Sea ice inhibits the development of wind‐generated surface gravity waves which are the dominant factor in upper ocean mixing and air‐sea fluxes. In turn, sea ice properties are modified by wave action. Understanding the interaction of ice and waves is important for characterizing both air‐sea interactions and sea ice dynamics. Current leading theory attributes wave attenuation primarily to scattering by ice floes. Here we use new in situ wave measurements to show that attenuation is dominated by dissipation with negligible effect by scattering. Time series of wave height in ice exhibit an ``on/off" behavior that is consistent with switching between two states of sea ice; a relatively unbroken state associated with strong damping (off), possibly caused by ice flexure, and very weak attenuation (on) across sea ice that has been broken up by wave action.

Plain Language Summary

Waves created by wind at the ocean surface are strongly attenuated when they travel across ice‐covered regions. Until now, this effect was thought to be the result of waves reflection off pieces of ice. Using new measurements of wave directions, we show that waves do not come for a broad range of directions, and scattering must be weak. Instead we find that attenuation is highly variable and related to the size of ice floes. We hypothesize that attenuation may be caused by cyclic deformation of the ice. When the waves are large enough to break the ice up, this deformation stops and the attenuation is much less. This finding is important for forecasting waves in ice‐infested waters as well as predicting seasonal sea ice extent.