Supplemental reading:

Lorenz (1967)

Held and Hou (1980)

Schneider and Lindzen (1977)

Schneider (1977)

Lindzen and Hou (1988)

As we noticed in our perusal of the data, atmospheric fields are far from being zonally symmetric. Some of the deviation from symmetry is forced by the inhomogeneity of the earth's surface, and some is autonomous (travelling cyclones, for example). Nevertheless, the zonally averaged circulation has, over the centuries, been the object of special attention. Indeed, the term ‘general circulation’ is frequently taken to mean the zonally averaged behavior. This is the viewpoint of Lorenz (1967).

You are urged to read chapters 1, 3, and 4 of Lorenz. Chapter 1 is a short and especially insightful discussion of the methodology of studying the atmosphere. As is generally the case in this field, there will be views in Lorenz which are not universally agreed on, but this hardly diminishes its value.

There are several reasons for focussing on the zonally averaged circulation:

1. Significant motion systems like the tropical tradewinds are well described by zonal averages.

2. The circulation of the atmosphere is only a small perturbation on a rigidly rotating basic state which is zonally symmetric.

3. The zonally averaged circulation is a convenient subset of the total circulation.

Our approach in this chapter will be to inquire how the atmosphere would behave in the absence of eddies. It is hoped that a comparison of such results with observations will lend some insight into what maintains the observed zonally averaged state. In particular, discrepancies may point to the rôle of eddies in maintaining the zonal average.