Helena tide gauges form part of a United Kingdom contribution to the Global Sea Level Observing System (GLOSS IOC 1998), and have been operated in their present form since 1993 ( Spencer et al. (1977), and explains why such a technical capability is important to global ocean observing. It describes some of the recent data obtained, which includes evidence for swells significantly larger than that observed by Cartwright et al. (1977), by making use of modern technical developments to make measurements right at the coast, and by employing a wave model to identify the origin of swells. The present paper can be considered as continuing the work of Cartwright (1971) and Cartwright et al. Deep-ocean wave modeling at that time was in its infancy. However, its conclusions were limited by the short duration of the measurements, by the fact that there were no substantial swell events in that period, and by the difficulty of assigning individual events to particular distant storms. The paper also described a first set of measurements at the island using a wave recorder (pressure sensor) deployed in 12 m of water in James Bay, approximately 180-m offshore of Jamestown, for 2 months in 1970–71. Helena, and explained the various issues to be considered when undertaking measurements of swells in shallow water. (1977) gave a historical background to the study of “rollers” at St. Both gauge sites are on the northwestern side of the islands in the lee of the southeast trade winds, and are, consequently, exposed to swells originating from the northwest.Ĭartwright et al. The increasing wave heights and lengthened wave periods characteristic of swells were observed in the first set of 1-Hz-sampled subsurface pressure (SSP) data obtained from the coastal tide gauges at English Bay (Ascension) and Jamestown (St. The swell had propagated from the remains of Hurricane Irene, which occurred about 1 week earlier in the central North Atlantic, holding approximately the same position for 2 days, thereby providing the sustained, intense directional wind forcing required for the generation of a swell ( Tricker 1964 Tucker and Pitt 2001). Helena Islands in the central South Atlantic ( Fig. A major “swell event” was observed around 26 October 1999 at Ascension and St. A deep-ocean swell can travel many thousands of kilometers from the storm that produces it and can cause considerable damage on distant coasts.
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