Patterns of Compound Drought and Heatwave events and their Atmospheric Circulation Drivers: Implications for Urban Areas in the Mediterranean region

Recent research reveals that extreme climate events arise from the complex interplay of multiple physical drivers across different spatial and temporal scales. Although traditional climate risk and impact assessments consider individual extreme events, it is the synergy of such events that can trigger cascading impacts especially for urban areas. Compound climate events are defined as the combination of multiple drivers and/or hazards that contribute to societal or environmental risks, while large-scale circulation drivers, such as atmospheric blocking, influence the occurrence and persistence of Compound Drought and Heat Wave (CDHW) events.

The scope of this study is to evaluate the current state of climate over the Eastern Mediterranean in terms of droughts and heatwaves, focusing on their compound impacts on urban areas, and identifying their large-scale atmospheric circulation drivers. The identification of the CDHW climatology is carried out through the improvement of a novel index tailored to the Eastern Mediterranean region. Multiple combinations are examined according to the generic definition of CDHW events (i.e.,heatwaves that occur during the period of drought events) and an optimum compound index is developed that successfully quantifies the interrelations of droughts and heatwaves.

Following a data-driven methodology, the role of large-scale atmospheric circulation drivers on the onset, duration and intensity of the CDHW events is examined using the synoptic climatology approach. The environment-to-circulation approach is selected; it employs the Self-Organizing Map (SOM) artificial neural networks for clustering and mapping the atmospheric circulation to understand the large-scale drivers responsible for CDHW events.

Extensive statistical analysis is carried out to evaluate their frequency, duration, intensity, and trends for the period 1974-2023. The analysis is based on the high-resolution state-of-the-art ERA5-Land reanalysis product at a 0.1° x 0.1° horizontal resolution, while it provides the hot-spot regions of CDHW high-impact phenomena over the Eastern Mediterranean region, with particular emphasis on urban areas.The analysis is extended for a future climate period (2041-20260) and is further supported with the use of remotely-sensed Land Surface Temperatures to examine the coupling between surface thermal extremes at the urban scale, heat waves and droughts.