Abstract
Tides are important, although I’ve always found them a bit of a dry topic (no pun intended), so I was interested to see if Tides: A Very Short Introduction could whet my appetite (pun intended) for more knowledge on all things tidal. I’m happy to say it did. As with all Very Short Introductions, the book is well-produced and has an abundance of black and white figures and short, punchy sections of text that allow for relatively easy digestion. As promised in the preface, the book generally avoids maths and instead relies on more intuitive physical explanations and examples to support the points it makes and appeal to students and non-specialised readers. It largely succeeds in this.
Chapter 1 (Watching the Tide) does a good job of explaining why knowledge of the tide, and the ability to predict its characteristics, is so important. As they note, the tide is the ‘greatest synchronised movement of matter on our planet’ (p. 1), and this movement is important for the navigation of coastal waters, for harnessing energy – tides producing mechanical energy that is inferior only to the energy produced by sunlight and the nuclear heat of the Earth’s core – and ecologically, in terms of mixing the sea and providing an ecotonal environment between land and sea. We then have an introduction to tidal rhythms, or harmonics, including semi-diurnal tides, tidal ranges, the 15-day cycle, and so on. Some key terms in relation to these are explained here, including spring tides, neap tides and tidal establishment. After establishing these rhythms, a short but important section discusses the ecotonal nature of the coast as a liminal space between marine and terrestrial environments, with the tide acting as a key flow connecting the two. This is particularly important to the marine organisms that live in intertidal zones, in terms of how much time they must spend exposed to air (emersion) and the various stresses this causes. This is a key driver of zonation of intertidal communities. And what fascinating communities they are! There follows some discussion of the sorts of organisms that can be found in intertidal ones, and I particularly liked that the authors touch on the ‘tidal memory’ that many of these organisms have, which (it seems) is controlled by their internal biological clocks. Being an ecologist, I would have liked a bit more on tidal communities and their unique natures, but it is of course difficult to satisfy everyone in a Very Short Introduction. The chapter concludes with a brief discussion of tidal surges and the peril these can cause.
Chapter 2 (Making Tides) gets into more detail on the processes driving the tides. A very brief history of tidal hypotheses, touching on Galileo, Johannes Keplar and Isaac Newton, is followed by one of the most important sections in the book: The Tide Generating Force. This section explains how the mutual gravitational attraction between the Earth and Moon essentially provides the force driving the tides. As promised, this is explained without recourse to mathematical formulae, but rather with some clear diagrams and straightforward but illuminating text. The authors then present a discussion of the equilibrium tide model, highlighting not only what it effectively explains, but also its limitations. The tide is, after all, quite complicated. Indeed, the authors note that ‘The tide behaves as a series of waves, with the same periods (or rhythms) as the tidal forcing, sweeping around the outside of the ocean basins in great circular movements’ (p. 23), the dynamics of which take a bit of working out. The chapter concludes with sections on the Coriolis effect and the ‘age of the tide’ effect (caused by tidal friction). I immediately went to try the authors’ practical example to show that (unlike the tide) the direction of water spiralling down a plughole is not influenced by Coriolis force, but was thwarted when I realised that I only have mixer taps in my house and as I was reading the book in lockdown, couldn’t go somewhere else. When things return to normal, I’ll get down to the library and experiment there.
Chapter 3 explores the measurement and prediction of tides. It is quite impressive that despite the complicated nature of tidal dynamics, tides can be accurately forecast for several years in advance. Measurement includes two components: vertical rise and fall (the tide) and tidal flows (tidal streams and currents). Ways of measuring both are discussed here, including the use of tide poles and tidal gauges, such as stilling wells and pressure gauges, along with the means of measuring the speed and direction of currents, which of course substantially vary spatially, especially with depth. There is discussion of the various instrumentation used for measurements, as well as how measured data are typically visualised. The chapter ends with a very useful section on the analysis of tidal data, covering the main analytical methods that will be essential for anyone studying physical oceanography, using worked examples to help develop understanding.
Chapter 4 is a relatively short chapter on tides in shelf seas (which are <200 m deep). This covers progressive (travelling) waves across continental shelves, and the standing waves created when such progressive waves bounce back from coasts. There then follows some detail on sea shelf resonance and tide amplification, with an example from Adelaide in South Australia. I feel that this section and the examples given will be very useful to students working through the mechanics and calculations of tides. The discussion is rounded off with a look at the effects of Earth rotation on progressive tides, and what happens when progressive tides enter relatively shallow water, including the rare phenomenon of ‘double high water’.
Chapter 5 (Tidal Bores) is dedicated to, yes, tidal bores (or tidal waves), how they form in narrowing and shoaling estuaries, the importance of both estuary shape and tidal range in determining their characteristics, and so on. This was an interesting chapter on an impressive aspect of tides, and the authors helpfully include some information on where bores are most common, should anyone desire to see (or surf) them, as well as advising that although bores can be predicted they are (as with all things tidal) a little complicated, so best to arrive early to avoid disappointment! The chapter also notes some of the most impressive (or well-known) bores around the world, with a sprinkling of anecdotal flavour that lightens the material a bit. There then follows a summary of five different types of tidal bore, and further detail on bore-associated phenomena such as whelps and the notorious rumbling noise that bores make. The chapter ends with a couple of brief paragraphs on the role that tidal bores play in estuarine ecosystems (being a unique type of flow), and a short discussion of other types of bore, along with tidal races.
Chapter 6 (Tides and the Earth) is about energy. The authors provide a nice overview of how tidal energy originates from the spin of the Earth and, because energy is lost through friction, energy transfer continually occurs to the tides from said spin, causing the Earth to slow in its rotation such that the day gets longer by around 1.9 milliseconds each century. This is then followed by discussion of tidal friction in shelf seas, including turbulence, and then of internal tides (where tidal motion occurs in layers of different density seawater). The chapter ends with a discussion of thermohaline circulation as a means of the tides redistributing solar heat, which of course falls unevenly across the Earth’s surface due to the curvature of the atmosphere.
Chapter 7 continues some of the themes from Chapter 6 in terms of turbulence and mixing, but focuses on shelf seas. I enjoyed the section on maelstroms, which tend to be associated with narrow tidal sea straits and have captured the imagination of artists for centuries. The discussion then moves onto heat storage and mixing, tidal mixing fronts, turbidity and mixing in estuaries. There is also some discussion of use of the tide to generate electricity, although this is quite brief and contains a slightly confusing segue into the design of turbines by students at a university in Mozambique. I would have liked this section to say more about potential benefits of the tide for harnessing energy, with a wider range of examples and possibilities, given its potential importance.
But never mind, because Chapter 8 (New Frontiers) provides an exciting climax to the tidal journey. The authors explore what might lie in store as researchers investigate tides in relation to deep-sea ecosystems (which we know little about due to their inaccessibility; I was intrigued by the material on deep sea sponges); tidal interactions with sunlight (and the ecological implications of this); tides back when the Earth was young (knowledge of which can help us age the Moon’s orbit, and determine how important the tides might have been in driving evolution and the colonisation of the land during the Devonian); and tides on other planets (especially the moons Io and Europa, which orbit Jupiter).
Overall, this is an interesting and well-written book that provides an invaluable introduction to all things tidal. Some difficult concepts are explained in approachable and engaging ways. One small criticism is that sometimes when a new section starts, the key terms are not clearly defined or explained, so I did find myself turning to the (very helpful) glossary quite a bit. This can be quite common in books where the terms are second nature to expert authors, but a bit more spoon-feeding might be required for the more general reader. Nonetheless, this is most certainly a book I would recommend to any oceanography students, or indeed physical geography students keen to get an understanding of tides and their importance in marine, coastal and estuarine environments.