Resonance and interactions of infragravity waves with sea ice

Abstract

Flexural-gravity waves propagating under continuous sea ice can potentially travel considerable distances with negligible reduction of their amplitude, particularly waves that have longer periods such as those in the IG (infragravity) band. This paper investigates resonance from that perspective, explicitly identifying where it might be influencing the formative composition and subsequent evolution of the flexural-gravity wave spectrum by moving energy to longer periods. Several possible manifestations of resonance are appraised, where the term is being applied loosely; some obvious and some less so. For example, the heaving, rolling and flexing of ice floes using NAVMI (nondimensionalized added virtual mass incremental) factors; the natural frequencies of finite sheets of fast ice constrained along some of their boundaries; the initiation and maintenance of trapped edge modes due to reflection at the shore followed by partial total internal reflection as the reflected waves venture out into deeper water again, with the attendant exiting leaky waves that are liberated as part of that process—each shaped by topographical subtleties of local terrain; and a proposition that ice sheets select a preferred frequency at which the wave moves at the same speed in ice as it does in open sea. A data set of observations from the Sea of Okhotsk that includes a broad span of IG waves is employed, as it is these flexural-gravity waves that have the greatest tenacity to propagate over vast distances. Because IG wave energy within the period range 15–70 s is a recurring feature of the measurements collected whilst the sea is ice-covered, this band is a primary focus of the paper. A conceptual model based upon the van der Pol oscillator is also briefly mentioned, to illustrate qualitatively how synchronization could act as a catalyst to some of the resonances that occur.