The Elementary Hadley Circulation

Abstract

In the most basic terms, the Hadley circulation can be thought of as a large-scale overturning of the atmosphere driven by latitudinal heating gradients, extending roughly between the Tropics of Cancer and Capricorn covering roughly half the surface area of the planet. Rising air occurs near the equator with subsidence in the subtropics. The circulation has a strong seasonal variability. It is manifested during the equinoxes as a pair of relatively weak cells with a common rising zone near the equator termed the Intertropical Convergence Zone (ITCZ). A much stronger cross-equatorial cell marks the solstitial seasons with rising motion in the summer hemisphere and widespread descending air in the winter hemisphere. The meridional circulations are instrumental in determining where tropical rainfall occurs and where the great deserts are located. Variability of the location and intensity of the Hadley circulation (or its regional manifestation such as the monsoons), through the ages has helped shape the history of mankind, either spawning regions of civilization by providing an abundance of rainfall for agriculture or destroying them by periods of drought. The variability of the Hadley circulation is also manifested on interannual times scales as an important component of the waxing and waning of El Nino in the Pacific Ocean, perturbing seasonal climates worldwide. The Hadley circulation was the first phenomenon to be described by using the physical insight of the natural system emerging out of the Renaissance. Both Halley (1686) and Hadley (1735) provided basic accounts of the physical processes that drive the meridional cells. However, a detailed examination of the phenomenon, using data sets that are now available, shows that many questions cannot be answered in the confines of the Halley-Hadley model. For example, what limits the latitudinal extent of the cells? What is the role of the Hadley system in balancing the planetary heat budget? What factors determine the vertical scale of the Hadley circulation? Why is there considerable longitudinal variability in the strength of the circulation? How does the ocean interact with the atmospheric Hadley circulation and is there an oceanic counterpart? An attempt is made to answer these questions from a fundamental physical perspective. It is found, for example, that the vertical transport of heat and the heat balance of the tropics in the ascending branch of the Hadley circulation are difficult to understand without considering "undiluted hot convective towers," first considered by Riehl and Mallcus (1958). An explanation of the depth of tropical convection follows by consideration of the magnitude of the sea surface temperature (SST) and the stability of the tropical atmosphere. Furthermore, both the atmosphere and the ocean meridional cells contribute to the poleward transport of heat. In the atmosphere, it is the instabilities of the Hadley cell (the middle latitude eddies or waves in the westerlies) that complete the transport of heat towards the poles. It is shown that the atmospheric Hadley circulation drives an oceanic circulation that acts as a negative climate feedback. Finally, a simple model of the combined ocean-atmosphere system is presented that underlines the importance of both the oceanic and the atmospheric Hadley circulations in balancing the heat budget of the planet.