Increasing compound warm spells and droughts in the Mediterranean Basin

Johannes Vogel,Eva Paton,Valentin Aich,Axel Bronstert

doi:10.1016/j.wace.2021.100312

Abstract

The co-occurrence of warm spells and droughts can lead to detrimental socio-economic and ecological impacts, largely surpassing the impacts of either warm spells or droughts alone. We quantify changes in the number of compound warm spells and droughts from 1979 to 2018 in the Mediterranean Basin using the ERA5 data set. We analyse two types of compound events: 1) warm season compound events, which are extreme in absolute terms in the warm season from May to October and 2) year-round deseasonalised compound events, which are extreme in relative terms respective to the time of the year. The number of compound events increases significantly and especially warm spells are increasing strongly – with an annual growth rates of 3.9 (3.5) % for warm season (deseasonalised) compound events and 4.6 (4.4) % for warm spells –, whereas for droughts the change is more ambiguous depending on the applied definition. Therefore, the rise in the number of compound events is primarily driven by temperature changes and not the lack of precipitation. The months July and August show the highest increases in warm season compound events, whereas the highest increases of deseasonalised compound events occur in spring and early summer. This increase in deseasonalised compound events can potentially have a significant impact on the functioning of Mediterranean ecosystems as this is the peak phase of ecosystem productivity and a vital phenophase.

Highlights

We analyse two types of compound events: 1) warm season compound events, which are extreme in absolute terms in the warm season from May to October and 2) year-round deseasonalised compound events, which are extreme in relative terms respective to the time of the year
The months July and August show the highest increases in warm season compound events, whereas the highest increases of deseasonalised compound events occur in spring and early summer
In creases in extreme events were observed in the past decades and are projected to continue in the 21st century in the Mediterranean Basin (Giannakopoulos et al, 2009; Hartmann et al, 2013; IPCC, 2019) for heat wave intensity and duration (Diffenbaugh et al, 2007; Fischer and Schar, 2010; Lionello et al, 2012; Christidis et al, 2015), as well as drought and aridity (Sousa et al, 2011; Dai, 2013; Cook et al, 2016; Samaniego et al, 2018; Spinoni et al, 2018)

Summary

Introduction

The Mediterranean Basin is a region prone to the effects of climate change and was characterized as one of the climate change hot-spots areas of the 21st century (Giorgi, 2006; Orlowsky and Sen eviratne, 2012; Lionello and Scarascia, 2018). Temperature increases at a faster pace in the Mediterranean compared to the global average due to regional feedback mechanisms enhancing changes in extreme temper atures (Diffenbaugh et al, 2007; Orlowsky and Seneviratne, 2012). Soil water content decreases in all seasons due to global warming, with the largest reductions in winter and spring (Samaniego et al, 2018)

Methods

Results

Discussion

Conclusion