John R. Marko

Improvements in Upward Looking Sonar-Based Sea-Ice Measurements: A Case Study for 2007 Ice Features in Northumberland Strait, Canada

Scientific and engineering studies of polar and marginal ice zones require detailed information on sea ice thickness and topography. Accurate information on sea ice thickness and topography data is required for basic ice-covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety- and climate change-issues. Since the early 1990s, upward-looking sonar (ULS) instrumentation have been developed and applied to provide under-ice topography data with high horizontal and vertical spatial resolution. Such internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. The ASL Model IPS4 Ice Profiler, which has been widely used in studies of the Arctic Ocean, as well as in numerous seasonal ice zones and in the Southern Ocean, is being upgraded to allow much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. With typical ping rates of 0.5 or 1 Hz, the enhanced capability of the Ice Profiler provides very high resolution measurements of ice keel drafts and the under-ice topography of sea-ice keel features. The Ice Profiler is often used in conjunction with an Acoustic Doppler Current Profiler which provides direct measurements of ice velocity. The combination of high resolution ice draft time series with ice velocities allows for computation of quasi-spatial ice drafts as a function of horizontal distance. The results from the first deployment of an upgraded Ice Profiler, operated just off the Confederation Bridge in Northumberland Strait, from November 2006 to April 2007, are presented and compared with the results of previous sea ice studies at the same location. The much larger onboard data capacity allows for realization of multiple targets for each ping and, on a subsampled basis, offers data on acoustic backscatter returns over the complete water column. This additional information is being analyzed to examine the nature and cause of occasional false target returns. In past measurement campaigns, there have been episodic occurrences of deep targets detected which are not consistent with sea ice features that can be reasonably expected to occur in Northumberland Strait. These features are often associated with occurrences of the largest (spring) tidal currents, leading to the hypothesis that these anomalous targets may be associated with velocity shears in the water column resulting from strong tidal flows past the bridge support structures. Based on these analyses, improvements to the target detection algorithm are being developed and tested.

Iceberg Movement Prediction off the Canadian East Coast

Iceberg models, as presently used in applications off the eastern coast of Canada, are described and summarized. These models can be divided into short, intermediate, and long-term categories, which are associated with: close-in ice management (within 30 km of drill sites); regional iceberg distribution forecasts, and; generalized 3 to 6 month advanced predictions of iceberg severity, respectively. Short- and intermediate-term models are usually further subdivided according to the underlying approach as dynamical, statistical or kinematic. The key features of major modelling category status are given below.

Upward looking ice profiler sonar instruments for ice thickness and topography measurements

Scientific and engineering studies in polar and marginal ice zones require detailed information on sea ice thickness and topography. Until recently, vertical ice dimension data have been largely inferred from aerial and satellite remote-sensing sensors. The capabilities of these sensors are still very limited for establishing accurate ice thicknesses and do not address details of ice topography. Alternative under-ice measurement methodologies continue to be major sources of accurate sea ice thickness and topography data for basic ice-covered ocean studies and, increasingly, for addressing important navigation, offshore structure design/safety, and climate change issues. Upward-looking sonar (ULS) methods characteristically provide under-ice topography data with high horizontal and vertical spatial resolution. Originally, the great bulk of data of this type was acquired from ULS sensors mounted on polar-traversing submarines during the cold war era. Unfortunately, much of the collected information was, and remains, hard to access. Consequently, the development of sea-floor based moored upward looking sonar (ULS) instrumentation, or ice profilers, over the past decade has begun to yield large, high quality, databases on ice undersurface topography and ice draft/thickness for scientific, engineering and operational users. Recent applications of such data include regional oceanographic studies, force-on-structure analyses, real-time ice jam detection, and tactical AUV operations. Over 100 deployments of moored and AUV-mounted ice profiler sonars, associated with an overall data recovery rate of 95%, are briefly reviewed. Prospective new applications of the technology will be presented and related to likely directions of future developments in profiler hardware and software.

Upward looking sonar-based measurements of sea ice and waves

With the recent reduction in summertime ice cover in the Arctic Ocean, year-long moored measurement programs require detailed information on sea ice thickness and topography data throughout most of the year, as well as ocean wave measurements during summer periods of major sea-ice retreat. This information is required for basic ice covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety- and climate change-issues. Since the early 1990s, upward looking sonar (ULS) instrumentation have been developed and applied to providing under-ice topography data with high horizontal and vertical spatial resolution. The internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. In the winter of 2007-2008, a new generation of ULS instrumentation was field tested, initially in Northumberland Strait near the Confederation Bridge separating the Canadian provinces of New Brunswick and Prince Edward Island. With typical ping rates of 1 Hz, the enhanced capability of the Ice Profiler provides very high resolution measurements of ice keel drafts and the under-ice topography of sea-ice keel features. The upgrades intrinsic to the ULS instrument feature much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. The offered combination of much increased dynamic range (via the 16 bit A/D converter) combined with the greatly expanded data storage capacity enables the instrument to operate at much lower gain levels. This facility allows extraction of information on the strength of the backscattering associated with sea-ice in contrast to the larger amplitude acoustic returns from open water, as well as detection of multiple targets from each regular 1 Hz ping. The instruments firmware also provides an ocean wave sampling mode in which a 2 Hz ping rate is used, typically over 20 minutes once each hour, from which non-directional wave spectra and wave parameters can be derived in post processing of the raw data. The new firmware allows the user to program the instrument to operate in up to 12 different sampling schemes over the course of the full deployment. For a typical Arctic Ocean deployment, this enables the instrument to be programmed to measure ocean waves in late summer and early autumn, then both waves and sea ice in autumn, sea ice in the winter and spring, sea ice and waves in the late spring and early summer. These features were utilized in the Northumberland Strait deployment, operated from Nov. 2007 to April 2008, to optimally detect the floating ice cover targets of interest, avoiding alternative false or null targets. Results are also presented on the measurement of ocean waves with wave heights of up to 3 m, and the early winter measurement of scattered ice keels in the presence of ocean waves.

Empirical linkages between Arctic sea ice extents and Northern Hemisphere mid‐latitude column ozone levels

Statistically significant correlations are demonstrated between annual mean column ozone data collected at mid-latitude sites [Kerr, 1991] and mean annual and winter sea ice extents east of Greenland and in the Barents and Kara Seas. These results are discussed with reference to the locations of the correlated parameters relative to the “Basic Pattern” of stratosphere-solar flux correlations [Labitzke and van Loon, 1992]. Possibilities for underlying linkage mechanisms are considered and related to recent decreasing hemispheric ozone level trends.

Investigating Propagation of Short-Period Ocean Waves Into the Periphery of Arctic Pack Ice Using High-Resolution Upward-Looking Sonar

Abstract Springtime fetch in the Cape Bathurst Polynya System may present opportunities for winds to generate waves capable of propagating into the thick pack ice formed over the winter. A waves-in-ice event at a study site located on the Canadian Shelf in the southern Beaufort Sea that occurred 22–23 May 2011 is presented and analyzed for wave energy attenuation and dissipation characteristics. The event was monitored near the ice edge and, therefore, presents information on attenuation of waves from the ice edge into the pack. Waves of T = 5 s, λ = 37.5 m were observed up to approximately 143 m and approximately 77 m away from the ice edge during two separate observation periods of ice edge wave propagation. We estimated reflection coefficients of 53% and 52% and wave attenuation coefficients of α = 2.4 × 10−2 m−1 and α = 5.4 × 10−2 m−1, respectively, for the two periods. Estimated attenuation rates are an order of magnitude greater than in comparable studies and are inconsistent with previous findings of a “rollover” effect in attenuation rates for short-period waves.

Iceberg severity off the east coast of North America in relation to upstream sea ice variability: An update

Earlier examination of strong correlations between mid-winter spatial extents in Davis Strait and large annual variations in the estimated numbers of icebergs passing south of 48°N motivated detailed studies of the origins of variability in iceberg numbers was conducted over twenty years ago [1]. This work established the critical role played by the processes that control the cyclonic movement of the icebergs from their primary West Greenland calving ground to the northern perimeter of Baffin Bay and, subsequently past the coastal shelves of North America. It was demonstrated that the effectiveness of these processes and year to year variations in their timing tended to overwhelm interannual variations in Greenland iceberg calving rates. A major connection between such fluxes and upstream ice extent was established through the effectiveness of extensive pack ice in lowering deterioration and melt rates of freely drifting icebergs during the last, late winter through early summer, segments of their drift trajectories. This role is exercised through the effectiveness of sea ice in lowering the local water temperature and reducing sea state parameters which essentially determine iceberg lifetime in these segments. Comparisons between historical International Ice Patrol (IIP) records of annual south of 48N iceberg numbers and a shorter record (post-1960s) of sea ice extents indicated strong correlations between the former and the January spatial extent of ice in Baffin Bay and Davis Strait north of 67N. Measures of this extent, designated as the Davis Strait ice index (DSII) were subsequently used on their own or with modifications as a basis for early season assessments and apportionments of resources by the IIP and others with interests in East Coast navigation. The iceberg severity off Newfoundland is revisited in the light of several major efforts to document and reorganize East Coast iceberg data-taking and analyses as well as the availability of, roughly, 20 more years of higher quality sea ice and iceberg data. The updated and extended analysis includes assessing impacts of suspected over-counting in early iceberg surveys and access to improved Canadian Ice Service (CIS) digital ice chart data, available from 1971 to the present, which allows better resolution of the sea ice data in marine areas upstream of 48N both in terms of areas of interest and in time. Examination of the 1971-2014 sea ice concentrations in Davis Strait for mid-January reveal that the post 2000 ice extents are consistent with the iceberg numbers being lower than those observed and inferred from the 1980-1999 sea ice concentrations. This extended analysis also discriminates between sea ice types, in particular first year ice vs. the thinner new to young ice categories for regions with three-tenths or more in ice concentration in the DSSII area of interest.

Broadband measurements of acoustic noise under a deteriorating fast ice cover

Results obtained from broadband recording of acoustic data beneath spring and summer ice in Barrow Strait are discussed relative to previous studies of ice noise and the feasibility of monitoring and predicting ice breakup. Recorded data indicate that the generally low frequency and short durations of impulsive ice cracking leave biological and wind-related noise sources as the principal contributors to average spectrum levels over most of the study period. Nevertheless, detailed analyses of ice-generated noise suggest that increases in cracking frequency and shifts in the probability distributions for the mean acoustic energy associated with cracking events may provide useful indicators or predictors of ice breakup.<<ETX>>