On the Mechanics of Pressure Ridge Formation in Sea Ice

ABSTRACT One of the most strild.ng features of the Arctic Ocean is the pressure ridges, which are linear accumulations of ice rising to several meters above the ocean surface and tens of meters below. These ridges present a formidable obstacle to both surface and submarine shipping and to transportation on the ice surface. In order to better understand these structures, a computer model of the ridge formation process has been developed that generate ridges whose are in good agreement with observed data. This model is described in a paper published in the Journal of Geophysical Research. Experience with the computer model has suggested additional simplifications that yield analytical models of certain features, such as force displacement relations, maximum height of ridges, and location of cracks in the ice sheet. These analytic models are the subject of the present paper. Significant dimensionless parameters are identified, and parametric studies are presented of the functional relationships between these parameters. The maximum height, crack location and required force are found as friction of the mechanical and geometrical properties of the ice sheet. The problem of forming a ridge from a refrozen lead is considered in detail. Initially, the ridge forms from the thin ice of the lead. Once the lead has closed, the ridge building process must either stop or continue by breading ice from the thick parent sheet. Typically, a force increase of two orders of magnitude is required to make the transition from building a ridge from refrozen lead ice to building it from the parent ice sheet. The large forces are toward the upper end of the range of forces available in the ice sheet, in agreement with the observation that most ridges are built from thin ice blocks. When forces are available and the parent ice is fractured, it is shown that ridges of limited height but unlimited lateral extent can occur. These correspond to the hummock fields that are occasionally observed. The proposed models agree with present observational knowledge and provide predictions that can be tested by future field experiment. INTRODUCTION From the time of man's earliest travels into the arctic, he has known of the existence of ice ridges. These ridges rise several meters above the ocean surface and may extend tens of meters below. Until recently, however, little has been known of the cross-sectional profile of ridges or of the mechanisms by which ridges are formed. Knowledge of ridge size and distribution is of interest because ridges present formidable obstacles to both surface and submarine transportation systems. The forces required to form ridges are an important part of the rheological behavior of pack ice since the compressibility of the pack is determined, in part, by the formation of ridge structures.