Observations of waves on an ice-covered ocean

A gravity meter read on floating ice, such as pack ice, shelf ice, or ice islands, shows a typical oscillation of about 20- to 60-sec period corresponding to motions of the order of a few hundred microns. This phenomenon has been reported in the literature by Crary, Hunkins, and others. The motion of an ice-covered ocean is related to long-period gravity waves on the open ocean. The motion of the ice is no doubt due to the combined effect of waves entering the ice field from the open ocean and local winds. Power spectrums over the band 10- to 100-sec period were computed for several records made at stations on pack ice (1 to 3 m thick), ice islands (20 to 50 m thick), and shelf ice (250 m thick). The spectrums have the same general form for the three different ice covers, and they show a sharp increase in spectral density with period in the band 10 to 100 sec. Records made during a storm and during a quiet period show an increase in wave motion due to local winds, especially at longer periods.