Climate extremes in Mediterranean metropolitan cities and atmospheric variability

The Mediterranean region is an area of increasing interest due to its unique climate. Nowadays, climate change has already evident consequences, such as the rise of extreme weather events, which significantly affect peoples’ life in the highly populated urban areas of the Mediterranean. Thus, in this study, ten metropolitan cities from the wider Mediterranean region with different climatic characteristics have been selected to study the frequency and the multidecadal trends of extreme events, as well as their possible connection with the large scale and synoptic scale atmospheric variability.

Four combined extreme indices have been evaluated on annual and seasonal basis for the period 1950-2018 using the high-resolution E-OBS gridded daily mean temperature and precipitation datasets (0.1° x 0.1°; v.19e) from the European Climate Assessment & Dataset (ECA&D, Klein Tank et al. 2002, www.ecad.eu). These combined extreme indices refer to the joint modes of temperature and precipitation extremes, concerning the co-occurrence of Cold/Dry days (CD), Cold/Wet days (CW), Warm/Dry days (WD), Warm/Wet days (WW), which can reflect extreme conditions better than temperature or precipitation statistics considered separately (Beniston, 2009; 2011). The links of the extreme events with the atmospheric variability are investigated based on large-scale teleconnection indices and spatiotemporal distribution of cyclonic activity. Toward this, the comprehensive climatology of Mediterranean cyclones assembled was used by applying a cyclone tracking algorithm (Murray and Simmonds, 1991; Flocas et al., 2011) with respect to the ECMWF ERA5 Interim mean sea level pressure fields since 1950.

The findings of the analysis showed distinct temporal and spatial variations of the combined extremes occurrences in the cities across the Mediterranean, which can be attributed to the effects of its complex topography, as well as to the non-uniform influence of the atmospheric variability. Specifically, the CD and WD indices have higher annual occurrences than the CW and WW, which indicates that the wider Mediterranean region experiences more dry days, either cold or warm, than wet days. The urban areas most affected by cold/dry events are located on the western Africa, while almost all urban areas around the Mediterranean coast are impacted by higher number of warm/dry events, with increasing trends.

References: Beniston M., 2009, Geophys. Res. Lett., 36, L07707; Beniston M., and Coauthors, 2011, Int. J. Climatol., 31, 1257-1263; Murray and Simmonds, 1991 Aust Met Mag 39 155 166; Flocas et al., 2010, J Climate, 23(19), 5243-5257