Long-term observations of the group structure of surface waves in ice

Several years of surface wave observations in the Chukchi Sea reveal wave groups are a common feature in open water and ice-covered conditions. The strength of the groupiness, here characterized by the group factor, is well correlated with the characteristic wave steepness, the spectral bandwidth, and the Benjamin-Feir Index. The general finding is enhanced groupiness in ice. However, the trends with wave characteristics are opposite from ice to open water, and suggest different mechanisms. In ice, groupiness increases with decreasing steepness, increasing bandwidth, and decreasing Benjamin-Feir Index. In open water, the trends indicate that both linear superposition of phase-coherent waves and nonlinear behaviour are important for the generation of wave groups. We hypothesize that in ice-covered conditions, directional spreading reduces the effective bandwidth in the dominant wave direction, possibly due to modified four-wave nonlinear transfer spreading high-frequency energy to lateral directions. This reduced effective bandwidth is then conducive to enhanced group formation by linear superposition. However, an increased high-frequency noise floor of the in-ice observations would also be consistent with the observed increase in omni-directional bandwidth. Without directional measurements, neither of these two processes can be favoured with certainty.