Introducing Oceanography

David N. Thomas

Description

Two thirds of our planet is covered by oceans and seas. Over recent decades developments in ocean science have dramatically improved our understanding of the key role oceans play in the Earth System, and how vital they are for regulating global climate. Humans depend on the oceans for many resources, but at the same time their impacts on the marine systems around the world are of increasing concern. Introducing Oceanography has been written by two leading oceanographers to provide a succinct overview of the science of the study of the seas for students and for the interested adult wanting a topical guide to this enormous and complex subject. The initial chapters describe the oceans and the forces at work within them. The authors then discuss the effects of light, the chemistry of the seas and the food web before surveying biological oceanography in the main oceanic regions. The final chapter looks at the methodology of ocean study. Copiously illustrated, this book is intended for those whose interest in oceanography has been stimulated, perhaps by media coverage of declining resources or climate change and who want to know more. Technical terms are kept to a minimum and are explained in a glossary.

Sujets

• Sciences et techniques

Informations

• Publié par: Dunedin Academic Press
• Date de parution: 01 juin 2021
• Nombre de lectures: 0
• EAN13: 9781780466613
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

Introducing Oceanography
SECOND EDITION
David N. Thomas David G. Bowers
Contents
Preface
1 The water of the oceans
2 Density and density flows
3 Flow in the oceans
4 Ocean waves
5 The tides
6 The edge of the ocean
7 Light in the sea
8 Life in the oceans
9 Chemistry of the oceans
10 Primary productivity in the oceans
11 Ocean food webs
12 Life at the ocean extremes
13 Changing oceans
14 Making measurements
Glossary
Further reading
Sourced illustrations
Dedication: For several years we both had the privilege of teaching a course on Shelf Sea Oceanography at Bangor University with Dr Sarah Jones (1962–2008). Sarah was an inspiration to the students as well as to us. We dedicate this book to her memory.
Figure i.i A tangled net – just a few of the interconnections between human health and activities in and around seas and ocean.

Preface
The study of the oceans, or oceanography, is of critical importance to all humans on the planet. Of the 195 nations in the world there are only 44 that are totally land-locked. But even those living cut off from the oceans depend on shipping to transport goods and services. The oceans are a rich source of food for billions of humans. There is nowhere on the surface of the planet that does not rely on the effects that the oceans have on climate, and even more importantly in driving major water, oxygen, carbon dioxide, nitrogen and other cycles of the elements fundamental to life on Earth.
We are just about to enter the United Nations Decade of the Ocean for Sustainable Development (2021–2030). Our oceans will be highlighted in a global campaign in a way they have never been before: the complex interactions between human activity and oceans/seas within the framework of the 17 UN Sustainability Development Goals (SDGs) will be challenge global leaders and stakeholders, and genuinely change how societies engage with the oceans in the future. There can be no doubt that the interactions between human health, well-being and activities in and around seas and oceans are complex ( Fig. i.i ). It is the unravelling of that complexity that will be the challenge, and a huge responsibility for the oceanographic community in its widest sense.

Figure i.ii Spilhaus projection of the World’s ocean showing sea surface temperature ranging from -1.8°C to 35°C. The surface velocity magnitude is shown to highlight the currents and eddies.
When we consider the oceans we understandably tend to consider specific oceans and seas that have immediate effect on our lives. However, much can be gained by thinking about the world’s oceans being connected through large-scale water movement. The remarkable Spilhaus projection shown in Figure i.ii conveys this concept wonderfully. When something happens in one part of the planet it is easy to see how there will be knock-on effects in other parts of the oceans.
It would be difficult to over-estimate the importance of the ocean to our own planet and to ourselves: now, in the past and in the future. Life on Earth began in the ocean, taking advantage of the relatively stable conditions and the supporting buoyancy of the water. During the Devonian period (420–360 million years ago), it is thought that some animals gradually left the sea to try out life on land. They were possibly helped in this great step by being stranded for a few hours by the tide (and rescued by the next tide), allowing them to gradually adapt to conditions out of water. The oceans are a key part of the climate control system of our planet – they distribute the heat of the Sun from low to high latitudes and absorb about half the carbon dioxide that we make. As we look to a future without fossil fuels, the ocean can provide a virtually infinite source of renewable energy in the form of tides, waves and the heat it has stored.

Figure i.iii The study of ocean processes varies from the tiny scale physics at the size of a bacteria cell through to global ocean circulation patterns over tens of thousands of years.
There is no escaping the importance of studying the oceans and understanding how they work. For oceanographers it is a privilege to work in such an important and exciting research field. As can be seen from Figure i.iii , processes in the oceans span time- and space-scales from fractions of millimetres and seconds through to those lasting over tens of thousands of years and influencing millions of square kilometres.
No individuals can study all aspects of oceanography by themselves. The oceans are complex chemical mixtures; the biological diversity and abundances are huge; a myriad of physical processes are vital for determining where the water goes and why. The oceans span the climate regions from the poles to the tropics. So, oceanographers include specialists in physics, chemistry, biology, geology, mathematical modelling, atmospheric scientists, and space scientists working with satellites, among others. People working on the possibilities of extraterrestrial life also look to the ocean for proxies of life further afield. To make any sense of their individual measurements it is often (mostly) vital that scientists from these different disciplines collaborate to synthesise their findings to understand the whole picture.
It is possible to study oceanography without ever going to sea. Earth-orbiting satellites can map the ocean surface and autonomous underwater vehicles are able to measure and photograph the ocean deeps. Automated instruments measure sea level and tides. Biologists and chemists work in laboratories on samples collected by others at sea. Computer models are continually growing more powerful; they can seek to mimic what is happening in today’s ocean and predict how it will look in the future.
Despite these state-of-the-art technologies that enable us to observe the oceans from afar, often the only way to understand the ocean is to take to the seas in a research vessel. Such oceanographic cruises are voyages of discovery and thrilling opportunities. They are hard work and can last many months (though some can be just a matter of hours or days). They can take the ships to regions of the world where wind and waves conspire to make life on board a ship very uncomfortable indeed. On the other hand, watching the sun set (or rise) on the horizon of a calm tropical ocean is an exhilarating and humbling experience.
In this short introduction we aim to give you a flavour of what oceanography is about. Naturally we can only touch on a few issues, but we hope to give you enough to make you want to find out more.
Note: all terms highlighted in bold are defined in the Glossary at the end of the book.
1 The water of the oceans
Oceanography is the scientific study of the oceans, and of the creatures that live in them. Roughly 70% of the surface of our planet is covered in water, nearly all of it the salt water of the ocean, with just a tiny fraction in freshwater lakes and rivers. It’s not at all clear how the water came to fill our ocean basins in the first place. Its origin may have been outside the Earth – for example, ice may have been deposited as our planet passed through the tail of a comet. This possibility explains the interest in landing on comets in recent space missions – to see if the composition of the ice in a comet tail matches that of our oceans. Another possibility is that water leached out from the rocks of the Earth as our planet cooled. These theories are, of course, not mutually exclusive and the ocean could have been formed by a combination of the two.
The Earth is the only place we know that has a liquid water ocean on its surface. In our solar system, the inner planets – Mercury and Venus – are far too hot for water to remain in liquid form and the outer planets – from Mars to Neptune – are far too cold. As we seek oceans on planets beyond our solar system, it will be necessary to look for bodies orbiting their sun in what is sometimes called the Goldilocks zone – the habitable belt around a star in which the temperature is just right for liquid water to exist.
A good place to start the study of oceanography is by considering the ocean dimensions. The Earth viewed from space appears as a blue planet because of all the surface water, but the oceans are actually not very deep compared to their horizontal extent. Figure 1.1 shows a map of ocean depths. There are mountains and valleys on the sea floor, just as there are on dry land, but the average depth of all the oceans is about 4 km. In comparison, the horizontal dimension – the width – of the oceans is of order 10,000 km. The ratio of the width to depth (a figure called the aspect ratio ), is therefore 10,000:4, or 2,500:1. That’s about the same as the aspect ratio of the page you are reading right now. Although oceans are very thin compared to their width, changes with depth are very important, as we shall see in this chapter. For example, the temperature of ocean water can change as much in going down a few kilometres from the surface at the equator to the ocean floor as it can in travelling several thousand kilometres along the surface from the equator to the poles.

Figure 1.1 Map showing the ocean depths.
Around the margins of the oceans there are shallow water bodies called shelf seas. These are much smaller in horizontal extent (say 250 km) and shallower (typically 100 m) than the oceans, but they have a similar aspect ratio. Examples of shelf seas are the North Sea in Europe and the Yellow Sea in Asia.
1.1 Salinity and temperature of ocean water
The most obvious difference between ocean water and water that comes out of the tap is that the ocean is salty . It is not obvious where the salt comes from: rivers flowing into the ocean appear to carry perfectly fresh, drinkable water. We return to this topic in chapter 9 but we can say here that salt is added to the oceans by river water (in very small concentrations) and (in much larger concentratio