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Stratospheric intrusion (SI) events are a topic of ongoing research, especiallybecause of their ability to change the oxidation capacity of the troposphereand their contribution to tropospheric ozone levels. In this work, a noveltool called STEFLUX (Stratosphere-to-Troposphere Exchange Flux) is presented,discussed, and used to provide a first long-term investigation of SI over twoglobal hot-spot regions for climate change and air pollution: the southernHimalayas and the central Mediterranean Basin. The main purpose of STEFLUX isto obtain a fast-computing and reliable identification of the SI events occurring ata specific location and during a specified time window. It relies on acompiled stratosphere-to-troposphere exchange (STE) climatology, which makesuse of the ERA-Interim reanalysis dataset from the ECMWF, as well as arefined version of a well-established Lagrangian methodology. STEFLUX resultsare compared to the SI observations (SIO) at two high-mountain WMO/GAWglobal stations in these climate hot spots, i.e., the Nepal ClimateObservatory-Pyramid (NCO-P, 5079 m a.s.l.) and Mt. Cimone (2165 m a.s.l.),which are often affected by SI events. Compared to the observational datasetsat the two specific measurement sites, STEFLUX is able to detect SI events on aregional scale. Furthermore, it has the advantage of retaining additionalinformation concerning the pathway of stratospheric-affected air masses, suchas the location of tropopause crossing and other meteorological parametersalong the trajectories. However, STEFLUX neglects mixing and dilution thatair masses undergo along their transport within the troposphere. Therefore,the regional-scale STEFLUX events cannot be expected to perfectly reproducethe point measurements at NCO-P and Mt. Cimone, which are also affected bysmall-scale (orographic) circulations. Still, the seasonal variability in SIevents according to SIO and STEFLUX agrees fairly well. By exploiting the fact thatthe ERA-Interim reanalysis extends back to 1979, the long-term climatology ofSI events at NCO-P and Mt. Cimone is also assessed in this work. The analysis of the35-year record at both stations denies the existence of any significant trendin the SI frequency, except for winter seasons at NCO-P. Furthermore, for thefirst time, by using the STEFLUX outputs, we investigate the potential impactof specific climate factors (i.e. ENSO, QBO, and solar activity) on SIfrequency variability over the Mediterranean Basin and the Himalayas

STEFLUX, a tool for investigating stratospheric intrusions: application to two WMO/GAW global stations