Spatial and temporal analysis of extreme weather events in the Mediterranean basin

The Mediterranean basin is designated as a climate change hot-spot area due to the extreme weather events that were observed in past decades and are expected to persist in the 21^st century. The future warming rates in the Mediterranean basin are expected to be twenty percent higher than the global average, reaching a fifty percent increase in the summer months. This increase in temperature can lead to a rise in the occurrence of warm spells combined with drought, which can have a detrimental socio-economic and ecological impact in the Mediterranean basin.

Research published in the journal Weather and Climate Extremes, led by Professor Eva Paton and graduate student Johannes Vogel from the Technical University of Berlin along with his colleagues Professor Axel Bronstert from the University of Potsdam and Dr. Valentin Aich from the Potsdam Institute for Climate Impact Research, quantified and studied the impact of the number of concurrent warm spells and drought in the last forty years in the Mediterranean basin. The occurrence of drought and heatwave at the same place at the same time was defined as a compound weather event in this study.

The research team studied extreme warm-season compound events and the year-round compound events (deseasonalised), which were intense relative to the time of the year. Heat spell was defined using a peak over threshold approach, whereas the drought was defined by using two indices, the standardized precipitation index and the standardized precipitation evapotranspiration index.

A significant increase in the occurrence of warm season and deseasonalised compound events was found in the last forty years (1979 to 2018) in the Mediterranean basin. Warm weather compound events increased consistently compared to the deseasonalised events. The western Balkan countries (Albania, Bosnia and Herzegovina, Croatia, Montenegro, and North Macedonia) exhibited the highest increase in compound events, followed by Italy, Morocco, France, and Spain. “This is consistent with previous findings indicating that temperatures at the hot tail, for example the highest percentiles, increase much faster than mean temperature, up to 6 ◦C for 1.5 ◦C mean warming due to surface moisture and atmospheric feedbacks in the Mediterranean Basin’” said lead author Johannes Vogel.

Along with the change in the occurrence of the compound events, Professor Paton and his team also investigated if either of the two individual components drove the compound events. An annual growth rate of around four percent was observed for deseasonalised compound events, along with an increased rate of warm spells. The change in the incidence of the drought was uncertain as it varied based on the applied definition. Altogether this study confirmed that changes in drought occurrence in the region were not due to the change in the precipitation but were caused due to the increase in warm-spell-led evapotranspiration.

The researchers also investigated the temporal distribution of these compound events and identified the highest number of warm-season compound events that happened in July and August. The deseasonalised compound events increased most in February, May, and June. The warm-season warm spell was maximum in August, and the deseasonalised warm spells were maximum in April. The evapotranspiration-based warm-season drought was maximum in July, and deseasonalised evapotranspiration-based drought increased in all months except March and November.

This study has corroborated prior findings that the extreme compound weather events of drought and warm spells have occurred in the last forty years in the Mediterranean and are likely to rise. These compound events are caused by warm spells, which lead to drought. Johannes Vogel emphasized that: “In late spring, compound events, which are extreme for that time of the year are rising. This is concerning because this is is a crucial agricultural phase. This highlights the importance of considering such events, which might be missed if only the absolute extremes of a year are investigated.” Thorough frequent monitoring of warm spells and drought changes is necessary to obtain valuable data and prepare for adequate risk management in the region.

Journal Reference:

Vogel, Johannes, Eva Paton, Valentin Aich, and Axel Bronstert. “Increasing compound warm spells and droughts in the Mediterranean Basin.” Weather and Climate Extremes 32 (2021): 100312. DOI: https://doi.org/10.1016/j.wace.2021.100312

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