Eddy-Influences on Hadley Circulations: Theory and Simulations with an Idealized GCM

An idealized GCM is used to investigate how the strength and meridional extent of the Hadley circulation depend on the planet radius, rotation rate, and thermal driving. Over wide parameter ranges, the strength and meridional extent of the Hadley circulation display clear scaling relations with regime transitions, which are not predicted by existing theories of axisymmetric Hadley circulations. For example, the scaling of the strength as a function of the radiative-equilibrium equator-pole temperature contrast is in two regimes, corresponding to two regimes in scaling laws of baroclinic eddy fluxes.

Over a wide range of flow parameters, albeit not always, the Hadley circulation strength is directly related to the eddy momentum flux divergence at the latitude of the streamfunction extremum. Simulations with hemispherically symmetric thermal driving span circulations with local Rossby numbers in the upper branch of the Hadley circulation between 0.1 and 0.8, indicating that neither nonlinear nearly inviscid theories nor linear viscous theories of axisymmetric Hadley circulations can be expected to be generally adequate.

The dependence of the Hadley circulation on eddy fluxes, which are themselves dependent on extratropical circulation characteristics such as meridional temperature gradients, suggests that tropical circulations depend on the extratropical climate. Implications of the results for theories of the tropical climate and of monsoon circulations are discussed.