The equations of fluid motion
In order to proceed further with our discussion of the circulation of the atmosphere,
and later the ocean, we must develop some of the underlying
theory governing the motion of a fluid on the spinning Earth. A differentially
heated, stratified fluid on a rotating planet cannot move in arbitrary
paths. Indeed, there are strong constraints on its motion imparted by the
angular momentum of the spinning Earth. These constraints are profoundly
important in shaping the pattern of atmosphere and ocean circulation and
their ability to transport properties around the globe. The laws governing
the evolution of both fluids are the same and so our theoretical discussion
will not be specific to either atmosphere or ocean, but can and will be applied
to both. Because the properties of rotating fluids are often counter-intuitive
and sometimes difficult to grasp, alongside our theoretical development we
will describe and carry out laboratory experiments with a tank of water on
a rotating table (Fig.6.1). Many of the laboratory experiments we make use
of are simplified versions of ‘classics’ that have become cornerstones of geophysical
fluid dynamics. They are listed in Appendix 13.4. Furthermore we
have chosen relatively simple experiments that, in the main, do nor require
sophisticated apparatus. We encourage you to ‘have a go’ or view the attendant
movie loops that record the experiments carried out in preparation of
our text.
We now begin a more formal development of the equations that govern
the evolution of a fluid. A brief summary of the associated mathematical
concepts, definitions and notation we employ can be found in an Appendix
13.2.