In this chapter, we discuss notions of typical lengths, typical times, typical speeds etc. It is by no means obvious what is meant by this, therefore a few words by way of introduction are appropriate. Many, not to say most variables can be expressed in terms of time (measured in seconds), length (measured in metres) and mass (measured in kilograms). Examples are speed (length divided by time), density (mass divided by volume, hence mass divided by length3) and pressure (mass divided by (length times time2)). The quantities mass, length and time are called the fundamental quantities. In a given situation, most users of models as well as the builders of the models themselves wish to concentrate on phenomena which have a specific limited range of any of the fundamental quantities, or of variables derived from them. For example, a tidal modeller would not be interested in small time scales of a few seconds or large time scales of many centuries, but something in between. An estuarine modeller would not be concerned with length scales of hundreds or thousands of kilometres but something much less. Most phenomena in ocean science are scale specific due principally to the dominance of the Coriolis effect, and the art of successful modelling is more often than not linked to the ability to screen out the unwanted in order to focus on what is desired. Dimensional analysis forms an essential element of this simplification. Once this simplification has taken place, the terms that remain constitute a simplified dynamic balance and these balances are useful in describing the fundamental movements of the sea.
KeywordsWind Stress Eddy Viscosity Potential Vorticity Dynamic Balance Gulf Stream
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