Balanced and Transient Dynamics of Deep and Shallow Hadley Circulations

Abstract:

We examine the balanced and transient dynamics of large-scale overturning circulations in the tropical atmosphere using idealized equatorial β-plane models. When deep diabatic heating in the Intertropical Convergence Zone (ITCZ) is prescribed as slowly varying with time, the deep Hadley circulation is of first order importance. In the absence of deep diabatic heating, the interior circulation associated with Ekman pumping cannot penetrate deep into the troposphere because the resistance of fluid parcels to horizontal motion (i.e., inertial stability) is significantly smaller than their resistance to vertical motion (i.e., static stability). In this scenario, only a shallow Hadley circulation exists. The meridional asymmetry between the winter and summer deep and shallow Hadley cells is attributed to the anisotropy of the inertial stability parameter, and as the ITCZ widens meridionally or as the forcing involves higher vertical wavenumbers, the asymmetry between the winter and summer cells increases. When we prescribe both a slow and fast "switch-on" deep diabatic heating, we learn that inertia-gravity waves propagate in the meridional direction away from the ITCZ and bounce back at critical latitutes in the zonally symmetric framework. When the assumption of zonal symmetry is relaxed, we find that inertia-gravity waves propagate away from the ITCZ in not only the meridional direction but also in the zonal direction.