A tide gauge in its most basic form is a simple graduated staff or other reference on which the water level can be estimated visually. Such readings are available for a few sites as far back as the 17th century. Measurement series made this way can be surprisingly accurate if carefully and faithfully done, and tide staffs are used to this day to provide an independent check on the functioning of many tide gauges. However, to be useful for most investigations of climate change, a record of water levels should be temporally dense and long (the longer the better), and without the limitations of visual estimates. In addition, the record must maintain internal consistency in spite of repairs, replacements, and changes of tide gauge technology. With these requirements in mind, what can be considered the modern epoch of sea level measurement began about 165 years ago with the use of a recording tide gauge at Sheerness in the United Kingdom. Other, more continuous series of sea level observations began soon after in the Baltic region and elsewhere in northern Europe. The longest continuous U.S. sea level record from a recording tide gauge began in San Francisco in 1854. However, there are not very many really long tide gauge records;
consider Fig. 3.1. This histogram shows the number of tide gauge records of various lengths.
It is clear from the cumulative percentages shown on the right-hand side of Fig. 3.1 that the vast majority (80% + ) of records are less than 60 years long and that only a relative handful cover the entire 20th century. In addition, the longest records are nearly all from sites in northern Europe and the United States, so the world's oceans are not well sampled. The lack of very many useful and very long records will be seen below to be one of the most important issues in determining a rate of global sea level rise for the 20th century.
In addition to any global change of sea level, a tide gauge records (of course) tides, effects of ocean circulation, meteorological forcing of the water, local or regional uplift or subsidence at the measurement site, vertical movements of the platform to which the gauge is mounted, and errors inherent to the gauges. With so many factors involved in maintaining a consistent record over an extended time such as a century or more, a cooperative international program is a necessity. The world's focal point for sea level data and information is the Permanent Service for Mean Sea Level (PSMSL), based at the Proudman Oceanographic Laboratory in the United Kingdom. The PSMSL manages sea level data from nearly 200 national authorities. It also sets standards, provides training in operation and maintenance of instrumentation, and carries out important sea level research. Their Web site found at http:// www.pol.ac.uk provides a wealth of information about sea level data, measurement technology, data analysis, and links to numerous important in situ and artificial satellite observing programs.
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 180 More
Record Length Bin, Years Figure 3.1 Distribution of tide gauge record lengths.
Tide gauges have taken many forms. As noted, the simplest measurement technique is to observe visually the level of the water on a graduated staff. Such readings are individually accurate to only a few centimeters, but if done frequently can give estimates of daily tidal range adequate for marine navigation and good estimates of annual mean water level. Much better results suited for scientific use are obtained by measuring water level in a so-called stilling well. A common form of this device used in the United States consists of a vertically standing 30-cm-diameter pipe that cones down to a 2.5-cm-inlet orifice. The small-diameter hole available for water entry and exit effectively limits the flow of water in and out of the well over time intervals (seconds) associated with waves, but does not interfere with longer period flow variations such as those from tides. The orifice thus serves as a mechanical low-pass filter that eliminates high-frequency variations of water level in the well. Water level in the stilling well has traditionally been determined by means of a float connected by a wire to a recording device. In more recent years, echo sounding of the distance from an audio or radar source to the water surface has been used successfully. Typically, the values of water level recorded are 6-minute averages of the actual measurements.
It is also possible to measure variations of water level from changes in the weight of the water overlying a special quartz crystal oscillator which responds in a manner proportional to the pressure change. Another means of measuring water level variations is a "bubbler" gauge. A flow of nitrogen or compressed air into a tube with one end in the water is adjusted continuously by a mechanism so that it releases bubbles at a constant rate. When the water level rises, the pressure must be increased to maintain the flow of bubbles. The pressure variations are thus a measure of water level. A small tank of gas can operate this system for many months. These systems are precise and simple and are often used to supplement other gauge types to provide short-term backup capability. For more and detailed information about tide gauge history and technology, consult the U.S. National Ocean Service Web site http:// www.opsd.nos.noaa.gov/ and the PSMSL site at http://www.pol.ac.uk/psmsl/ training.
In common with most scientific measurements, it is easier to measure short-term variations of water level than very long-period ones. Bubbler and quartz crystal systems can drift with time through transducer drift, and so are not suitable for investigation of long-term sea level rise at a site unless equipped with ancillary datum control equipment. Float-type and echo sounding systems have their own problems, but are well suited to monitoring long-term sea level trends if properly maintained. A critical aspect of this maintenance is ensuring that the gauge, usually mounted on a pier, is not giving measurements contaminated by settling or other movement of the structure. In addition, the gauge must be remounted or replaced in a known manner that enables continuity of the record to be maintained. To solve these problems, one or more geodetic benchmarks are installed near the pier on presumably stable ground.
Periodic (usually annual) geodetic surveys are made to a gauge to determine if any vertical changes in the gauge mount have occurred. Replacing or moving a gauge without introducing a "step" discontinuity in the record is also made possible using the geodetic reference benchmarks. The PSMSL refers to gauge records for which a complete geodetic history is available as Revised Local Reference (RLR) sites. The PSMSL states that with only a few exceptions, RLR sites are the ones that should be used for scientific applications.
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