Refraction of swell by surface currents

Swell with λ = 500m emitted by a point-source in the Southern Ocean.
TitleRefraction of swell by surface currents
Publication TypeJournal Article
Year of Publication2014
AuthorsGallet B., Young W.R
JournalJournal of Marine Research
Date Published2015/03
Type of ArticleArticle
ISBN Number0022-2402
Accession NumberWOS:000347185800004
Keywordsbuoy; ocean; pacific; propagation; wave refraction

Using recordings of swell from pitch-and-roll buoys, we have reproduced the classic observations of long-range surface wave propagation originally made by Munk et al. (1963) using a triangular array of bottom pressure measurements. In the modern data, the direction of the incoming swell fluctuates by about +/- 10 degrees on a time scale of one hour. But if the incoming direction is averaged over the duration of an event then, in contrast with the observations by Munk et al. (1963), the sources inferred by great-circle backtracking are most often in good agreement with the location of large storms on weather maps of the Southern Ocean. However there are a few puzzling failures of great-circle backtracking. For example, in one case, the direct great-circle route is blocked by the Tuamoto Islands and the inferred source falls on New Zealand. Mirages like this occur more frequently in the bottom-pressure observations of Munk et al. (1963), where several inferred sources fell on the Antarctic continent. Using spherical ray tracing we investigate the hypothesis that the refraction of waves by surface currents produces the mirages. With reconstructions of surface currents inferred from satellite altimetry, we show that mesoscale vorticity significantly deflects swell away from great-circle propagation so that the source and receiver are connected by a bundle of many rays, none of which precisely follow a great circle. The +/-10 directional fluctuations at the receiver result from the arrival of wave packets that have travelled along the different rays within this multipath. The occasional failure of great-circle backtracking, and the associated mirages, probably results from partial topographic obstruction of the multipath, which biases the directional average at the receiver.

Short TitleJ. Mar. Res.

Because of refraction by surface currents, a storm and a receiving station are connected not only by the great-circle route, but by a multipath: a bundle of rays with an angular width which is much larger than the angular width of the storm. Wave packets travel on these many rays before reaching the receiver, which leads to strong temporal fluctuations in the incoming direction measured at the receiver.

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