One of the striking and largely unexplained characteristics of the low latitude troposphere is the mid-troposphere minimum in the equivalent potential temperature. This minimum creates an apparently stable gradient of equivalent potential energy within the middle and upper troposphere, which in turn requires the concept of penetrative deep convection to explain the ubiquitous deep convective cloud systems of the tropics.
Using tropospheric ozone as a quasi-conservative tracer of large scale transport it has been shown [Kley et al., 1997] that dry, ozone rich, subtropical air from the upper troposphere is advected to the tropics where it is entrained into rising convection around the 330K potential energy level. The mixing of dry with convective, high humidity, air lowers the thermodynamic moist static energy (Q) and equivalent potential energy of convective elements, thereby reducing their neutral buoyancy levels.
We use data from eperiments over the Atlantic ocean, central Pacific and Arabian sea to suggest that the minimum of Q in the tropics can be consistently explained by a circulation which, similar to the return flow of the Hadley cell, transports subtropical air to the tropics. However, dissimilar to the Hadley cell, the circulation transports air to the middle tropical troposphere. The conclusion of this paper is that the minimum of Q and of equivalent potential energy around 700 hPa in the inner tropics is the result of advection and mixing of extratropical upper tropospheric air with humid tropical air.
Reference:
Kley, D., H.G.J. Smit, H. Voemel, H. Grassl, V. Ramanathan, P.J. Crutzen, S. Williams, J. Meywerk and S.J. Oltmans. Tropospheric Water Vapor and Ozone Cross Sections in a Zonal Plane Over the Central Equatorial Pacific
Quart.J.Roy.Met.Soc. 123, 2009-2040 (1997)