All textbooks describe the large-scale meridional circulation in the tropics in terms of the Hadley cell, which is an overturning circulation with its lower branch in the atmospheric boundary layer toward the tropical seasonal mean convective center, its upper branch in the upper troposphere (10 - 12 km) away from the convective center, its ascending branch in the convective center, and its descending branch spreading over a large area outside the convective center. This is depicted by the blue lines in following figure. My analyses of in situ observations and global reanalyses, along with other studies, have revealed that, in addition to this Hadley-type, deep overturning circulation, there is a shallow meridional circulation (SMC) with its upper branch away from the convective center in the lower troposphere (2 - 5 km), as depicted by the red line in the same figure. Such a SMC exists over the tropical eastern Pacific and West Africa, and possibly in other part of the tropics.

My current research on this subject strives to understand the dynamics of this SMC (in collaboration with Prof. David Nolan of U Miami), document biases and errors in the presentations of the SMC by simulations and reanalyses of current global climate models, explore the climatic impact of the SMC such as its role in the West African monsoon (in collaboration with Prof. Chris Thorncroft of SUNY Albany) and its water vapor transport.

My publications on this subject:

Nolan, D.S., S.W. Powell, C. Zhang, and B.E. Mapes, 2010: Idealized Simulations of the Intertropical Convergence Zone and its Multi-level, Cross-Equatorial Flows. J. Atm. Sci., submitted.(manuscript)

Thorncroft, C.D., H. Nguyen, C. Zhang, and P. Peyrille, 2010: Annual Cycle of the West African Monsoon - Regional Circulations and Associated Water Vapor Transport. Quart. J. Roy. Met. Soc., submitted. (manuscript)

Hagos, S., and C. Zhang, 2009: Diabatic Heating, Divergent Circulation and Moisture Transport in The African Monsoon System. Quart. J. Roy. Met. Soc., DOI: 10.1002/qj.538. (reprint).

Zhang, C., D.S. Nolan, C.D. Thorncroft, and H. Nguyen, 2008: Shallow meridional circulations in the tropical atmosphere. J. Climate, 21, 3453-3470.(reprint)

Nolan, S.D., C. Zhang, and S.-H.Chen, 2007: Dynamics of the Shallow Circulation Around ITCZ Regions. J. Atm. Sci., 64, 2262-2285. (reprint)

Zhang, C., P. Woodworth, and G. Gu, 2006: The seasonal cycle in the lower troposphere over West Africa from sounding observations. Q.J.Roy.Meteorol.Soc., 132, 2561-2584. (reprint)

McGauley, M., C. Zhang, and N.A. Bond, 2004: Large-scale characteristics of the atmospheric boundary layer in the eastern Pacific cold tongue/ITCZ region. J. Climate, 17, 3907-3920. (reprint )

Zhang, C., M. McGauley, and N.A. Bond, 2004: Shallow meridional ciruclation in the tropical eastern Pacific J. Climate, 17, 133-139. (reprint)