Double tropopause formation in idealized baroclinic life cycles: The key role of an initial tropopause inversion layer

Recent studies have shown that double tropopauses exist in all seasons, and at all longitudes, in the midlatitudes. As of yet, the key mechanism responsible for their formation is not known. In this study, we explore the connection between double tropopauses and midlatitude baroclinic eddies. This is investigated in the context of idealized life cycle experiments. The key finding of this study is that large areas of double tropopauses form spontaneously at the nonlinear stage of the life cycle evolution, provided an extratropical tropopause inversion layer is present in the balanced initial temperature profile. We also show that the areas covered with double tropopauses grow as the strength of the initial tropopause inversion layer is increased. Without such a layer, as in canonical examples of baroclinic life cycles much studied in the literature, no double tropopause formation occurs. In agreement with observations, double tropopauses in our life cycle experiments form predominantly in areas of cyclonic flow at upper levels. However, the air masses that end up between the two tropopauses are found to originate from high latitudes. This appears to differ from a recently published case study, where the air between double tropopauses was shown to originate partly from low latitudes. Such a discrepancy suggests that more than one pathway may exist to advect air masses between the two tropopauses.