Transient Aspects of the Hadley Circulation

This paper examines the transient dynamics of large-scale, zonally symmetric overturning circulations in the tropical troposphere. The dynamics are discussed in the context of idealized analytical solutions of the meridional circulation equation arising in an equatorial beta-plane model of the Hadley circulation. This partial differential equation for the meridional circulation can be solved by first performing a vertical transform to obtain a set of horizontal structure equations, and then performing a horizontal Hermite transform to obtain a set of second order ordinary differential equations in time. The solutions of these ordinary differential equations contain terms for the slow, quasi-balanced part of the response and terms for the transient, zonally symmetric, inertia-gravity wave part of the response. When the ITCZ is located off the equator, the quasi-balanced part of the response reveals a basic asymmetry between the winter and summer hemispheres, with the winter hemisphere side containing most of the quasi-balanced compensating subsidence. These basic dynamical aspects of the Hadley circulation are revealed in the upper tropospheric water vapor patterns observed by the 6.7 micron water vapor channels on the GOES satellites over the Atlantic and eastern Pacific.