The term atmospheric general circulation, as used in this book, connotes a statistical representation of the three‐dimensional, time varying flow in the global atmosphere, including the cycling of zonal momentum, energy, water vapor, and other trace constituents.

In this chapter, we revisit one of the classical topics of atmospheric dynamics: the maintenance of the zonal mean zonal flow $\left[u\right]$ relative to the rotating Earth.

On a rotating planet, the zonally symmetric zonal wind and temperature fields are in thermal wind balance. By applying this dynamical constraint, it is possible to go beyond the consistency arguments for steady state balances in Eqs. (3.21) and (5.20) and deduce how the flow will evolve in response to specified, time varying distributions of diabatic heating rate, frictional drag, and the eddy transports of zonal momentum and heat. In this zonally averaged version of the primitive equations, which dates back to Eliassen,1 the mean meridional circulations play a critical role in enforcing the constraint that the zonal wind and temperature fields remain in thermal wind balance as the flow evolves.

This chapter introduces some of the fundamental concepts that underlie our understanding of the general circulation of planetary atmospheres: radiative–convective equilibrium, a mechanical energy cycle, a thermodynamic heat engine, stratification – how it develops and why it matters, the dynamical response to horizontal and vertical heating gradients, the influence of rotation, the far‐reaching effects of frictional drag.

Wave–mean flow interaction has played a central role in studies of the general circulation, dating back to the foundational works of Rossby, Starr, and collaborators. In the early studies the waves were usually referred to as “eddies” (as in “turbulent eddies”) without regard for the specific kind of instability or forcing mechanism that gave rise to them. Starr was particularly intrigued with the countergradient transports of angular momentum equatorward of the tropospheric jet stream.1

Parts II, III, and IV are exclusively concerned with the zonally averaged circulation. All representations of the eddies and the transports that they produce are based on zonally averaged statistics.

Total energy connotes the sum of the internal and mechanical (i.e., internal plus potential plus kinetic) energy, where the kinetic energy is ordinarily neglected, as justified in Exercise 5.4. Observational studies of the long‐term mean global energy balance dating back to the 1950s demonstrate the central role of the poleward eddy heat transports. Using space‐based measurements of radiative fluxes through the top of the atmosphere, it is now possible to partition the total poleward transport of energy between the atmosphere and the oceans and to monitor seasonal and nonseasonal variations in energy storage in the oceans.

The total energy per unit mass of an air parcel is the sum of its internal, potential, and kinetic energy. It can be shown (see Exercise 6.1) that integrated over a column of unit area, the sum of the potential $P$ plus internal $I$ energy is given by $P+I=\int c_{p}Tdp$.

The first studies of the mass balance of atmospheric trace constituents were focused on water vapor. The earliest of these studies were motivated by the fact that the release of latent heat of condensation in precipitation is an important heat source in the global energy budget, the subject of Chapter 5. These early studies also provided new insights into the hydrologic cycle, particularly over land, and were helpful in explaining the observed salinity distribution in the ocean.

Part I consists of two chapters. The first describes the observational basis for general circulation, documents its salient features, and introduces the reader to the kinds of models that are being used to simulate it.

When plotted as partial zonal averages in Fig. 16.1, the seasonality of the zonal mean circulation in the eastern and western hemispheres of the tropics is quite different. In the eastern hemisphere (from the Greenwich Meridian eastward to the Date Line), the zonal mean circulation is dominated by the seasonally reversing Australasian monsoon, which is strong and nearly synchronous with the annual cycle in the meridional profile of insolation. In contrast, in the western hemisphere, the seasonality is not as pronounced and the annual cycle is lagged by about two months relative to the solstices.