Evolution of Persistent Wave Groups

Abstract

During the near-field leg of the Hawaiian Ocean-Mixing Experiment (HOME-NF), short, steep surface wave groups were observed that elicited strong group-forced responses in the wave-filtered surface current field, as reported by Smith. Some of these wave groups persisted for 17 wave periods, yet were only about 1 wavelength long in the along-wind direction. Here, the authors consider the evolution of wave groups of the form observed and find that this persistence is consistent with linear dispersion in spite of the very compact form. The key aspects enhancing persistence are 1) that the wave crests within the group are oriented at an angle with respect to the group envelope and 2) they are much wider in the crosswind direction than along-wind (in the example examined in detail, about 5 times). According to a simplified model, groups with the observed 5-to-1 aspect ratio and this ?slant-wave? structure can persist for up to 20 wave periods, consistent with the observations (cf. 8 periods for a collinear wave group). The maximum persistence increases in proportion to the across-wind length of the group.