Limits on the Extent of the Solsticial Hadley Cell: The Role of Planetary Rotation

Abstract The role of planetary rotation in limiting the extent of the cross-equatorial solsticial Hadley cell (SHC) is investigated using idealized simulations with an aquaplanet general circulation model run under perpetual-solstice conditions. Consistent with previous studies that include a seasonal cycle, the SHC extent increases with decreasing rotation rate, and it occupies the entire globe for sufficiently low planetary rotation rates. A simple theory for the summer-hemisphere extent of the SHC is constructed in which it is assumed that the SHC occupies regions for which a hypothetical radiative–convective equilibrium state is physically unattainable. The theory predicts that the SHC extends farther into the summer hemisphere as the rotation rate is decreased, qualitatively reproducing the behavior of the simulations, but it generally underestimates the simulated SHC extent. A diagnostic theory for the summer-hemisphere SHC extent is then developed based on the assumptions of slantwise convective neutrality and conservation of angular momentum within the Hadley cell. The theory relates the structure of the SHC in the summer hemisphere to the distribution of boundary layer entropy in the dynamically equilibrated simulations. The resultant diagnostic for the SHC extent generalizes the convective quasi-equilibrium-based constraint of Privé and Plumb, in which the position of rain belts is related to maxima in the low-level entropy distribution.