Motion‐induced stresses in pack ice

Abstract

We consider motion‐induced stresses in pack ice through the analyses of a variety of observations collected during the Sea Ice Mechanics Initiative study conducted in the Beaufort Sea during 1993. Motion‐induced components of in situ stress from stress gauge data are compared to stresses calculated as residuals based on a force balance argument using observed wind, current, and ice motion data. There is reasonable qualitative and quantitative agreement between the observed and calculated motion‐induced stresses in the north‐south direction if the residual stress is assumed to be distributed over a horizontal distance of ∼10 m. To obtain a general agreement with the magnitudes of the observed and calculated stresses in the east‐west direction, the residual stress must be considered to be distributed over a horizontal distance of ∼50 m. There are three significant stress events determined by the force balance calculations, but only the one event in the north‐south direction has a strong corresponding signal in the stress gauge data. There is very little indication of the two events in the east‐west direction in the gauge data. Numerical simulations of the distribution of motion‐induced stresses within a floe show that significant variations in the character of the stresses can occur over short horizontal distances throughout the floe. Hence a seeming lack of a clear correspondence between the observed and calculated stress may be due to our inability to properly recognize the modified signature of the event at the specific locations of the stress gauges. The results suggest that to effectively develop an understanding of the role that point stress measurements can play in developing our understanding of the process of ice deformation, it may be necessary to couple the stress measurements with models of the patterns of motion‐induced stresses within a floe. Finally, we consider the relationship between the residual stress and the differential motion of the ice pack as seen by a cluster of drifters on various floes. The three main stress events seen in the residual stresses all occurred during periods of convergence of the floes. However, we have tested various relationships between stress and strain, and they indicate that there should have been additional stress events as a result of other periods of substantial convergences of the ice pack. This suggests the possibility that the residual stresses were not locally generated.