Hadley Circulation Dynamics
The equations that govern the Hadley circulation are reviewed, and the observed 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 thermodynamic balance, and is important for regulating the circulation’s strength when heating differences between seasons are close (within ~5°) to the equator. 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 the summer cell is half the intensity of that when continents are included. The strengthening of the winter cell occurs through an increase in low-level wind speeds, which enhances the zonal momentum flux from the surface into the atmosphere. The development of strong monsoon circulations in the Northern Hemisphere summer and the convergence zones of the Southern Hemisphere (South Pacific [SPCZ], South Atlantic [SACZ], and South Indian Ocean [SICZ] convergence zones) shifts mass out of the subtropics, lowers the zonal mean subtropical highs, and weakens the summer cell.
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