Characteristics of the tropical tropopause layer 2

Among Jim Holton's most prominent legacies is his lasting contribution to an understanding of the processes near the tropical tropopause that determine the quantity of water vapor entering the stratosphere, the "water vapor puzzle" as he called it in a 1984 book chapter. A fundamental question in the water vapor puzzle is the relative roles of stratospheric pumping (the Brewer-Dobson circulation, driven by breaking waves in the extratropical stratosphere) and local convection in determining the characteristics of the tropical tropopause.

Grappling with this question, Holton's 1995 review paper on stratosphere-troposphere exchange set the stage and since then several papers have promoted the concept of a "tropical tropopause layer" (TTL) influenced both by stratospheric and tropospheric processes. The definitions of the TTL, however, are almost as numerous as the papers seeking to describe it. In this work, we review the observed properties of the tropical upper troposphere and lower stratosphere including temperature structure and variability, clouds, radiation, ozone, water vapor, isotopologues, and other trace constituents. We also review theoretical considerations that inform a discussion of the TTL, and finally suggest a way to synthesize the observations in a definition of the TTL.