The Deep and Shallow Hadley Circulations

This presentation examines a single theory that explains the dynamics of large-scale overturning circulations in the tropical atmosphere on the equatorial beta-plane, namely the deep and shallow Hadley circulations. The dynamics are discussed in the context of idealized analytical solutions of the meridional circulation equation arising in the zonally symmetric model of the Hadley circulation. Under certain simplifications of its coefficients, this elliptic partial differential equation for the meridional circulation can be solved by first performing a vertical transform to obtain a horizontal structure equation, from which arises the concept of a spectrum of Rossby lengths. When deep convection is present in the ITCZ, the deep Hadley circulation is of first order importance. In the absence of deep convection, Rossby lengths are large and Rossby depths are small, therefore the interior circulation associated with Ekman pumping cannot penetrate deep into the troposphere. Therefore, only a shallow Hadley circulation exists. This shallow overturning circulation is characterized by meridional velocities of up to eight meters per second at the top of the boundary layer, agreeing well with observations in the eastern Pacific shown in Zhang et al. (2004).