Thermal equilibrium requires that, for the Earth-Atmosphere system as a whole, net radiative heating must vanish. Although it applies globally, this requirement need not hold locally. In fact, net radiation (Fig. 1.34c during DJF) shows that low latitudes experience radiative heating: They receive more energy through absorption of SW than they lose through emission to space of LW. Conversely, middle and high latitudes experience radiative cooling, especially in the winter hemisphere: They lose more energy through emission to space of LW than they receive through absorption of SW. To preserve thermal equilibrium, these local imbalances in the radiative energy budget must be compensated by a poleward transfer of heat. Accomplished mechanically, the latter transfers energy from tropical regions, where it offsets the surplus of radiative energy, to extratropical regions, where it offsets the deficit of radiative energy. The poleward transfer of heat is accomplished by the general circulation of the Earth-atmosphere system, 60% of it by the circulation of the atmosphere (see Fig. 17.10).

The simplest mechanism to transfer heat poleward is a steady, zonally symmetric circulation between the equator and poles. Such motion is driven by atmospheric heating in the tropics and cooling in the extratropics. Atmospheric heating is concentrated at low latitude, where it derives from latent heat release inside centers of convection (Fig. 9.41b). Together with radiative cooling at higher latitude, it forces vertical motion across isentropic surfaces (Secs. 2.5, 3.6). The latter must be compensated by horizontal motion.