Pressure variations produced at the ocean bottom by hurricanes

Abstract

Low-frequency pressure variations (0.1 to 3 hz) associated with the near passage of six hurricanes have been detected by a deep (5.7 km) ocean bottom hydrophone (OBH) located approximately 260 km northeast of Antigua, British West Indies. Recordings made during one hurricane, Arlene, that passed almost directly over the OBH site in August 1963 have been analyzed in detail. Two principal pressure spectral components are observed: (1) normal microseisms with prominent spectral peaks at periods of 2.8, 4.6, and 10 sec for Arlene, and (2) a shorter-period component with periods between 0.5 and 1.0 sec but usually at 0.9 sec. Maximum microseisms at the OBH occur many hours after peak hurricane winds have passed the point of closest approach to the hydrophone; hence, generation does not take place directly beneath the storm. The onset of normal microseisms from Arlene occurs at about the same time at Guadeloupe and the OBH, and this closely parallels increased regional ocean wave activity. The predominant periods for wind waves and swell from hurricane Arlene are approximately twice the periods of the 2.8- and 4.6-sec microseismic spectral peaks respectively, suggesting that the microseisms are produced by the interaction of ocean waves as described by Longuet-Higgins. The interaction does not take place within the storm itself. The microseismic spectral peak at a period of 10 sec may be produced by the direct action of the 10-sec swell on local shorelines. Maximum amplitudes for the high-frequency pressure component coincide with the time of a hurricane’s closest approach to the OBH. Thus, it is concluded that this signal is produced directly beneath a hurricane and may serve as a means of tracking its motion.