e-mail: M.Smithson@pol.ac.uk
The ability to measure and predict tides has been recognised as being important for many centuries. Accurate predictions, making use of tidal measurements, have been possible for well over 100 years and since the advent of the personal computer it has been possible for oceanographers to make tidal predictions for any location and any time in a few seconds. However, the study of tides is probably one of the oldest branches of oceanography and as such, many people in the physical oceanography community consider that there is little going on in the field, or little work left to be done. Major advances in technology in recent years accurate satellite instrumentation have meant that there has been a revival of interest in the study and measurement of tides. Long-term studies of sea-level rise, global climate change investigations, ocean circulation studies such as the World Ocean Circulation Experiment (WOCE) and marine geoid determination make use of these new satellites and all require accurate tidal information.
Pelagic tidal constants (that is, tidal harmonic constants derived from sea bed pressure recorders remote from land) are important for several reasons. They give definitive measures of the tide harmonics at specific open ocean locations which are far more accurate than any computer model, and hence provide reliable "ground-truth" for testing such models, as well as means to improve them. They are free from the local effects of coastal topography and shallow water which make many of the port tide-gauges, whose constants are compiled by the IHO, unsuitable for extrapolation to the open ocean. They may be directly compared with satellite altimetry because the altimeter signal is free from contamination by land returns. Being produced by scientists (at considerable expense) for scientific rather than shipping interests, and their results being scattered among the oceanographic literature, pelagic tidal constants deserve and require a special compilation of their own.
In 1992 such a compilation was produced as IAPSO Publication Scientifique No. 35, Pelagic Tidal Constants 3, on behalf of the IAPSO Commission on Tides and Mean Sea Level, and was an update of IAPSO Publication Scientifique Nos. 30 (Pelagic Tidal Constants, Cartwright, Zetler and Hamon, 1979) and 33 (Pelagic Tidal Constants 2, Cartwright and Zetler, 1985). These previous publications had a total of 205 sets of tidal constants (Q1, O1, P1, K1, N2, M2, S2, K2, where available) from pelagic pressure measurements. The second edition was essentially an appendix to the first. The 1992 publication was a compilation of both the earlier editions and 143 new records which had come to light (either published or supplied to the editor privately) since the publication of the earlier editions. A brief description of the measurements was also given, e.g. the type of recording instrument and sensor, details of the programme or experiment to which the record related and information on depth and length of deployment. The location of all these data are shown by the crosses in the upper part of the enclosed figure. The numbers in parentheses refer to the regions as defined in the publication. It is noticeable immediately that most of the measurements are in the north Atlantic and north-east Pacific Oceans with the south Pacific and Southern Oceans under-represented.
These data were made available as files "pelagic.dat" and "pelagic.hel" in directory pub/psmsl on a Unix Workstation, Internet number 192.171.134.7, and can be obtained by anonymous ftp. Since 1992 a number of new records have become available and have been included in this dataset. The middle figure shows where these are located. In particular, there is a cluster off the south coast of Alaska and some welcome measurements in the Drake Passage. The 1992 publication and an unpublished appendix containing these new data are available from the author. The lower figure shows the locations of measurements which have been obtained, either as raw pressure data or as computed harmonic constants which will be included later this year. Again these are in the northern hemisphere.
It is hoped that this dataset provides a useful source of information for oceanographers and other geoscientists involved in tidal studies. Any new data (either as pressure time series or tidal constants, published or unpublished) will be gratefully received by the author for inclusion. Measurements from the southern hemisphere (especially the Pacific Ocean) are particularly welcome.
