Aspects of the Mechanics and Modeling of Thermally-Induced Stresses in Arctic Pack Ice as Related to Under-Ice Ambient Noise

Abstract

Thermally-induced stresses tend to produce distinct noise spikes at frequencies >500 Hz. However, the exact relationship between stress variations and under-ice noise oscillations are unclear. Surface heating and cooling of ice floes produce vertical strain rate variations which would tend to bend the ice. However, buoyancy and gravitational forces keep typical ice floes flat, so bending stresses develop. Thus, the overall problem of thermal stress in sea ice deals with the propagation of heat through the ice and relating the thermally-induced strain rate to stress at a given depth in the ice. This involves understanding the rheology relating stress to strain. Specific relationships for a thermal stress model for freshwater and sea ice are presented and discussed. Numerical model results are presented, and the calculated stress variations are compared to concurrent under-ice noise variations. These comparisons provide some interesting results. One is that thermal heating can often produce fracturing well below the surface of an ice floe, accounting for some anomalous noise spikes in higher frequency noise records after the passage of atmospheric warm fronts. Also, the sporadic nature of under-ice noise during cooling at night is related to the non-linear, rheological characteristics of sea ice.