The Deep and Shallow Hadley Circulations

This talk examines the dynamics of the deep and shallow Hadley circulations. The dynamics are discussed in the context of idealized analytical solutions of the transverse circulation equation that arises in the balanced model of the Hadley circulation. Zhang et al. (2004) have presented evidence for a shallow meridional circulation in the eastern Pacific. This shallow overturning circulation resembles the deep overturning circulation in many respects, but its cross-equatorial return flow occurs just above the top of the boundary layer. Nolan et al. (2007) have interpreted this shallow meridional circulation as a large-scale sea-breeze, driven by north-south sea surface temperature gradients when deep convection is absent in the ITCZ region. The purpose of the present research is to discuss several other dynamical aspects, which, in addition to sea surface temperature gradients, appear to play an important role in understanding this shallow circulation. The three dynamical aspects discussed here are: (i) Ekman pumping out of the boundary layer in the high vorticity region of the ITCZ; (ii) low inertial stability in the equatorial region, causing most of the Ekman pumping to be returned across the equator; (iii) a spectrum of small Rossby depths, causing the return flow to be trapped just above the boundary layer.