfjord area (western Barents Sea, Arctic) in late winter (March 2003)

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7–10/10) prevailed and ice thickness ranged over <0.1–1.6 m covered by <0.1–0.6 m of snow. Air temperatures ranged between −1.8 and −27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0–5 m depth) were not stratified (T=−1.9 to −2.0°C and S=34.2–34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 μg chlorophyll a l−1), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2–241.3 and 5.3–16.4 μg l−1, respectively, the C/N ratio over 11.2–15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0–1.8 ind. m−2 for Apherusa glacialis, 0–0.7 ind. m−2 for Onisimus spp., and 0–0.8 ind. m−2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181–2,487 ind. m−3 (biomass: 70–2,439 μg C m−3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34–1,485 ind. m−3), contributing 19–65% to total abundances, followed by copepod nauplii (85–548 ind. m−3) and the cyclopoid copepod Oithona similis (44–262 ind. m−3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.

Occurrence of Anonyx sarsi (Amphipoda: Lysianassoidea) below Arctic pack ice: an example for cryo-benthic coupling?

During a winter expedition to the western Barents Sea in March 2003, benthic amphipods of the species Anonyx sarsi were observed directly below pack ice. Only males and juveniles [16.0–37.0 mm long, 16.2–120.8 mg dry mass (DM)] were collected. Guts contained macroalgal fibres, fish eggs and flesh from large carrion. Amphipods had very low levels of total lipids (2.7–17.2% DM). Analysis of lipid biomarkers showed that some of the specimens had preyed on pelagic copepods. Individual respiration rates ranged over 0.4–1.7 ml O2 day−1 (mean: 1.2 ml, SD: 0.5 ml). Individual ammonia excretion rates varied between 7.8 μg and 49.3 μg N day−1 (mean: 30.7 μg, SD: 15.2 μg). The atomic O:N ratio ranged over 35 to 71 (mean: 55, SD: 14), indicating lipid-dominated metabolism. Mass-specific respiration ranged over 9.8–16.6 ml O2 day−1 g DM−1 (mean: 13.1 ml, SD: 2.2 ml). The metabolic rates of A. sarsi were twice as high as those of the truly sympagic amphipod Gammarus wilkitzkii, which is better adapted to the under-ice habitat by its energy-saving attached lifestyle. It is concluded that males and juveniles of A. sarsi were actively searching for food in the water column and at the ice underside, but that the nutritional status of the amphipods in late Arctic winter was generally very poor.

Carnivorous feeding and respiration of the Arctic under-ice amphipod Gammarus wilkitzkii

The dominant Arctic under-ice amphipod Gammarus wilkitzkii consumes a wide range of food items. The carnivorous feeding activity and energy budget of this large species were studied using three different approaches. Maximum potential ingestion rates Imax estimated from an allometric function taken from the literature and based on body mass were 2.1±0.4% of body carbon day–1. Based on respiration measurements, the specific ingestion rates required to meet metabolic demands were lower (1.4±0.4% of body carbon day–1). Feeding experiments, in which co-occurring pelagic calanoid (Calanus hyperboreus) or sympagic harpacticoid (Halectinosoma sp.) copepods were offered as prey, yielded actual ingestion rates of 8.0±5.6% of body carbon day–1 and 0.1±0.1% of body carbon day–1, respectively. These results indicate that predatory feeding on pelagic copepods may constitute an important food source for G. wilkitzkii. Abundances of G. wilkitzkii at the ice underside (median: 1.6 ind. m–2), Calanus spp. in the upper metre below the ice (2.6 ind. m–3), and Halectinosoma sp. in the lowermost 2–3 cm of the ice (393.5 ind. m–2) were determined from several multi-year pack-ice floes in the northern Greenland Sea and Fram Strait. Potential predation impact of G. wilkitzkii was estimated by combining information on ingestion rates with population densities. It was very high on Calanus spp. in the under-ice water layer (61.5% of the under-ice standing stock day–1), but comparatively low on Halectinosoma sp. in the bottom of the ice (3.8% of standing stock day–1). The observation of G. wilkitzkii preying on pelagic copepods in the under-ice water layer represents a hitherto unknown but obviously significant process and a new direction in the cryo-pelagic coupling in the Arctic marine ecosystem.

Under-ice amphipods in the Greenland Sea and Fram Strait (Arctic): environmental controls and seasonal patterns below the pack ice

During three "Polarstern" cruises to the ice-covered Greenland Sea (spring 1997, summer 1994, autumn 1995) studies on the under-ice habitat (morphology, hydrography, ice-algal biomass) and on the macrofaunal, autochthonous under-ice amphipods (species diversity, abundance) were carried out in order to describe environmental controls and seasonal patterns in this community. In spring, the ice underside was rather smooth and whitish, while in summer melting structures and sloughed-off ice-algal threads were observed, in autumn detritus clumps accumulated in depressions at the ice underside. Only in summer, a thin layer of warm (up to –0.6°C) and less saline (as low as S=6.3) water was found at the ice–water interface above Polar Water. Integrated ice-algal biomass was highest during autumn (2.6 mg chl a m–2) and lowest during summer (1.2 mg chl a m–2). Four species of under-ice amphipods occurred in spring and summer (Apherusa glacialis, Onisimus glacialis, O. nanseni, Gammarus wilkitzkii), but only the last species was observed at the ice underside in autumn. A. glacialis and G. wilkitzkii were equally abundant in spring; A. glacialis dominated in summer. The highest total abundance of amphipods occurred during summer (31.9 ind. m–2), compared to lower abundances in spring and autumn (5.3 and 1.1 ind. m–2, respectively). A factor analysis revealed seasonal patterns in the data set, which mainly influenced A. glacialis, and species-specific relations between several environmental factors and the distribution of under-ice amphipods. Abundance of A. glacialis was closely related to the under-ice hydrography and ice-algal biomass, whereas the other amphipod species were more influenced by the under-ice morphology. It is therefore stated that the observed thinning of the Arctic sea ice and the resulting increased meltwater input and change in morphology of floes will have a profoundly adverse effect on the under-ice amphipods.

The Experimental Study on The Fluid Resistance of trapped Ice Floes

The Okhotsk Sea of Hokkaido is covered with drift ice in winter. Drift ice that rushes into Saroma Lagoon cause heavy damage to scallops and oysters, and aquacultural facilities. So, an ice boom that prevents drift ice from rushing into the lagoon, especially one with a net at the center of lagoon mouth to completely shut out the drift ice, started being constructed in 1994, and 80 % is now completed. The ice force acting on this ice boom is determined by fluid force and is closely related to the shape of the ice underside. This paper reports on the results we obtained from model experiments on the fluid force of water acting on ice floe and a rubble field of ice.

Grazing of Arctic under-ice amphipods on sea-ice algae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 160:93-99 (1997) - doi:10.3354/meps160093 Grazing of Arctic under-ice amphipods on sea-ice algae Iris Werner* Sonderforschungsbereich 313, University of Kiel, Institute of Polar Ecology, Wischofstr. 1-3, D-24148 Kiel, Germany *E-mail: iwerner@ipoe.de Feeding of the Arctic under-ice amphipods Apherusa glacialis, Onisimus spp. and Gammarus wilkitzkii on sea-ice algae was investigated in laboratory experiments. The specific ingestion rates of algal biomass declined with increasing size of the species (A. glacialis, 51; Onisimus spp., 11; G. wilkitzkii, 2 ng chl a equival. mg-1 dry mass d-1). Juveniles of Onisimus spp. and G. wilkitzkii had specific rates that were 1 to 2 orders of magnitude higher than those of the respective adults. Calculations of the carbon budget and observations of amphipods behaviour indicate that A. glacialis is predominantely herbivorous, whereas Onisimus spp. are herbivorous/detritivorous, and G. wilkitzkii is herbivorous/detritivorous/carnivorous. The grazing impact on the ice-algal standing stock at the ice underside in summer, estimated for 2 different areas in the Arctic, was low (1.1% d-1 in the Laptev Sea, 2.6% d-1 in the Greenland Sea), indicating that food is not a limiting factor in this habitat. Ingestion rate · Grazing impact · Sea ice · Amphipoda · Arctic Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 160. Publication date: December 15, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Roughness of Weddell Sea ice and estimates of the air‐ice drag coefficient

The roughness of a sheet of sea ice encodes its deformational history and determines its aerodynamic coupling with the overlying air and underlying water. Here we report snow surface, ice surface, and ice underside roughness computed from 47 surface elevation profiles collected during a transect of the Weddell Sea. The roughness for each surface, parameterized as the standard deviation of the surface elevation, segregates according to whether or not a floe has been deformed: deformed ice has greater roughness than undeformed ice. Regardless of deformational history, the underside roughness is almost always greater than the snow surface and ice surface roughnesses, which are nearly equal. Roughness spectra for all three surfaces and for both deformed and undeformed ice roll off roughly as k−1 when the wavenumber k is between 0.1 and 3 rad m−1. The snow surface and underside spectra roll off somewhat faster than k−1, and the ice surface spectra roll off somewhat slower than k−1. Both top and underside Arctic ice roughness spectra, on the other hand, have been reported to roll off faster than k−2. We speculate that the excess spectral intensity at high wavenumbers in the Antarctic ice surface spectra results from the small‐scale roughness that the ice sheet had on consolidation. This excess high‐wavenumber spectral intensity persists in the ice surface spectra of second‐year ice. Evidently, once formed, the ice surface remains unchanged on the microscale until the entire ice sheet melts. With a remote measurement of roughness, we should be able to decide whether an ice floe is deformed or undeformed. Our spectral analysis hints that remote sensing may also be able to differentiate between first‐year and second‐year ice. From the snow surface spectra, we compute a roughness scale ξ that parameterizes the air‐ice momentum coupling and lets us estimate the neutral stability drag coefficient referenced to a height of 10 m, CDN10. Typical CDN10 values are 1.1–1.4 × 10−3 over undeformed ice and 1.3–1.8 × 10−3 over deformed ice.

Statistical properties of Arctic sea ice morphology derived from sidescan sonar images

Using a transparent overlay technique, the authors have classified a 140km long stretch of sidescan sonar imagery of the Arctic sea ice underside (700m swath width) into regions occupied by unique ice-types. The categories used were multi-year ice, first year ice, pressure ridge, apparently open (‘dark’) lead, apparently refrozen (‘bright’) lead and randomly deformed ice. The technique was validated by comparing these classifications with data obtained simultaneously using a narrow beam upward-looking sonar. The pattern of ice-types thus observed was tested statistically for associations between adjacent regions, the distribution of lead widths, spacings and orientations and the distribution of ridge spacings and orientations. Significantly non-random sequences of ice-types were observed, lead orientations and spacings were clustered and were set to preferred values and ridge spacings fitted a negative exponential distribution. The extent of uniformly deformed ice (with no apparent linearity) was much greater on the underside of the ice than has ever been observed on the upper surface.

Inferring Ice/Ocean Surface Roughness from Horizontal Current Measurements

Abstract Turbulence measurements in the underice boundary layer from two Arctic drift stations are used to develop a method for estimating the small-scale roughness, zo, of the ice underside from horizontal current and current variance, sampled at one level. Horizontal variance is shown to be well correlated with turbulent kinetic energy (TKE). Measurements also indicate that at depths where turbulence is fully developed to the surface roughness, shear production of TKE is approximately in balance with viscous dissipation, so that the magnitude of local horizontal stress is proportional to flow variance. A similarity model is used to extrapolate local stress to the interface, and zo is estimated from the logarithmic profile for current speed. The method has application for using remote data buoys, equipped with “smart” current meters, for mapping the underice roughness.

The Underside of Arctic Sea Ice Imaged by Sidescan Sonar

Despite extensive submarine operations in the Arctic, little is yet known about the three-dimensional morphology of the ice underside. Several studies have been done using upward-looking sonar1–7, which yields a linear profile of ice draft from which the distributions of features such as pressure ridges and leads may be derived. Such a profile gives no information about the spatial morphology or appearance of the ice bottom surface. In contrast, the upper ice surface has been well documented from airborne and satellite-borne sensors such as synthetic aperture radar, passive microwave, infrared and visual imaging. Here I report the first extensive set of imagery of the underside of sea ice in the Arctic Ocean. Sidescan sonar images show a distinct difference between the appearance of first-year ice and of multi-year ice. First-year ice has a smooth bottom except for a network of cracks, while multi-year ice is covered with an array of blisters or bulges. The clear discrimination between ice types enables sidescan sonar to be used as a means of validating airborne passive microwave data.