Spatial-temporal variation of winter warm spells in Italy over the period 1993-2022 

Global warming and the associated climate change, unequivocally attributable to human activities and greenhouse gas emissions, will very likely intensify in the near future, resulting in the increased occurrence of extreme weather events, such as heatwaves. Summer heatwaves are widely studied to explore their thermodynamic precursors and their effects on health and natural ecosystems, while winter warm spells (WWS), defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) as “a sequence of at least six consecutive days when the daily maximum air temperature exceeds the calendar day 90th percentile of the probability density distribution of the reference period", are still scarcely studied. In this contribution, the temporal and spatial variability of WWS that occurred over the Italian Peninsula during the period 1993-2022 is investigated. The identification of WWS is carried out by examining the wintertime (December, January, February) maximum daily temperatures measured in eight Italian airport sites, belonging to different Köppen-Geiger climatological classes. The WWS events involving the whole Italian territory or only northern/central/southern Italy are detected. It is interesting to note that although exceeding the 90th percentile of the daily maximum temperature is quite frequent, only one winter warm spell that affected the entire Italian territory is detected over the period 1993-2022. In the period under investigation, the synoptic conditions associated with WWS over Italy or a portion of the peninsula are, on average, characterised by anticyclonic systems centred on the western Mediterranean, responsible for persistent high-pressure conditions over Italy, subsidence and, therefore, exceptional warming. Finally, the period length threshold used for the detection of WWS is reduced from six to three days. The outcomes suggest that, in orographically heterogeneous areas such as Italy, the definition of WWS provided by ETCCDI allows for capturing only synoptic scale events, losing information on moderate warm spells, which can have important implications on health and natural ecosystems. Therefore, for regional studies on complex terrain, it would be advisable to reduce the time threshold for the identification of WWS to three days. This study is supported by the Boundary layer Air Quality-analysis Using Network of Instruments (BAQUNIN) project, funded by ESA, which allowed the establishment of one of the first observatories in the world to involve several passive and active ground-based instruments installed in multiple locations and managed by different research institutions. Moreover, this research is part of the activities envisaged in the “uRban hEat and pollution iSlands inTerAction in Rome and possible miTigation strategies” (RESTART) project, funded by the Italian Ministry for University and Research as a Project of National Interest (PRIN2022). RESTART aims to explore the urban heat island and the urban pollution island in Rome (Italy), providing a series of mitigation strategies, including tailored nature-based solutions, and ready-to-use guidelines for the improvement of well-being and liveability in urban environments.