Formation of tropical cold-point tropopause by baroclinic eddies in a dynamic-core GCM

The role of baroclinic eddies in the formation of the cold-point tropopause (CPT) is investigated by performing a series of dynamic-core GCM integrations with Held-Suarez type thermal forcings. The simple dynamical system forms a distinct cold level at the top of the tropical troposphere, which is very similar to the CPT in the atmosphere. Temperature at the modeled CPT is well below (~10° K colder) the prescribed radiative equilibrium temperature (Te) to which the model's temperatures are kept relaxed, and this is mainly caused by tropopause upwelling resulting from deposition of easterly momentum in the subtropical upper troposphere by extratropical synoptic-scale waves (zonal wavenumber 4-9). Parameter studies, by varying meridional temperature gradient and vertical stratification of Te profile, further show that the modeled CPT temperature decreases with increasing extratropical baroclinicity. Upwelling at the tropical upper tropopause, convergence of Eliassen-Palm flux, and residual circulation present all consistent changes.

Axisymmetric simulation, which has no eddies, exhibits very weak dynamical cooling at the tropical tropopause. This weak indication of CPT is largely insensitive to the baroclinicity in Te profile, confirming the role of eddy in the formation of the tropical CPT, in particular the mechanism of eddy-driven overturning circulation. Although the influences of large-scale stationary eddies were not taken into account, these results highlight the importance (at least non-negligible role) of the mid-latitude baroclinic eddies in the formation of the cold-point in the tropical tropopause layer.