Manual on sea level measurement and ... - unesdoc - Unesco

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Sea Level Measurement and InterpretationWave activity with a period of a few minutes can becaused by non-linear effects; e.g. when the wavesencounter a current or a change in bottom topography.These longer-period waves occur because the height ofsuccessive waves is not uniform; they occur in groupsof higher or lower waves. This leads to the popularmisconception that every seventh wave is the highest.In fact, the wave groups are not of equal length butthey do produce non-linear effects that have periodsrelated to the period of the wave groups. The mostsignificant effect of this, as far as the study of sea levelis concerned, is that the wave groups produce ‘set-up’of the sea level near the coast. The degree of ‘set-up’depends on many factors, of which the shape of thebeach is the most critical. Set-up can be of the order ofa few tens of centimetres during a severe storm.Waves have directional properties as well as a magnitude.Many early recordings were only concerned withwave height, because instruments capable of measuringdirection were not available. Wave riders from thisera were moored to the sea bed on a flexible couplingand contained accelerometers which were integratedtwice to obtain wave height. However, modern mooringsare now available which are capable of measuringpitch and roll of the surface buoy, from which directionalinformation can be derived.Coastal tide gauges tend not to be located optimallyto measure wave conditions in the nearby deep ocean.However, they can at times provide useful informationwith the correct (pressure) gauge technology. Vassieet al. (2004) provide a recent description of the useof pressure tide gauges to measure swell at oceanislands.2.3 SeichesSeiches are periodic variations in the surface level usuallyset in motion by a disturbance such as a strong wind or current,a sudden change in atmospheric pressure or even atsunami. In lakes and gulfs their period is controlled by thedimensions of the basin and their lifetime is determined byfrictional effects. Typical periods are in the range of a fewminutes to a few hours (between wind waves and tides),and typical amplitudes are centimetric to decimetric. Theycan be seen on tide gauge records from almost all regions.Seiches have largely been ignored in most sea level studies,owing to their primarily local origin, but knowledgeof them is important for coastal and harbour engineeringas well as for harbour operations, where small-amplitudeseiches may be associated with strong currents at theentrance of the harbour. On the other hand, they can havea major effect on other sea level studies. For example, iftheir amplitude is large enough, and if the sampling rate ofthe tide gauge is insufficiently high, then their energy canbe aliased into tidal and other sea level signals.2.4 TidesThe oceans respond to the gravitational attraction of theMoon and the Sun, and the solar radiation, to producethe tides, which are normally the predominant signalsin sea level records. The tides are easy to distinguishfrom other components of sea level variation (e.g. stormsurges) because they have well defined periods, whereasother processes tend to occur at irregular intervals.An examination of the forces causing the tides leadssome way towards an understanding of their nature. Thisexamination is usually via discussion of the EquilibriumTide (Doodson and Warburg, 1941; Forrester, 1983;Pugh, 1987; Open University, 1989). The gravitationalattraction of the Moon and Sun on the Earth producesa semi-diurnal (2 cycles per day) ‘tidal bulge’, whichis usually oriented at an angle to the equator producingthe diurnal (1 cycle per day) tidal components. Thediurnal and semi-diurnal waves both have a planetaryspace scale. As the Earth rotates about its axis, signalscontaining the above periods, but usually dominated bythe semi-diurnal component, should appear in the sealevel record. A lunar day is slightly longer than a solar dayby approximately 50 minutes, leading to lunar and solartides of differing periods which interact over 14 days toproduce the Spring-Neap cycle.Study of the celestial motion of the Earth–Moon–Sunsystem leads to a more complex form of the tidal potential(or Equilibrium Tide) in which the main constituentsare modulated at periods of 1 month, 1 year, 8.85 years,18.61 years and 21,000 years. The effect of the modulationis to split the tides into additional constituents butwith periods close to 1 and 2 cycles per day. This groupingis termed ‘tidal species’.The tidal potential so far discussed explains only the diurnaland semi-diurnal species of the tide, but can be extendedto include ter-diurnal (third of a day period) tides and tidesof even shorter period. A power spectrum of a tidal recordclearly shows that higher-order species do exist, exceptsometimes when measurements are made at an oceaniclocation. These ‘compound tides’ are primarily generatedby the main tidal components in shallow water as theyencounter frictional forces. They have periods of 2, 4 and 6cycles per day (and even 12 cpd in very shallow areas), witheach species demonstrating separate tidal characteristics.The tidal regime varies enormously in different partsof the world. In most regions the tide is dominated bysemi-diurnal components, reflecting the importance ofthe main semi-diurnal terms in the Equilibrium Tide.However, there are many areas where the tides are predominantlydiurnal (e.g. Persian Gulf), and some wherethe regime is ‘mixed’ (i.e. the diurnal and semi-diurnalcomponents have a comparable magnitude). Examplesof these various regimes are shown in Figure 2.2.4IOC ong>Manualong>s and Guides No 14 vol IV

Sea Level Measurement and InterpretationFigure 2.2 Tidal characteristics at five stations, showing different regimes: diurnal, mixed, semi-diurnal with strongspring-neap modulation in the Indian Ocean, semi-diurnal with smaller amplitudes at a N.Atlantic site, and shallowwater distortions.While the temporal characteristics of the tide in the realocean are similar to those of tidal potential (EquilibriumTide), their spatial characteristics are very different. Thisdifference is caused by the dynamical response of theocean basins, causing the tides to propagate as progressivewaves and to generate standing waves in some areas.Tides in the deep ocean have amplitudes of typically1 m or less, considerably lower than the amplitudes oncontinental shelves where local resonances can producelarge amplitudes. In all oceans (deep oceans as well as theenclosed sea areas of continental shelves) there are regionsof no tide, called amphidromic points, which are a consequenceof the standing waves.Tide gauges, such as those described in this manual,remain the primary source of tidal knowledge in coastalregions, although new techniques are under continuousdevelopment (section 8). The tides of the deep oceancan also now be well measured, with the use of bottompressure recorders (Cartwright et al., 1980; Filloux, 1980;Spencer and Vassie, 1997), and more recently by means ofaltimeter satellites (Shum et al., 1997).2.4.1 Tidal AnalysisThe model that has been derived for the Equilibrium Tide isnot completely without use, as it does provide the knowledgethat the tide is composed of a finite number of constituentsof calculable frequency. It also provides a measureof their relative amplitudes so that we have an idea whichconstituents are important in the real tide.The analysis consists in reducing a set of measurements,which amounts to 8,760 hourly values in anormal year, to a manageable set of parameters whichcompletely specify the tidal component of the record.The tides can then be removed to reveal the remainingIOC ong>Manualong>s and Guides No 14 vol IV5

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