For many years the Flinders University of South Australia has offered a first year topic Earth Sciences in two parts. The first semester topic, Earth Sciences 1A, covered the place of the Earth in the universe, aspects of geology, and an introduction into geophysics and hydrology. Meteorology and oceanography were covered in the second semester topic Earth Sciences 1B.
Beginning in 2000 the two topics are delivered as Earth Sciences 1, which continues as the first semester topic with identical content, and Marine Sciences 1 as the second semester topic. Marine Sciences 1 still contains extensive material on meteorology and physical oceanography but also contains an elementary introduction to aspects of marine biology.
These notes represent the topic content for physical oceanography. In addition, two introductory lectures place the atmospheric and oceanographic aspects of the topic in the context of the exact sciences; they are an abbreviated version of the first two lectures given at the beginning of the semester.
The sciences of meteorology and oceanography study the results of these processes in the atmosphere and in the ocean.
Figure 1
The living and non-living components of the interactive system Earth are shaped through interaction - even the structure of its non-living components is determined by the presence of life.
This is most pronounced for the atmosphere; its composition is determined by the presence of life (fig. 1).
In the 1960s the British chemist James Lovelock was engaged by the space agency NASA of the USA in its search for extra-terrestrial life, particularly on Mars and Venus. Noticing the large difference in atmospheric composition between Earth and the other two planets - and in particular the fact that the atmospheres of Venus and Mars are in a dead equilibrium while the atmosphere of Earth is in a dynamic equilibrium, which without life would immediately revert to the dead equilibrium - he and Lynn Margulis, a microbiologist from the USA, developed the gaea concept or hypothesis.
They point out that the presence of life has far reaching consequences for the planet as a whole. The development of forest, for example, reduces the albedo (the reflectivity of the Earth's surface) considerably. As a result the Earth is several degrees warmer than it would be without the presence of life. The gaea hypothesis therefore states that the planet Earth is a living organism itself, and oceans, land, air and all lifeforms are different organs of a living body.Whether one accepts the gaea hypothesis in its extreme formulation or not, it is beyond doubt that the gaea hypothesis is a scientific hypothesis and can withstand the many attempts to turn it into a "new age religion". The fact remains that Earth as it is today is determined in its physical state (the distribution of water and ice, the composition of the atmosphere, the weathering processes of rocks, and much more) by the presence of life.
Figure 2Modern science has been extremely successful in explaining the Earth by dividing it into compartments (fig. 2) which can be studied separately. It is not well equipped to approach Earth as an interrelated organism. This "western" way of analyzing and understanding the world is also reflected in the structure of western languages, which compose sentences through subject-object relationships wich always establish a clear master-servant hierarchy. A sentence such as: "The engineer improves the environment" says that there is an environment, of which the engineer is not a part; he is the master of the environment. Other cultures do not divide the world into compartments, and their languages describe the world in totally different ways. There are many examples of American Indian languages which do not know the concept of subject and object; and if the Australian Aborigines say: "We are the land, and the land is us." they express their view that dividing the world into compartments can make you loose sight of important interactions between the various "spheres."
Keep in mind that meteorology and oceanography are just two compartments of a system with many living and non-living interactive components and that studying processes through meteorology and oceanography is only one way of studying the world. Nevertheless, the success of western science in explaining how the physical world works should not be dismissed lightly, and we shall follow its methods.
With this proviso, let us proceed to the study of physical processes in nature and look at three examples: convection, eddies and waves.
A fluid can be stratified, which means that its density can vary. For a fluid to be in a stable state, its density has to decrease from the bottom upwards to the top.
Figures: 4 - 5 - 6
Convection occurs when this condition is not satisfied. Instability occurs when the density of the fluid is higher at the top than at the bottom. The lighter fluid then rises to the top, the denser fluid sinks to the bottom until stability is achieved. The resulting movement is called convection.
Convection represents a balance of forces between gravity and friction. A third force is required to establish the initial instability. The space and time scales of convection depend on the viscosity of the fluid. The examples given in the figures show convection in the "solid" earth, atmosphere and ocean (figs. 4, 5 and 6).
Figures: 7-8-9
Eddies are the results of instabilities in fluid motion. They involve a somewhat more complicated balance of forces than what we intend to study here, but they are such common features that it is instructive to look at some examples and compare again the "solid" earth, the atmosphere and the ocean. The similarity of eddies in the atmosphere and in the ocean will be discussed in more detail in Lecture 1 later in this course. In this context it is worth noting that the "solid" earth undergoes very similar processes, although on much longer time scales.
In the atmosphere (fig. 7) and in the ocean (fig. 8) eddies can be generated from wind shear or current shear, ie when the fluid moves in the same direction but with different speed. The high viscosity of the "solid" earth often prevents eddy formation even when there is shear in the movement of the mantle or crust. Folding (fig. 9 is observed instead.
Figures: 10-11-12
Waves are a balance of forces where the forces vary periodically in strength and produce periodic fluid motion as a result. They are an efficient means to transport energy over large distances.
There will be ample opportunity later in this topic to study the interaction of forces in wave motion in detail.
At this point we use waves as another example which demónstrates again that the "solid" earth (fig. 10), the atmosphere (fig. 11) and the ocean (fig. 12) are three different types of fluid in motion.
There are many ways in which waves can be generated. The examples shown in the figures represent different force balances. What they have in common is that their properties can be understood and their behaviour predicted on the basis of the Laws of Physics.
In today's world human activity - be it industrial, commercial or recreational - will shape our environment more than ever before. Active environmental management on a global scale has become a necessity.
Human activity cannot override the Laws of Nature. Active environmental management has to be based on these laws, it cannot succeed if it attempts to oppose them.
The sciences of meteorology and oceanography investigate and explain how the Laws of Physics determine processes in the atmosphere and oceans. They form the basis for any environmental management.
Responsible environmental management takes into account many factors, such as economical, social and historical considerations; but it cannot ignore the Laws of Physics.
