Walter B. Tucker

Evidence for rapid thinning of sea ice in the western Arctic Ocean at the end of the 1980s

Examination of springtime ice drafts obtained from submarine profiles in a narrow band of the western Arctic Ocean from offshore Alaska to 89°N indicates that the mean ice draft decreased 1.5 m between the mid-1980s and early 1990s. No similar trend was evident in ice drafts near the North Pole. The 1980s drafts were composed largely of ice exceeding 3.5 m, while the early 1990s drafts contained more ice in thinner categories. The differences in drafts between the two periods appear to be related largely to ice dynamics effects associated with the presence and strength of the Beaufort Gyre, which weakened considerably in the early 1990s.

Indirect measurements of the mass balance of summer Arctic sea ice with an electromagnetic induction technique

Abstract In the framework of the Surface Heat Budget of the Arctic (SHEBA) study, indirect, non-invasive ice mass-balance measurements were carried out at a drifting station in the northern Chukchi Sea between May and August 1998. Ice thickness was derived from electromagnetic induction (EM) measurements of apparent conductivity along 13 profiles (60−900 m long). As shown through sensitivity studies with a one-dimensional model, the apparent conductivity data from individual points can be inverted to yield estimates of ice thickness and ablation with an accuracy of approximately 0.05 m (for 2 m thick level ice). Ablation rates were 8−18 mm d−1, with total ablation amounting to roughly 0.9−1.2 m. Measurements of thickness and melt rates along different profiles in undeformed multi-year ice corresponded closely, indicating that the sampling statistics are adequate. The roughness of undeformed ice has been found to increase during the summer due to deepening of melt ponds and enhanced bottom melt. Ice under melt ponds was disproportionately thinner, most likely a result of thicker snow cover reducing winter accretion.

Microwave and physical properties of sea ice in the winter Marginal Ice Zone

Surface-based active and passive microwave measurements were made in conjunction with ice property measurements for several distinct ice types in the Fram Strait during March and April 1987. Synthetic aperture radar imagery downlinked from an aircraft was used to select study sites. The surface-based radar scattering cross section and emissivity spectra generally support previously inferred qualitative relationships between ice types, exhibiting expected separation between young, first-year and multiyear ice. Gradient ratios, calculated for both active and passive data, appear to allow clear separation of ice types when used jointly. Surface flooding of multiyear floes, resulting from excessive loading and perhaps wave action, causes both active and passive signatures to resemble those of first-year ice. This effect could possibly cause estimates of ice type percentages in the marginal ice zone to be in error when derived from aircraft- or satellite-borne sensors.

Physical characteristics of summer sea ice across the Arctic Ocean

Sea ice characteristics were investigated during July and August on the 1994 transect across the Arctic Ocean. Properties examined from ice cores included salinity, temperature, and ice structure. Salinities measured near zero at the surface, increasing to 3–4‰ at the ice-water interface. Ice crystal texture was dominated by columnar ice, comprising 90% of the ice sampled. Surface albedos of various ice types, measured with radiometers, showed integrated shortwave albedos of 0.1 to 0.3 for melt ponds, 0.5 for bare, discolored ice, and 0.6 to 0.8 for a deteriorated surface or snow-covered ice. Aerial photography was utilized to document the distribution of open melt ponds, which decreased from 12% coverage of the ice surface in late July at 76°N to almost none in mid-August at 88°N. Most melt ponds were shallow, and depth bore no relationship to size. Sediment was pervasive from the southern Chukchi Sea to the north pole, occurring in bands or patches. It was absent in the Eurasian Arctic, where it had been observed on earlier expeditions. Calculations of reverse trajectories of the sediment-bearing floes suggest that the southernmost sediment was entrained during ice formation in the Beaufort Sea while more northerly samples probably originated in the East Siberian Sea, some as far west as the New Siberian Islands.

Morphological investigations of first-year sea ice pressure ridge sails

Abstract Sea ice pressure ridge sail heights and the dimensions of ice blocks that comprised the sails were measured for 30 ridges in April, 1980. The ridges were located from 30 to 200 km offshore in the Prudhoe Bay, Alaska region. Sail height was found to be a function of the thickness of the ice in the ridge. A reasonable relationship shows that height is dependent on the square root of block thickness. The data also verify that the ratio of sail height to ice block thickness is much larger for ridges composed of thin ice than for those composed of thick ice. Ridge width and cross-sectional area are also found to be related to block thickness. For the largest blocks in the ridge, block surface area is related to thickness squared, as would be expected from plate deflection theory. Examination of lateral variation of height and width along the ridge shows an expected large variation, particularly with height. No geographic variations of individual pressure ridge morphology were discernible from these data, but a laser profile taken during the study period shows that the mean heights and numbers of ridges decrease as the distance from the coast increases.

Stress measurements in drifting pack ice

Abstract Accurate measurements of in-situ pack ice forces are necessary to improve ice forecasting models and to estimate loads on offshore structures. Two months of in-situ ice stress measurements were obtained in the pack ice of the eastern Arctic during the fall of 1988. Sensors were placed to examine both the horizontal and vertical distributions of ice stresses in multiyear ice. Stresses in the multiyear ice 200 m from the edge of the floe reached 150 kPa during extreme deformation events. Within a few meters of the edge and in adjacent first-year ice, they exceeded 350 kPa on several occasions (400 kPa in one instance) during local ice failure events. Thermally induced stresses at shallow depths in the multiyear ice were caused by rapid temperature changes and could be nearly as large as stresses observed during deformation. The vertical distribution of stresses varied with the type of deformation event, but the largest values were always observed in the upper half of the ice sheet. Stresses due to deformation were rapidly attenuated away from the edge of the floe. Near the edge, however, recorded stresses agreed well with those observed in the adjacent first-year ice. These two locations also experienced twice daily oscillations of about 50 kPa which are apparently tidal or inertially induced.

Seasonal changes in Arctic sea-ice morphology

Abstract The morphology of the Arctic sea-ice cover undergoes large changes over an annual cycle. These changes have a significant impact on the heat budget of the ice cover, primarily by affecting the distribution of the solar radiation absorbed in the ice-ocean system. In spring, the ice is snow-covered and ridges are the prominent features. The pack consists of large angular floes, with a small amount of open water contained primarily in linear leads. By the end of summer the ice cover has undergone a major transformation. The snow cover is gone, many of the ridges have been reduced to hummocks and the ice surface is mottled with melt ponds. One surface characteristic that changes little during the summer is the appearance of the bare ice, which remains white despite significant melting. The large floes have broken into a mosaic of smaller, rounded floes surrounded by a lace of open water. Interestingly, this break-up occurs during summer when the dynamic forcing and the internal ice stress are small During the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment we had an opportunity to observe the break-up process both on a small scale from the ice surface, and on a larger scale via aerial photographs. Floe break-up resulted in large part from thermal deterioration of the ice. The large floes of spring are riddled with cracks and leads that formed and froze during fall, winter and spring. These features melt open during summer, weakening the ice so that modest dynamic forcing can break apart the large floes into many fragments. Associated with this break-up is an increase in the number of floes, a decrease in the size of floes, an increase in floe perimeter and an increase in the area of open water.

Physical Properties of Sea Ice Discharged from Fram Strait

It is estimated that 84 percent of the ice exiting the Arctic Basin through Fram Strait during June and July 1984 was multiyear ice and that a large percentage of this ice is ridged or otherwise deformed. While freeboard and thickness data, together with salinity measurements on cores, usually sufficed to distinguish between first and multiyear floes, preliminary identification could usually be made on the basis of snow cover measurements with snow cover being much thicker on multiyear ice. Cores from the top half meter of multiyear floes were generally very much harder and more transparent than cores from first-year floes. Age estimates of multiyear floes, based on petrographic and salinity characteristics of cores, did not exceed 4 to 5 years for any of the floes that were observed exiting Fram Strait.

Observations and modeling of thermally induced stresses in first-year sea ice

During spring 1992, ice property, geophone, meteorological, and stress data were collected on first-year ice southwest of Cornwallis Island within the Canadian archipelago. One of the goals of the study was to specify the average characteristics of the ice, use these characteristics in a model of thermally induced stresses in the ice, and examine the fracturing associated with the occurrence of those stresses. The results of simulations with a thermal stress model indicate that stress variations within the ice can be reasonably approximated by a viscoelastic rheology. The rheology takes into consideration thermally induced strains generated locally as well as strains generated elsewhere and then mechanically transmitted through the ice. The geophone data showed both ice-borne and water-borne propagation paths for individual fracturing events. The data imply a detection radius out to 500–600 m for the ice-borne signatures of fractures. An investigation of a region after fracturing showed that (1) fracturing occurred in an area with a 10- to 15-cm snow cover, (2) the snow cover had been scored down to the surface of the ice, and (3) cracks in the ice were found under each location where the snow had been scored. The cracks were 5–6 m long and at least 15 cm deep. A review of these and other experimental results draw us to the conclusion that the forces required to produce fractures in response to natural forcing is greater for first-year floes than for multiyear floes.

An update on portable hot-water sea ice drilling

Abstract A portable hot-water drilling system designed for measurment of sea-ice thickness at small horizontal scales is described. The major components of this system are a fuel-oil-fired water heater, a generator to provide power to the oil burner, and a gasoline-powered water pump. The convenience and performance of this system are superior to that of a propane-fired system described in an earlier report.