Abstract. In recent decades, an increasing persistence of
atmospheric circulation patterns has been observed. In the course of the
associated long-lasting anticyclonic summer circulations, heatwaves and
drought spells often coincide, leading to so-called hotter droughts.
Previous hotter droughts caused a decrease in agricultural yields and
an increase in tree mortality. Thus, they had a remarkable effect on carbon
budgets and negative economic impacts. Consequently, a quantification of
ecosystem responses to hotter droughts and a better understanding of the
underlying mechanisms are crucial. In this context, the European
hotter drought of the year 2018 may be considered a key event. As a first step
towards the quantification of its causes and consequences, we here assess
anomalies of atmospheric circulation patterns, maximum temperature, and
climatic water balance as potential drivers of ecosystem responses which are
quantified by remote sensing using the MODIS vegetation indices (VIs)
normalized difference vegetation index (NDVI) and enhanced vegetation index
(EVI). To place the drought of 2018 within a climatological context, we
compare its climatic features and remotely sensed ecosystem response with
the extreme hot drought of 2003. The year 2018 was characterized by a climatic
dipole, featuring extremely hot and dry weather conditions north of the Alps
but comparably cool and moist conditions across large parts of the
Mediterranean. Analysing the ecosystem response of five dominant land cover
classes, we found significant positive effects of climatic water balance on
ecosystem VI response. Negative drought impacts appeared to affect an area 1.5
times larger and to be significantly stronger in July 2018 compared to
August 2003, i.e. at the respective peak of drought. Moreover, we found a
significantly higher sensitivity of pastures and arable land to climatic
water balance compared to forests in both years. We explain the stronger coupling and
higher sensitivity of ecosystem response in 2018 by the
prevailing climatic dipole: while the generally water-limited ecosystems of
the Mediterranean experienced above-average climatic water balance, the less
drought-adapted ecosystems of central and northern Europe experienced a
record hot drought. In conclusion, this study quantifies the drought of 2018
as a yet unprecedented event, outlines hotspots of drought-impacted areas in
2018 which should be given particular attention in follow-up studies, and
provides valuable insights into the heterogeneous responses of the dominant
European ecosystems to hotter drought.
Maximum temperatures over Slovenia and their relationship with atmospheric circulation patterns
