Dynamical formation of the tropical cold point tropopause in an idealized GCM

The formation of the tropical cold-point tropopause (CPT) is examined using a dry primitive equation model driven by the Held-Suarez forcing. Without realistic moist and radiative processes, the simple dry model successfully reproduces the zonal-mean structure of the CPT. The modeled CPT is appreciably colder (~10 K) than the prescribed equilibrium temperature, and it is maintained by upwelling in the tropical upper troposphere and lower stratosphere (UTLS). The transformed Eulerian-mean analysis shows that the upwelling is primarily driven by convergence of synoptic-scale waves originating from the midlatitude troposphere and propagating into the subtropical UTLS. A strengthened CPT is also observed with increased baroclinicity in the equilibrium temperature. Contribution by the planetary-scale waves is minor in this simulation as no planetary-scale wave sources are prescribed. A transient simulation starting from an axisymmetric steady state – where the CPT is absent – presents that evolution of the CPT is closely coupled to the wave driving and zonal-mean flow response in the stratosphere. This also suggests that the formation of the CPT is likely an important process determining the vertical extent of the westerly jet in the lower stratosphere. Although a more comprehensive modeling test is required for the real atmosphere, this result implies that synoptic-scale waves may play a non-negligible role in the formation and maintenance of the CPT, particularly in the seasons when planetary-scale wave activities in the lower stratosphere are weak.