The Static Stability of the Tropopause Region in Adiabatic Baroclinic Life Cycle Experiments

The tropopause inversion layer (TIL) is a region of enhanced static stability just above the WMO-defined thermal tropopause. It is a ubiquitous feature in midlatitudes and is well characterized by observations. However, it is still lacking a satisfactory theoretical explanation. This study utilizes adiabatic baroclinic life cycle experiments to investigate dynamical mechanisms that lead to TIL formation. As the baroclinic wave grows, a strong TIL forms above anticyclonic anomalies, while no TIL is found above cyclonic anomalies; this is consistent with previous results. However, during the early growth phase there is no TIL in the global or zonal average: positive and negative anomalies cancel out exactly. The zonal and global mean TIL only emerges during the mature stage of the life cycle, after the onset of wave breaking. The TIL predominantly occurs equatorward of the jet and the vertical structure bears resemblance to the TIL in midlatitudes; there is no equivalent to the subpolar TIL. Life cycles without significant wave breaking develop neither a global nor a zonal mean TIL. No global mean TIL is found in any life cycle if the dynamical tropopause definition is used. In addition, a new mechanism of dynamical TIL formation is presented, suggesting that the TIL in the global and zonal mean is linked to a strongly skewed distribution of relative vorticity after wave breaking.