Abstract
Heatwaves are extreme weather events that have become more frequent and intense in Europe over the past decades. Heatwaves are often coupled to droughts. The combination of them lead to severe ecological and socio-economic impacts. Heatwaves can self-amplify through internal climatic feedback that reduces local precipitation. Understanding the terrestrial sources of local precipitation during heatwaves might help identify mitigation strategies on land management and change that alleviate impacts. Moisture recycling of local water sources through evaporation allows a region to maintain precipitation in the same region or, by being transported by winds, in adjacent regions. To understand the role of terrestrial moisture sources for sustaining precipitation during heatwaves, we backtrack and analyse the precipitation sources of Northern, Western, and Southern sub-regions across Europe during 20 heatwave periods between 1979 and 2018 using the moisture tracking model Water Accounting Model-2layers (WAM-2layers). In Northern and Western Europe, we find that stabilizing anticyclonic patterns reduce the climatological westerly supply of moisture, mainly from the North Atlantic Ocean, and enhances the moisture flow from the eastern Euro-Asian continent and from within their own regions—suggesting over 10% shift of moisture supply from oceanic to terrestrial sources. In Southern Europe, limited local moisture sources result in a dramatic decrease in the local moisture recycling rate. Forests uniformly supply additional moisture to all regions during heatwaves and thus contribute to buffer local impacts. This study suggests that terrestrial moisture sources, especially forests, may potentially be important to mitigate moisture scarcity during heatwaves in Europe.
Highlights
Heatwaves are extreme weather events where exceptionally high temperature persists for a number of consecutive days
Anticyclonic patterns that signify atmospheric blocking can be clearly observed in Northern and Western Europe, which diverts over 10% of the westerly climatological moisture supply from the North Atlantic Ocean
The reduced contribution from the ocean is replaced by moisture from the terrestrial sources, such as those recycled within the regions and from the eastern Euro-Asia continent
Summary
Heatwaves are extreme weather events where exceptionally high temperature persists for a number of consecutive days. Heatwaves are regional phenomena often linked to high pressure systems as a result of changes in the meandering jet streams (Horton et al 2016). High pressure systems that occur during heatwaves are referred to as atmospheric blocking hereafter. Atmospheric blocking triggers extreme weather conditions as they prevent high and low pressure systems from mixing with each other (Lau and Kim 2012). The regional atmospheric blocking commonly traps warm air within it and is the enabling conditions that inhibit precipitation during heatwaves (Horton et al 2016). Apart from the effect of atmospheric blocking as a highpressure system, how it alters the moisture transport or the amount of available moisture that can precipitate in the affected regions during heatwaves remains unexplored
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