Winter warm spells over Italy: Spatial-temporal variation and large-scale atmospheric circulation

This article analyses the winter warm spells (WWS) that occurred in central Mediterranean over the period 1993-2022, examining the daily maximum temperatures collected at eight airport sites located in the Italian Peninsula, belonging to different climate zones. According to the definition proposed in 1999 by the Expert Team on Climate Change Detection and Indices (ETCCDI), a WWS is a sequence of at least six consecutive days when daily maximum air temperature exceeds the calendar day 90th percentile centred on a 5-day window for a base period. WWS occurring over the entire Italian territory or only over northern/central/southern Italy have been identified and related to the peculiar synoptic conditions. It was found that December is the month most prone to WWS and, on average, WWS last 9.4 days in northern Italy, 6.6 days in central Italy, and 8.5 days in southern Italy. Over the period under investigation, the Italian Peninsula experienced only one common event characterized by persistent high-pressure systems associated with air subsidence over western Mediterranean and, therefore, with exceptional warming. Finally, it has been proven that the definition of WWS proposed by ETCCDI allows to capture synoptic scale events but, in orographically complex areas such as Italy, underestimates moderate spells, which generally might have a duration of at least 3 days. Consequently, it is important to consider the possibility of reducing the period length threshold used for the detection of WWS when orographically heterogeneous regions are studied. In central Mediterranean and in orographically heterogeneous areas such as Italy, the definition of winter warm spells (WWS) provided by the Expert Team on Climate Change Detection and Indices allows to capture mainly synoptic scale events, losing information on moderate warm spells. When orographically heterogeneous regions are studied, it is important to consider the possibility of reducing the period length threshold used for the detection of WWS. image