GFD-II-Ch.6. Instability. 2011-02-17 1
Chapter 6: Instability
Sec. 6.1: Instability
The ocean-atmosphere system is forced by the solar radiation. This forcing is usually very
constant. Yet, observations show substantial fluctuations in both the atmosphere and the ocean
at various time scales, such as the atmospheric cyclones, storms, marine eddies and El Nino.
This variability occurs because the mean state is physically unstable to infinitesimal
The simplest example of instability is the gravitational instability of a ball on a supporting
surface. In principle, the ball is in an equilibrium state as long as the local supporting surface is
flat. In practice, however, only the equilibrium state is physically achievable only when the
nearby supporting surface is a well. In the case when the supporting surface concaves
upwards, the ball will practically not stay in equilibrium because any infinitesimal perturbation
will push the ball off the equilibrium position.