7. Tidal mixing

‘Tidal mixing’ describes tidal mixing in shelf seas, where the water is shallow and tidal currents can be much faster than in the deep ocean. Most of the energy lost from the tide through friction is first converted into turbulence, which then makes a very effective mixing mechanism, stirring the Sun’s heat downwards. Shelf seas at temperate latitudes in summer are divided into stratified regions and vertically mixed regions, depending on the tidal streams’ strength and the water depth. The transition from one to the other happens rapidly and creates a tidal mixing front. Tidal mixing in estuaries is also discussed along with the harnessing of tides to generate electricity.

4. The tide in shelf seas

‘The tide in shelf seas’ describes progressive waves, standing waves, and what happens when a shelf sea is in resonance, using the example of the Gulf of St Vincent off the south Australian coast. It also considers the effect of Earth rotation and tides in shallow water, where the rare feature is double high water or double low water. The great ocean basins are bordered by shallow seas lying on the continental shelves. Shelf seas are generally less than 200 metres deep and vary in width from almost nothing to hundreds of kilometres. It is in these shallow seas and the rivers that flow into them that the most spectacular tides are found.

3. Measurement and prediction

‘Measurement and prediction’ outlines the ways in which tides and tidal currents can be measured, and considers the standard method of analysing tidal data using a curve-fitting procedure known as harmonic analysis. Tides, unusually for natural events, can be forecast with great accuracy for years in advance. The ability to do this and make people safer at sea is probably the greatest practical success story of the science of physical oceanography. There are two aspects of the problem to be considered: the rise and fall of the level of the sea, which we shall call the tide; and the horizontal flow of the water, called tidal streams or currents.

8. New frontiers

‘New frontiers’ considers where the future lies for tidal studies. On our own planet there are discoveries to be made in difficult-to-reach places such as deep-sea ecosystems. The interaction between tides and sunlight in shallow water has been barely explored. Innovative computer models allow us to reproduce the tide in the early ocean. Tidal forces are not confined to Earth. Tidal flexing of the icy moons of Jupiter appears to have created a liquid water ocean on Europa. It is possible that this ocean has the right conditions for life, and space probes planned for the next few years will be able to test this possibility.

1. Watching the tide

The tide is the ocean’s response to the variation in gravitational pull of the Moon and the Sun over the surface of the Earth. ‘Watching the tide’ considers the importance of tides, from tide knowledge for safe navigation of ships in coastal waters to tides as a source of mechanical energy in the form of potential and kinetic energy. Tides also mix the sea, maintaining vertical circulation of the ocean and pumping heat from the tropics to the poles. Tidal rhythms (or harmonics), including diurnal and semi-diurnal tides, are discussed along with tidal range, neap tides, spring tides, and meteorological effects called surges, which can cause serious flooding.

2. Making tides

‘Making tides’ explains the forces that generate tides and the scientific discoveries that helped to explain them. Every point in the ocean experiences a tide-generating force changing in strength and direction, with rhythms set by the motions of the Earth, Moon, and Sun. In general, this force will have east–west and north–south components and each of these will change in different ways with time. The most important effect is that set by the spinning Earth within the Moon’s orbit. The deflection of moving objects on a spinning Earth to the right in the northern hemisphere and to the left in the south is called the Coriolis effect.

6. Tides and the Earth

‘Tides and the Earth’ explains that the energy in tides comes, ultimately, from the Earth’s spin. Tidal streams, rubbing against the seabed, lose energy through friction and, to make up for this loss, energy is transferred into the tide from the Earth’s spin. As a result, Earth’s rotation is gradually slowing and the day is lengthening. Most tidal friction happens in shelf seas, where the currents are strongest and the water is shallow, but there is an additional loss of energy in the body of the deep ocean, through the creation of waves called internal tides, which mix the interior of the deep ocean.

5. Tidal bores

A tidal bore is perhaps the most spectacular tidal phenomenon that can be readily observed. When a large tide enters a shallow, funnel-shaped estuary with a gently sloping bottom, its waveform is distorted and this can lead to an impressive rolling ‘wall of water’, travelling upriver. ‘Tidal bores’ explains that estuary shape and a large tidal range are important for tidal bore formation. Tidal bores can be smooth, non-breaking ‘undular’ waves or a variety of breaking forms of increasing violence. Famous examples are seen along the Qiantang River in China, the Amazon River in Brazil, and the River Severn in the UK. The impact of tidal bores on estuarine processes and ecosystems is also discussed.