The Tropopause Inversion Layer in baroclinic Life-cycle Experiments

The Tropopause Inversion Layer (TIL) is an ubiquitous feature of the mid-latitude tropopause region. Although it is well characterized by observations, it is still lacking a satisfying theoretical explanation. In this study we present results from conservative baroclinic life-cycle simulations on a sphere: a strong and realistic TIL forms as a consequence of conservative dynamics only. Furthermore the TIL is a robust feature of the zonal mean fields of mature baroclinic life-cycles, and the contribution of variability to the zonal mean is shown to be negligible. We also discuss the sensitivity of the results to model resolution and introduce an aspect ratio criterion in order to resolve apparent contradictions between previous studies. Furthermore we investigate the sensitivity with respect to the tropopause criterion: a robust TIL exists only above the lapse-rate based tropopause; in the domain average, the dynamical tropopause does not exhibit a TIL at all. In our analysis we discuss the evolution of the TIL during a baroclinic life-cycle, with particular emphasis on the wave-breaking phase. The formation of a TIL is associated with a net increase in average tropopause height. An analysis of zonal mean PV and N² fields in the tropopause region further reveals good qualitative agreement with earlier results from idealized PV-inversion. We interpret our results as an indication for the dominance of dynamical mechanisms in TIL formation and conclude with a brief discussion of the relevance of our results for a statistical equilibrium climate under radiative forcing.