Lewis H. Shapiro

Aufeis in the Ivishak River, Alaska, mapped from satellite radar interferometry

Abstract Aufeis deposits form every year on many rivers on the North Slope of Alaska; and, through repeated episodes of overflow and freezing, they may reach a thickness of more than 3 m. The presence of aufeis in a drainage basin tends to stabilize river discharge in the same way as a glacier does, by providing melt water during hot dry periods. This distribution of aufeis deposits along the rivers is discontinuous because of variations in channel geometry and under-ice water supply, and monitoring their growth during the winter is generally impractical. However, experiments using satellite radar interferometry (SRI) to map changes in the extent of aufeis near the junction of the Ivishak and the Echooka Rivers in the northern Brooks Range during the winter have produced promising results. Interferograms of the area were made from pairs of images acquired by the European Space Agencys First Earth Remote Sensing Satellite in January through March 1994. The results indicate that aufeis deposits can be mapped on interferograms as areas in river valleys in which the radar phases are poorly correlated and radar backscatter values frequently change. The ability of SRI in monitoring subtle winter aufeis processes fills a major gap in the study of aufeis by remote sensing.

Correlation of nearshore ice movement with seabed ice gouges near Barrow, Alaska

Nearshore patterns of ice motion near Barrow, Alaska, were monitored between 1973 and 1979 with a sea ice radar system. Side scan sonar surveys of the seafloor were made at the same location during the summers of 1977 and 1978. The two data sets provide information on ice motion, ice gouging, and the rate and character of seabed deformation by ice. Four ice stages (open water, freeze-up, winter, and breakup) can be defined on the basis of the frequency and patterns of ice motion and ice cover processes as recorded by the radar system. In all four ice stages the dominant ice motion was nearly parallel to the coast. Side scan sonar surveys show that the principal ice gouge directions nearshore were at a high angle to the coast and to the predominant direction of ice drift in both years. Gouge density was greatest in a narrow zone centered about the 10-m isobath which reflects the distribution of deep ice keels and the pattern of reworking of the seafloor by waves and currents. We believe the majority of observed gouges were formed by keels of multiyear ice floes during an event that occurred in July 1975, 2 years before the first seabed survey. Persistence of the gouge pattern from 1975 until 1977 is attributed to persistence of the ice cover and limited reworking by waves and currents, or additional gouging. The observed decrease in gouge density between 1977 and 1978 is attributed to (1) seafloor reworking and gouge infilling by storms and (2) the absence of ice conditions that were conducive to creating fresh gouges.

Ice motion and driving forces during a spring ice shove on the Alaskan Chukchi coast

An ice shove along the Alaskan Chukchi Sea coast occurred in June 2001, affecting the communities of Barrow and Wainwright, some 150 km apart. Aerial photography before and after the event allowed measurement of ice displacement vectors near Barrow where up to 395 m of ice motion was accommodated almost entirely in discrete ridges up to 5 m high. The forces required to build these ridges are estimated at 35−62 kN m -1 , and driving forces of the whole event are investigated. Most ice deformation at or near the beach coincided with local onshore winds, but the event was preceded by the compaction of pack ice in the central Chukchi Sea and the closure of the coastal flaw lead, driven by the larger-scale wind field acting over several days beforehand. Whether this acted to impart pack-ice stress to the coast or simply to create a critical fetch of consolidated ice is uncertain. The near-melting near-isothermal state of the ice may have been a complicit factor and affected the behavior of the land-fast ice. Coastal morphology and bathymetry affected the location of deformation. This study highlights the range of scales at which processes act and culminate to have implications for Arctic communities.