大气环流课件：HADLEY CIRCULATION DYNAMICS.pdfVIP

Chapter 2 HADLEY CIRCULATION DYNAMICS Seasonality and the Role of Continents Kerry H. Cook Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York14853- 1504, U.S.A. Abstract The equations that govern the Hadley circulation are reviewed, and the ob- served circulation is described. Atmospheric general circulation model (AGCM) simulations are used to evaluate the dominant zonally averaged momentum and thermodynamic balances within the Hadley regime. A diagnostic application of the governing equations is used to identify the mechanisms of the Hadley circulation’s seasonal evolution between equinox and solstice states. A “vertical driving” mechanism acts through the thermo- dynamic balance, and is important for regulating the circulation’s strength when heating differences between seasons are close (within ~5º) to the equa- tor. A “horizontal driving” mechanism acts through the horizontal momentum equations and is more effective off the equator. Unlike the results from axi- symmetric models in which the prescribed heating is always close to the equator, the horizontal forcing mechanism is responsible for most of the Hadley circulation seasonality in the reanalysis and GCM simulations. The presence of continental surfaces introduces longitudinal structure into tropical diabatic heating fields, and pulls them farther from the equator. The winter Hadley cells in a simulation with continents are much stronger than in a simulation with no continents, and