Under‐ice noise resulting from thermally induced fracturing of the arctic ice pack: Theory and a test case application

Abstract

Abstract. A theory is presented that relates thermally induced fracturing of pack ice to under-ice noise level variations. It begins with the governing equations for the thermomechanics of pack ice. The thermomechanics relates thermally induced strain rates to the stresses within various vertical layers of the floe. In addition, paradigms are developed which specify the relative quantity of fracturing and stress relief in the floe as the tensile yield strength of the ice is exceeded. The thermomechanics is complemented by an acoustic propagation model that relates the number of fracture events at a given time to the acoustic levels at arbitrary frequency and depth below the ice. The acoustic theory assumes that each fracture acts as a simple monopole source, the fractures are evenly distributed horizontally, and the energy of each fracture propagates through the ice and the water column on the basis of the governing equations for elastic waves in a horizontally stratified medium. The results indicate that noise episodes resulting from fracturing occurring over the top 40 cm of a 160 cm thick floe will propagate over distances of up to 100 km. However, noise episodes associated with fracturing occurring in the lower 100 cm of the floe will only propagate over a range of -10 km. The thermomechanics and acoustic propagation theories were used to develop a numerical model for predicting under-ice noise levels for a given thermal forcing of floes within the arctic ice pack. The model was used to simulate stresses in a multiyear floe and under-ice noise levels at 500 Hz at 305 m below the floe. Model-predicted ice stresses and under-ice noise levels compare quite well to observed stresses and noise variations during the fall of 1988 in the eastern Arctic Ocean. The model predicts that most of the thermally induced, under-ice noise at 500 Hz was a result of fracturing occurring between 5 and 30 cm below the ice surface for a 1.6 m thick multiyear floe.

Measurements of open-water arctic ocean noise directionality and transport velocity

“…The measurements were taken during late summer and early fall between 2008 and 2014 off the North Slope of Alaska, when no local ice fields are present. While ice presence is indeed a feature of the Arctic, which makes it unusual compared to most other ocean basins (Milne and Ganton, 1964;Diachok and Winokur, 1974;Makris and Dyer, 1991;Lewis, 1994;Uscinski and Wadhams, 1999;Stein et al, 2000;Johannessen et al, 2003), the open waters of the Arctic Ocean are fascinating in their own right because they remain, at present, areas without significant anthropogenic noise contributions from shipping at low frequencies (Aulanier et al, 2017;Halliday et al, 2017;Ivanova et al, 2020). In addition, the lack of long-distance swell eliminates most coastal surf noise (Wilson, Jr. et al, 1985).…”

Configuration of PIPS for Thermal Stress Calculations

“…We have undertaken an assessment of incorporating the thermomechanics of pack ice (Lewis, 1998) to allow PIPS3 to predict under-ice noise variations (Stein et al, 2000). The objectives of this year's work are the initial modifications of the PIPS3 thermodynamics (Bitz and Lipscomb, 1999) to include aspects of thermomechanics and quantifying the impact of truncation error for the larger time steps and vertical discritization of the PIPS3 and ice salinity profiles that are allowed to vary in space and time.…”