An assessment of stratospheric intrusions in Italian mountain regions using STEFLUX

The Mediterranean basin is considered a global hot-spot region for climate change and air quality, especially concerning summer-time ozone (O-3). Previous investigations indicated that the Mediterranean basin is a preferred region for stratosphere-to-troposphere exchange (STE) and deep stratospheric intrusion (SI) events. The Lagrangian tool STEFLUX, based on a STE climatology that uses the ERA Interim data, was hereby used to diagnose the occurrence of deep SI events in four mountain regions over the Italian peninsula, spanning from the Alpine region to the southern Apennines. By using near-surface O-3 and relative humidity (RH) observations at three high-mountain observatories, we investigated the performance of STEFLUX in detecting deep SI events. Both experimental and STEFLUX detections agreed in describing the seasonal cycle of SI occurrence. Moreover, STEFLUX showed skills in detecting long-lasting SI events, especially in the Alps and in the northern Apennines. By using STEFLUX, we found positive tendencies in the SI occurrence during 1979-2017. However, in contrast to similar studies carried out in the Alpine region, the negative long-term (1996-2016) trend of O-3 in the northern Apennines did not appear to be related to the SI's variability.