Attributing stratosphere-troposphere mass exchange to tropopause folds: a 45-year trajectory-based climatology and trend analysis in ERA5

Abstract. Tropopause folds (TF) are one of the main mechanisms of inducing cross-tropopause mass transport, but it is difficult to directly assess the amount of transport due to folds. In this study we perform a multidecadal quantification of stratosphereto-troposphere (STT) and troposphere-to-stratosphere (TST) mass transport associated with TFs, over the 1980–2024 period using ERA5 reanalysis on model levels. STT and TST events are obtained using trajectory calculations, and the crossing location and time are shown 5 to occur close in time and space with TFs. To quantify the TF-attributed portion of mass exchange, we use the local false discovery rate framework, a method of performing hypothesis testing given a large number of samples, that allows the assignment of a probability to each transport event quantifying likelihood of association to folds. Using this framework we find that globally, 58.3% of STT and 30.1% of TST are fold-associated. Latitudinally resolved analysis shows that the fold-associated transport fraction peaks in the subtropics for both transport types. Trend analysis shows that over the 10 period of 1981–2024 seasonal mean TF frequency has generally increased over the globe, while total STT/TST has generally declined. However, the fold-attributed STT/TST is largely flat, suggesting that the STT/TST decrease is predominantly due to non-fold processes.