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.

In Chapter 1, we presented a survey of the general circulation encompassing both Northern and Southern Hemispheres and winter and summer seasons. In this chapter, we focus on the Northern Hemisphere winter season DJF, which arguably exhibits the most distinctive patterns in terms of zonally varying jets, storm tracks, and climatological‐mean stationary waves.

This chapter documents and offers a dynamical interpretation of the annual mean tropical circulation. It is made up of six sections. The first documents the patterns of rain rate, vertical velocity, and low cloud coverage. The second and third document and interpret the upper and lower tropospheric circulations in terms of equatorially trapped planetary waves introduced in Chapter 10 and relate them to the observed rain rate distribution.

In this chapter, we consider the leading mode of year‐to‐year climate variability, the El Niño–Southern Oscillation phenomenon, widely referred to as ENSO. El Niño connotes the episodic weakening of the equatorial Pacific SST cold tongue.1 Southern Oscillation refers to a “seesaw” in sea‐level pressure (SLP) between the eastern and western ends of the tropical Pacific Ocean.

From a global perspective, the dynamics of wave–mean flow interaction in the stratosphere is dominated by Rossby waves, as described in the previous chapter. However, the tropical stratosphere is a notable exception, which merits a chapter of its own.

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 datasets and analysis tools for diagnosing the zonally varying general circulation that became available during the 1970s made it possible, for the first time, to clearly discern the signature of low frequency variations. This new capability sparked interest in phenomena that had been known to long‐range weather forecasters dating back to the early twentieth century statistical studies of Exner and Walker, but had not hitherto been studied in the context of advancing our understanding of the general circulation.

Warm core tropical vortices are distinctly different from any of the motion systems considered in previous chapters. In the literature they are referred to as tropical depressions, tropical storms, or tropical cyclones, in order of increasing intensity. Tropical cyclones (TCs) are also known by local names such as typhoon and hurricane.

The last two chapters were devoted to the seasonal cycle in the tropical general circulation and to ENSO‐related interannual variability. In this chapter, we consider the variability on the intraseasonal timescale, defined here as fluctuations with periods ranging from 20 to 90 days (or frequencies ranging from 1 to 5 cycles per season).

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 governing equations for the tropical and extratropical general circulations differ in two respects: one relating to the relative importance of the terms in the horizontal equation of motion and the other to the terms in the thermodynamic energy equation. The extratropics are nearly in geostrophic balance.