Crack formation and breakout of shore fast sea ice in Mordvinova Bay, south-east Sakhalin Island

Abstract

Recognizing that cyclical flexural fluctuations in sea ice sheets stimulated by waves can potentially initiate and perpetuate cracks that may ultimately lead to the destruction of an ice cover, we discuss some winter observations collected beneath the fast ice cover of Mordvinova Bay in the south-east of Sakhalin Island during 2018–2019 when a large crack formed that subsequently detached the offshore sea ice from the inshore ice plate by a wide lead. Energy density spectra show the ramifications of surface-gravity waves from the open ocean—primarily at swell frequencies—penetrating far into the ice from offshore, together with comparatively long, resonant leaky wave modes that gradually dissipate as they propagate alongshore but present a diminishing standing wave envelope of antinodes perpendicular to the coast. By evaluating amplitudes over different ranges of period, three swell wave events are assessed to determine whether they alone could have induced enough bending in the sea ice plate to cause breakage, acknowledging that this would normally occur much closer to the peripheral ice edge where the stresses are largest or as the swells run ashore. The foreshortened seaward standing curvature profile of leaky wave antinodes is also considered as a prospective source of cracking. Whilst neither pathway is found conclusively to be independently responsible for breaking the ice in this case, it is conjectured that the combined influence of swell and leaky waves acting together could cause the observed crack to form. In this case, fracture would occur at a distance of less than about four or so kilometres from the shore, where the antinodal curvatures induced by the standing leaky modes are sufficient. Conceding that the unrelenting flexing of the ice plate by waves and swell could affect the longevity of the sea ice as well, fatigue-precipitated weakening is explored. By determining the value of the mechanical stress at which an ensuing rupture of the fast ice could happen, it is shown that ice destruction can eventuate approximately 17 h after the swell first arrives at the ice edge from the Sea of Okhotsk, appreciating that breakup will depend on prevailing and prior meteorological conditions, the form and state of the sea ice including any preexisting defects, and the specific periods of swell waves that infiltrate the Mordvinova Bay ice sheet.