Wave propagation through fields of pack ice

Abstract

Experimental studies of penetration of sea waves and swell into fields of loose pack ice were carried out by means of a ship-borne wave recorder, during a voyage into the Weddell Sea in R.R.S. John Biscoe in 1959—60. This reconnaissance study has provided the first systematic data within an ice field of the variation of wave amplitudes and period over the normal wave spectrum of 4 to 24 s. Although observations were confined to a single ship, a reasonably constant background of swell, together with varied ice conditions, has made it possible to draw certain conclusions for waves and swell of relatively small amplitudes. The penetration of long ocean swell, of periods from 11 to 23 s, into ice fields consisting of large floes of more than half a wavelength across takes place by bending of the floes. The results suggest that the fraction of the wave energy penetrating such an ice field is proportional to A 4/A3, where h is the thickness of the ice floes and A the wavelength of the swell. For periods of less than 10 s, floes of around 1.5 m thick and 40 m or less in diameter approximate to rigid floating plates. For these periods, the main energy cut-off took place when floe diameters were about one-third of the wavelength; little loss of energy occurred when floes were less than one-sixth of the wavelength across, while no detectable penetration took place when the floes were half a wavelength or more in diameter. Consideration of the results, together with limited evidence available from tide and gravimeter observations, shows that most long waves penetrate polar ice fields with little loss of energy. Discussion of the energy required to bend large ice floes indicates that long-period swell is propagated through regions covered by pack ice with little loss of energy only when the energy required to bend the floes is at least an order of magnitude smaller than the total energy of the waves.