A spatial imbalance between radiative inputs and outputs exists for the earth-ocean-atmosphere system. The earth loses energy at all latitudes due to outgoing infrared (IR) radiation. Near the tropics, more solar radiation enters than IR leaves, hence there is a net input of radiative energy. Near Earth’s poles, incoming solar radiation is too weak to totally offset the IR cooling, allowing a net loss of energy. The result is differential heating, creating warm equatorial air and cold polar air (Fig. 11.1a).
This imbalance drives the global-scale general circulation of winds. Such a circulation is a fluiddynamical analogy to Le Chatelier’s Principle of chemistry. Namely, an imbalanced system reacts in a way to partially counteract the imbalance. The continued destabilization by radiation causes a general circulation of winds that is unceasing.
Because buoyancy causes warmer air to rise and colder air to sink, you might guess that equator-topole overturning would exist (Fig. 11.1b). Instead, the real general circulation has three bands of circulations in the Northern Hemisphere (Fig. 11.1c), and three in the Southern. In this chapter, we will identify characteristics of the general circulation, explain why they exist, and learn how they work.