Elements of Observing System 321 Tide Gauges

The measurement of changes in sea level to understand the mechanisms responsible for phenomena such as the tides and the catastrophic floods due to storms and tsunami was performed by the observers of the Ocean from ancient times. It is now realized that sea level changes are important on all timescales from seconds (due to wind waves) through to millions of years (due to the movement of continents). The devices employed to make sea level changes (relative to the level of the land where the instrument is located) are usually called tide gauges. It is based on the principles of well-known float gauge in a stilling well, the measurement of subsurface pressure, or of the time-of-flight of a pulse of sound, or of a pulse of radar. The classical and most reliable method of measuring the sea level is by tide staff, but it is prone to manual errors. Subsequently, the float based tide gauges have been used extensively for long time. However, such systems require supporting structures, shelters and regular maintenance. The other commonly used types are pressure sensor gauges (differential/absolute) in which sensors are mounted directly in the sea. However, this require knowledge of atmospheric pressure (in case of absolute pressure sensor), seawater density and gravitational acceleration to make the conversion from pressure to sea level. In spite of the above lacuna, the instruments have many practical advantages as sea level recorders. In late 1990s, radar devices, which were mainly used in process technology, were introduced into hydrometry. Though satellite based altimeter provides mean sea level anomaly in the open ocean with coarse temporal resolution, the information from gauges is essential for understanding local mean sea level trends and extremes. Also, gauges data are required to provide precise calibration of radar altimetry. Apart from this, tide gauges have a long history and healthy future (IOC manual 2006) with many applications both in operational and scientific research.

The observed sea level consists of periodic geophysical forces such as mean sea level, a tidal signal and meteorological residuals. Each of these components is controlled by separate physical processes and the variations of each part are essentially independent of the variations in the other parts. Tides are the periodic movement of the seas which have coherent amplitude and phase relationship to some periodic geophysical force. The dominant forcing is the variation in the gravitational field on the surface of the earth due to the regular movements of the earth-moon and earth-sun systems. These cause gravitational tides. There are also weak tides generated by periodic variations of atmospheric pressure and on-shore/off-shore winds which are called atmospheric tides. Meteorological residuals are the non-tidal components of sea level which remain after removing the tides by analysis. They are irregular, as are the variations in the weather. Sometimes the term "surge residual" is used but more commonly surge is used to describe a particular event during which a very large non-tidal component is generated. Mean sea level is the average level of the sea, usually based on hourly values taken over a period of at least a year. For geodetic purposes the mean sea level may be taken over several years. More elaborate techniques of analysis allow the energy in seal level variations to be split into a series of frequency or spectral components. The main concentration of energy is in the semidiurnal and diurnal tidal bands, but there is a continual background of meteorological energy which becomes more important for longer periods or lower frequencies.

The Global Sea Level Observing System (GLOSS) (http://www.gloss-sealevel. org/) was established in 1985 by IOC to provide oversight and coordination for global and regional sea level networks in support of oceanographic and climate research. GLOSS remains under the auspices of the IOC and is one of the observing components of JCOMM. GLOSS is an example of a global coastal observing network and has the largest participation of member states (~70) among the existing observing elements in GOOS. Tide gauge data from the GLOSS networks are assembled and archived at two data centers (Merrifield et al. 2009). The British Oceanographic Data Center (BODC, http://www.bodc.ac.uk/) is responsible for delayed mode datasets. The main archive for historic, monthly-averaged, sea level records from tide gauges from around the world is available at Permanent Service for Mean Sea Level (PSMSL, http://www.pol.ac.uk/psmsl/) (Woodworth and Player 2003). Figure 3.1 shows the present status of reporting of the sea level gauges in the GLOSS Core Network (Merrifield et al. 2009).

Estimates of twentieth century sea level rise are primarily based on the historical tide gauge data maintained by the PSMSL. Church et al. (2004) estimated monthly distributions of large scale sea level variability and change over the period 1950-2000 using historical tide gauge data and altimeter data sets. Annual averages of the global mean sea level (millimeter) as derived from analyses of tide gauges shows a global rise of 1.8 ± 0.3 mm/year during 1950-2000. Tide gauges have also been used to monitor the stability of satellite altimeter sea surface height observations, long term sea level trends at coastal stations, navigation, hydrography, flood warning, tsunami warning and other coastal engineering applications.

Fig. 3.1 Status of reporting of the sea level gauges in the GLOSS Core Network in 2009. Near real-time stations (blue) provide data typically within 1 h of collection; fast delivery (green) within one month. Delayed mode low frequency data within 5 years (yellow) or greater (orange) include monthly averages provided to the Permanent Service for Mean Sea Level (PSMSL). (Source: Merrifield et al. 2009)

Fig. 3.1 Status of reporting of the sea level gauges in the GLOSS Core Network in 2009. Near real-time stations (blue) provide data typically within 1 h of collection; fast delivery (green) within one month. Delayed mode low frequency data within 5 years (yellow) or greater (orange) include monthly averages provided to the Permanent Service for Mean Sea Level (PSMSL). (Source: Merrifield et al. 2009)

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