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.

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.

The tropical atmosphere encompasses the latitude belt equatorward of the subtropical anticyclones at the Earth’s surface and the tropospheric jet streams at the tropopause level. As shown in Section 2.6.1, the meridional extent of the tropics decreases with increasing rotation rate.

The balance requirement approach covered in Part II provides a reasonable explanation of how the atmosphere satisfies the various budget constraints imposed by the conservation of mass, momentum, total energy, and mechanical energy, but it does not go very far in addressing such questions as:

• Why is there a single pair of tropospheric jet streams located around 30∘N/S?

• Why do the eddies transport angular momentum poleward, across 30∘N/S, maintaining the trade‐wind and westerly wind belts?

• Why do the diagnoses based on the angular momentum balance in Chapter 3 and the energy balance in Chapter 5 yield the same configuration of mean meridional circulations?