Measurements of surface waves have been made intermittently from 1953 to 1975 at Ocean Weather Stations India (59°N 19°W) and Juliett (52°30'N 20°W) using Ship-borne Wave Recorders. Measurements made during the first decade of the project were analysed, and published in the late 1960s measurements made in the second decade have been analysed and the results are published in this report; they may be taken as representative of wave conditions existing over a typical year, divided into the four seasons. Estimates of extreme heights having various return periods are also made.

There are significant increases in wave height deduced from these analyses compared with those in the previous studies; they were more severe in the 1970s than in the 1960s all conceivable sources of error have been investigated to try to understand the reasons for the differences, but none of any significance has been found.

It therefore seems necessary to accept that wave conditions can be more severe than had previously been believed.

Because of space limitations, the figures cannot be presented here, but will be available in a report from the Institute of Oceanographic Sciences.

Background to Surface Wave Measurements at Ocean Weather Stations

Measurements started in 1953, using a Ship-borne Wave Recorder (Tucker, 1956) mounted on OWS Weather Explorer (formerly a Flower Class Corvette), one of the four British vessels in service at that time. They occupied four stations, Alpha, India, Juliett and Kilo in rotation with Dutch and French ships. Because of this sparse occupation level by a wave-recorder-carrying vessel, it was not possible to obtain a continuum of records. Indeed, it took many years before the ships had been on station long enough to accumulate sufficient records to produce an acceptable number for every calendar month, from any one station, to allow wave climate data to be published; this stipulation was made to allow a uniform representation throughout the year. The situation is exacerbated by the fact that the recorded wave period is apparently altered by an incalculable amount when the vessel itself moves relative to the water if the sea waves were all to come from one know direction and the speed and direction of the vessel were known, there would be no problem, but when wave energy comes from a range of directions, as is always the case, and the vessel has to move to keep station, the apparent period, and to a lesser extent the height, is irretrievably modified. The acceptable criteria in these analyses were that the vessels were within about 20 nm of their normal positions and that the station-keeping speed was 2 knots or less; this latter one makes only 10% modification to the lowest period waves, about 4 s, considered, and proportionately less to longer-period waves. In addition, electronic devices have been known to fail or lose their calibrations, and when any doubt at all existed, a recording was deemed to be unusable for this purpose. The mean depths were 2000 m at India and 300m at Juliett.

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