We are all familiar with the fact that the weather differs from the climate, the difference arising because the atmospheric flow is unsteady, and we talk more about this in chapter 6. The same applies to the ocean, only more so: the large-scale currents in the ocean are almost all unstable, rather like a river flowing over rapids, and tend to break up into smaller mesoscale eddies, as illustrated in the lower panel of figure 2.6. The resulting eddies are the oceanic analogue of atmospheric weather, although because of differences in the physical properties of the
two systems ocean eddies tend to be much smaller, with scales of 50-300 km, compared to 500-3,000 km in the atmosphere. In spite of their small scale, the ocean eddies are energetic—the total kinetic energy associated with the eddies in the ocean is about ten times larger than the kinetic energy in the mean currents. Nevertheless, the eddies do not completely destroy the mean flows or make them meaningless, although they can obscure them from easy view and straightforward measurement. In fact, even though eddies dominate the energy budget, they don't dominate the global-scale transport of important properties (such as heat and salinity), and recognizable large-scale oceanic flows remain as a residual after appropriate averaging. Understanding how the eddies and the large-scale mean currents interact remains a daunting challenge in physical oceanography, and the reader may wish to contemplate for a moment the two views of the ocean, juxtaposed in figure 2.6. A full reconciliation of these contrasting views and an understanding of how they fit together are perhaps now coming into our reach, although tantalizingly beyond our grasp.
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