How sea ice can affect coastal swells, infragravity waves and leaky wave modes: Spectral adaptation from modulation

Abstract

Several months of seafloor-mounted, pressure-transducer data collected in Mordvinova Bay off the south-east coast of Sakhalin Island in the Sea of Okhotsk are used to interpret surface-gravity waves across a wide range of periods, during intervals when the sea surface was free of sea ice, covered by drift ice of 0.5–1 m thickness, and when both drift ice and consolidated shore fast sea ice was present. A substantial fraction of the data set is shown to be frequency modulated by the diurnal tide and, to a lesser extent, by the semidiurnal tide. Whereas amplitude modulation by the tide has been found in the past in the open ocean, frequency modulation is less common and neither tidal amplitude modulation nor angle modulation have been observed and recorded under sea ice to the authors’ knowledge. Three theoretical oceanographic modalities that could excite the modulation are discussed. Although the sea ice affected the wind-generated waves and swells in the measured energy spectra as predicted, namely by preferential attenuation that causes spectra to become narrower by progressively reducing shorter periods with distance travelled, its effect on waves of longer periods, i.e. 1–10 min, was unanticipated. Oscillations in this band, present as infragravity (IG) waves, leaky waves and an edge wave mode are shown to be affected as well; especially spectral peaks corresponding to IG/leaky waves, which respond to the sea ice by shifting to a slightly longer mean period and becoming broader. The period shift is explained by the spectral sharpening previously mentioned, whilst the broadening is conjectured to be associated with changes to the frequency modulation.