Rollover of Apparent Wave Attenuation in Ice Covered Seas

Wave attenuation from two field experiments in the ice-covered Southern Ocean is examined. Instead of monotonically increasing with shorter waves, the measured apparent attenuation rate peaks at an intermediate wave period. This rollover phenomenon has been postulated as the result of wind input and nonlinear energy transfer between wave frequencies. Using WAVEWATCH III (R), we first validate the model results with available buoy data, then use the model data to analyze the apparent wave attenuation. With the choice of source parameterizations used in this study, it is shown that rollover of the apparent attenuation exists when wind input and nonlinear transfer are present, independent of the different wave attenuation models used. The period of rollover increases with increasing distance between buoys. Furthermore, the apparent attenuation for shorter waves drops with increasing separation between buoys or increasing wind input. These phenomena are direct consequences of the wind input and nonlinear energy transfer, which offset the damping caused by the intervening ice. Plain Language Summary A long standing question about the non-monotonic attenuation rate observed in waves propagating in ice covers is re-visited. This phenomenon has been discussed for over four decades without a conclusive explanation. Using recent data from two independent and very different field experiments in the Antarctic marginal ice zone, and the most updated global wave model WAVEWATCH III (R), the reason for this "rollover" of attenuation is determined. The wind input and nonlinear transfer of wave energy between different wave frequencies offset the damping caused by the ice cover, resulting in the apparent drop of attenuation especially for short waves. Therefore, as the distance between two observation points increase, the decay of high frequency wave energy between the two points decreases. With increasing separation between the two observation points, the wave period for rollover increases and the attenuation rate for all periods drops. When using field data to measure the attenuation rate, care must be taken to consider input from the existing energy sources between the two measuring points.