Low frequency 0.045-Hz swell from the Southern Ocean

Abstract

Unusual, low-frequency, one-dimensional energy spectra of surface gravity waves from measurements with a buoy in deep water (167 m) near Cape Town, South Africa, are discussed here. Classical treatment of the time series of spectra showed that the wave source was 2,800 nautical miles distant and probably in the Southern Ocean. Examination of South African Weather Bureau surface pressure analyses of the South Atlantic depict an intense, slow moving cyclone, with geostrophic wind velocities >50 knots at the same time and place as the origin of the waves. The wave spectra are highly unusual in that they reveal quasi-regular swell at very low frequencies with spectral energy densities an order of magnitude larger than those normally present at these frequencies. The wave event and its associated meteorological conditions are also discussed.