Double Tropopause Events over the Mediterranean Region: Climatology, Trends and Dynamical Context from ERA5

Double tropopauses are recurrent features of the extratropical upper-troposphere–lower-stratosphere (UTLS), but their climatology and long-term evolution over the Mediterranean region remain poorly characterized. Here, ERA5 pressure-level data for 1979–2025 are used to investigate the seasonal distribution, vertical structure, trends, and geographical redistribution of double-tropopause events over the Mediterranean sector. Lapse-rate tropopauses were identified using a discrete pressure-level implementation of the WMO criteria. The analysis focuses on double-tropopause profiles with the first lapse-rate tropopause at or above 8 km (DT–UTLS). To improve the validation of high second-tropopause cases, the temperature and geopotential profiles used for lapse-rate tropopause detection were extended up to 10 hPa, allowing a more complete evaluation of the WMO 2-km layer above LRT2 on the available ERA5 pressure-level grid. DT–UTLS occurrence shows a marked seasonal structure, with winter maxima over the southern part of the domain and a relative northward displacement in summer. The detected events exhibit a layered vertical structure, with a lower first lapse-rate tropopause and a second tropopause several kilometres higher. Long-term changes are spatially heterogeneous and seasonally asymmetric. The most robust frequency signal is a summertime decline over the north-eastern part of the study domain, centred near the Black Sea–northern Anatolia sector and the adjacent eastern/northern Mediterranean margin, accompanied by a westward redistribution of the JJA occurrence regime. In DJF, the geographical distribution shifts significantly southward. A positive winter trend in the subtropical belt is obtained with the standard DT–UTLS definition, but its statistical significance weakens under a stricter exact-2-km sensitivity test and is therefore interpreted with caution. Detrended associations with upper-level circulation and event-conditioned vertical-structure diagnostics provide dynamical context for the observed variability, but are not interpreted as causal attribution. These results identify Mediterranean DT–UTLS occurrence as a useful indicator of the seasonally evolving UTLS transition zone and its long-term regional redistribution.