Supplemental reading:

Holton (1979), sections 9.2, 9.3

Pedlosky (1979), sections 7.1–7.3

Remarks

In Chapters 8–11 we examined the wave properties of the atmosphere under a variety of circumstances. We have also considered the interactions of waves with mean flows. In Chapter 7, we noted that eddies would have to be involved in transporting heat between the tropics and the poles. Thus far our study of waves has not provided much insight into this matter. As it turns out, vertically propagating stationary Rossby waves do carry heat poleward. This heat transport, while not insignificant, is largely a byproduct of the fact that the wave momentum flux acts to reduce shears, and geostrophic adjustment involves a concomitant reduction of meridional temperature gradients. Unfortunately, we will not have time to study these mechanisms in these notes. However, the quasi-geostrophic framework established in Chapter 12 greatly simplifies such studies. This framework will be used to study how travelling disturbances arise in the atmosphere. We will also sketch (in Chapter 14) some results which suggest that these travelling disturbances play the major role in determining the global, temperature distribution. The generation of such disturbances involves hydrodynamic instability, and before diving into this problem in a meteorological context, it will be useful to examine stability in simpler situations.

Before beginning this topic we should recall possible wave (eddy) sources considered thus far:

1. Direct forcing as produced by tidal heating, flow over mountains, or flow through quasi-stationary inhomogeneities in heating.

2. Resonant free oscillations. These are presumably preferred responses to any ‘noise'.