altimeter (satellite) A satellite-mounted radar system used to determine sea surface height above a fixed level, based on the time it takes for a radar pulse to travel from the satellite to the sea surface and back. Today, heights can be determined with accuracies of a centimetre.
amphidrome/amphidromic system A point of no vertical rise and fall of the tide, created when two tide waves, travelling in opposite directions, cancel each other at all times. The currents in the two waves, however, add at the amphidrome.
capillary waves, see gravity and capillary waves.
Coriolis effect An apparent deflection of moving objects on a rotating planet. On the Earth, the deflection is to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis effect makes winds blow around pressure systems and creates tidal amphidromes.
co-tidal chart A map showing co-tidal lines (along which high water occurs at the same time) and co-range lines (along which the tidal range is constant).
deep-sea sponge grounds Mass occurrences of large sponges in the deep sea (>200 metres water depth) whereby sponges dominate the seabed fauna. Aggregations may be monospecific (one species) or multispecific.
diurnal inequality A difference in the level of the two high waters (or two low waters) on the same day.
diurnal tide A tide in which there is just one high and low water each lunar day. Purely diurnal tides are rare but are observed along some coasts, where resonance amplifies the diurnal harmonics of the tide.
entrainment In oceanography and fluid dynamics, something incorporated into a flow may be described as having been entrained. For example, air bubbles are entrained into turbulent tidal bore fronts, eroded sediments are entrained into tidal flows, and nutrient rich deep waters are entrained into surface waters by the vertical mixing action of internal tides.
equilibrium tide The tide that would occur in an ocean which covered the Earth (with no land masses present) and which was able to respond instantaneously to changes in the tide-generating force.
equinoctial tides The largest semi-diurnal tides occur at the equinoxes, in March and September, when the Sun lies in the plane of the Earth’s equator.
gravity and capillary waves Water waves which move in response to the pressure gradient force due to the surface slope between the crest and the trough are sometimes called gravity waves, because gravity is an essential part of making the wave. In contrast, the small ripples sometimes seen on the water surface rely on surface tension, rather than gravity, for their restoring force and are called capillary waves.
harmonics The rhythms which contribute to the tide. A pure harmonic is a regular, sinusoidal, up-and-down motion of fixed amplitude and period as illustrated in
Chapter 3 of this book.
hydraulic jump An abrupt rise in the water surface that occurs at the transition between two different flow regimes. In the case of tidal bores, a hydraulic jump occurs as a result of the discontinuity of flow at the point where the incoming tide meets the river outflow. It travels upriver with the flooding tide.
internal tide Tidal motion imposed on layers of different density in the ocean. Internal tides often form when tidal streams in stratified water flow over submarine mountains, such as those found at mid-ocean ridges.
neap tides, see spring and neap tides
pressure gradient force A horizontal force created by differences in pressure in the sea. When the sea surface is sloping relative to the horizontal it creates a pressure gradient force acting down-slope.
progressive and standing waves Water waves which travel over the seabed are called progressive waves. The speed at which they travel in water shallow compared to their wavelength just depends on the depth of water. Two equal progressive waves travelling in opposite directions create a standing wave in which there are nodes at intervals of half a wavelength: places of no vertical tide but fast tidal streams, where the elevations in the two waves always cancel but the currents add.
refraction A change of wave direction caused by a change of wave speed. Waves travelling in shallow water have speeds determined by the water depth. The parts of the wave travelling in deeper water will travel faster than those in shallower water, with the effect that the wave crest will swing round towards alignment with the depth contours.
resonance/resonant period The response of a body when it is forced at a regular period close to its natural period of oscillation. Tides in the ocean are enhanced because the oceans are close to resonance with the period of the tidal forcing.
semi-diurnal tide A tide in which high waters occur at intervals of, on average, twelve hours and twenty-five minutes, or about half a day.
shallow-water waves, or long waves, have long wavelengths compared to the depth of water in which they travel and their speed is equal to √(gd) where g is the acceleration due to gravity and d the water depth. In contrast, waves travelling in water which is deep compared to their wavelength travel at a speed which depends on their wavelength, with the long waves outstripping the shorter ones. See also gravity and capillary waves.
shelf sea A sea, generally less than 200 metres deep, covering a continental shelf.
spring and neap tides The fortnightly variation in tidal range caused, in the case of semi-diurnal tides, by the relative positions of the Moon and Sun with respect to the Earth. When the Sun and Moon make a straight line with the Earth their tidal forces combine and large spring tides result. When the Sun and Moon make a right angle with the Earth, smaller neap tides occur. There is a spring–neap cycle in the diurnal tide too, but this is controlled by the declination of the Moon and Sun.
standing waves, see progressive and standing waves
storm surge An increase in sea level caused by meteorological effects, for example an onshore wind or a low pressure system. Storm surges on top of a large spring tide can cause serious flooding.
tidal locking The situation in which an astronomical body orbiting another takes as long to rotate about its own axis as it does to orbit its partner. The orbiting body then constantly shows the same hemisphere to its partner (‘synchronous rotation’).
tide waves Water waves with period equal to the semi-diurnal or diurnal tidal period and created, directly or indirectly, by tidal forces. Tide waves are long compared to the depth of the ocean and travel at a speed which just depends on the depth of water.
thermocline Depth range with a relatively strong vertical gradient of temperature, marking the transition between an upper layer of warm water and a lower layer of cold water (in a stable state). Similar terms exist for the salinity and density fields (halocline and pycnocline, respectively).
tidal bore A hydraulic jump that forms on the incoming tide and travels upriver in some funnel-shaped estuaries with a large semi-diurnal tidal range. A tidal bore may be undular (a smooth, non-breaking wave) or turbulent (breaking). See also hydraulic jump and whelps.
tidal prism The volume of water that flows into an estuary or bay during the flood tide.
tidal straining The action of a vertically sheared tidal flow on a horizontal density gradient. On the ebb tide in a well-mixed estuary, for example, the faster surface currents pull less dense water over denser water further out to sea creating stratification. The stratification will generally (but not invariably) be destroyed when ebb turns to flood. The term is also used to mean flexing of the solid Earth by tidal forces.
tide-generating force The tide-generating force created by the Moon on the Earth is the difference between the gravitational attraction of the Moon at a point and the gravitational attraction of the Moon at the centre of the Earth. An equivalent definition applies to the Sun’s tide-generating force and indeed any two astronomical bodies. It is the horizontal component of the tide-generating force that raises tides in the ocean.
tsunami An ocean wave of very great wavelength generated by the abrupt displacement of a large volume of water by, for example, seismic disturbances (earthquakes), submarine landslides, volcanic eruptions, comet/asteroid impacts, glacier calving, etc. Tsunami are not tidal phenomena and should not be referred to as ‘tidal waves’.
turbulence A random motion which causes fluctuations in the velocity of a tidal stream or other flow and which transfers parcels of water between different parts of the flow. Turbulence derives its energy from the mean flow and the energy is ultimately dissipated as heat in the smallest turbulent eddies.
water mass A body of water with a distinctive range of temperature and salinity, which is identifiable based on these and other physical and biogeochemical properties. Water masses acquire their characteristics at the sea surface owing to climatic conditions at particular locations. They sink and spread along an appropriate density surface where they can be traced based on their properties.
whelps A train of secondary waves, typically of smaller amplitude, following the lead wave of a tidal bore, particularly one of undular type (see tidal bore).