Abstract
AbstractThe 2018 drought and heatwave over northern Europe were exceptional, with unprecedented forest fires in Sweden, searing heat in Germany and water restrictions in England. Monthly, daily, and hourly data from ERA5, verified with in situ soil water content and surface flux measurements, are examined to investigate the subseasonal‐to‐seasonal progression of the event and the diurnal evolution of tropospheric profiles over Britain to quantify the anomalous land surface contribution to heat and drought. Data suggest the region entered an unprecedented condition of becoming a “hot spot” for land‐atmosphere coupling, which exacerbated the heatwave across much of northern Europe. Land‐atmosphere feedbacks were prompted by unusually low soil water over wide areas, which generated moisture limitations on surface latent heat fluxes, suppressing cloud formation, increasing surface net radiation, and driving temperatures higher during several multiweek episodes of extreme heat. We find consistent evidence in field data and reanalysis of a threshold of soil water content at most locations, below which surface fluxes and daily maximum temperatures become hypersensitive to declining soil water. Similar recent heatwaves over various parts of Europe in 2003, 2010, and 2019, combined with dire climate change projections, suggest such events could be on the increase. Land‐atmosphere feedbacks may play an increasingly important role in exacerbating extremes, but could also contribute to their predictability on subseasonal time scales.
Highlights
The summer of 2018 saw a combination of drought and heat concentrated over northern Europe
The analysis is designed to determine whether locations in northern Europe moved into a regime where land surface feedbacks exacerbated drying and warming during 2018
We have used a combination of high-quality reanalyzes and in situ measurements of volumetric soil water content (VWC), temperature, and surface fluxes to demonstrate the existence of a threshold in the range of VWC below which the sensitivity of the atmosphere to drying soils substantially increases, providing a potentially strong positive feedback mechanism by which the land surface may exacerbate heatwaves during drought conditions
Summary
The summer of 2018 saw a combination of drought and heat concentrated over northern Europe. Modeling studies indicate that most of northern Europe remains in an energy-limited regime, even during the warmer summer months, not experiencing L-A feedbacks (Dirmeyer et al, 2009; Schwingshackl et al, 2018; Seneviratne et al, 2010) It is not usually a “hot spot” of L-A coupling like the Great Plains of North America or the Sahel region of Africa (Koster et al, 2006). One COSMOS-UK location, Sheepdrove, has no flux estimates but is very near a UKCEH flux site Such an array of measurements provides ground truth at a number of locations around northern Europe to validate the behavior of ERA5 data, which provide complete coverage over the domain. Transpiration is a major component of latent heat flux, and its changes are strongly linked to root zone VWC, hysteresis and strongly varying time scales with depth have been found to yield less robust relationships with extreme heat (Benson & Dirmeyer, 2020)
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