Synoptic climatology of extreme heat events in the Apennines (Italy)

This study, using a dataset consisting of in-situ daily temperature records and ERA5 reanalysis data, analyses the extreme temperature events occurred in the Apennine Mountains (Italy) over the period 1961-2022. The available dataset has been employed to meet the following two main goals: i) to assess the linear trends of the heat waves (hereafter, HWs) and warm spells in terms of number of events, duration, and severity by applying the Seasonal Kendall test; ii) to shed light, on a seasonal basis, on the synoptic climatology of such events. From the linear trend analysis, it emerged that the Apennines, as many other regions of the world, experienced an increasing trend in extreme temperature episodes. In the last 30-year reference period (1991-2020), the number of regional extreme heat events increased by 134% in summer and 102% in spring compared to the 1961-1990 period, while in winter and autumn the increase in warm spells is smaller and generally not statistically significant in terms of duration and severity. Using Principal Component Analysis and k-means clustering, several synoptic-scale patterns that can trigger extreme hot conditions in the study area have been identified. In the last 30-year period, notable changes in the synoptic climatology of extreme heat events have been detected in summer, as well as in spring and autumn. Specifically, in summer the large-scale patterns characterised by a cyclonic area over the eastern North Atlantic (over the British Islands or off the coasts of Ireland) and by a ridge from North Africa to the eastern Europe provide a larger relative contribution to the total number of HWs. Such patterns promote the advection, over the study area, of hot subtropical air masses, mainly at mid-tropospheric levels. In addition, a relationship between HWs that occurred in the Apennines and the Sea Surface Temperature (SST) anomalies in the North Atlantic has been identified. In the days preceding the HWs, a negative SST anomaly in the eastern North Atlantic is generally observed. This feature can be considered as one of the factors that trigger, amplify, and prolong summer extreme heat events in the Central Mediterranean.

Such results supply new insights about the links between extreme heat events in the central Mediterranean area and large-scale atmospheric types as well as useful tools to improve the predictability of HWs and warm spells at both meteorological and climatological time scales. Finally, this study demonstrates the importance of having reliable and quality-checked long-term climatological time series in mountainous areas. If these sites were properly managed guaranteeing sufficient spatial and temporal coverage, it would be possible to carry out capillary studies to investigate how climate change affects high altitudes and, therefore, how it impacts mountain ecosystems and human activities.