Untangling the Annual Cycle of the Tropical Tropopause Layer with an Idealized Moist Model

The processes regulating the climatology and annual cycle of the tropical tropopause layer (TTL) and cold point are not fully understood. Three main drivers have been identified: planetary-scale equatorial waves excited by tropical convection, planetary-scale extratropical waves associated with the deep Brewer–Dobson circulation, and synoptic-scale waves associated with the midlatitude storm tracks. In both observations and comprehensive atmospheric models, all three coexist, making it difficult to separate their contributions. Here, a new intermediate-complexity atmospheric model is developed. Simple modification of the model’s lower boundary allows detailed study of the three processes key to the TTL, both in isolation and together. The model shows that tropical planetary waves are most critical for regulating the mean TTL, setting the depth and temperature of the cold point. The annual cycle of the TTL, which is coldest (warmest) in boreal winter (summer), however, depends critically on the strong annual variation in baroclinicity of the Northern Hemisphere relative to that of the Southern Hemisphere. Planetary-scale waves excited from either the tropics or extratropics then double the impact of baroclinicity on the TTL annual cycle. The remarkably generic response of TTL temperatures over a range of configurations suggests that the details of the wave forcing are unimportant, provided there is sufficient variation in the upward extent of westerly winds over the annual cycle. Westerly winds enable the propagation of stationary Rossby waves, and weakening of the subtropical jet in boreal summer inhibits their propagation into the lower stratosphere, warming the TTL.