800 years of summer European-North Atlantic jet stream variability and its impact on climate extremes and human systems

Abstract

<p>Climate extremes over the mid-latitudes are driven by a combination of thermodynamical and dynamical factors. In Europe, the primary dynamical driver of summer climate extremes is the position of the jet stream over the Europe-North Atlantic (EU) region. In certain configurations, the EU jet creates a summer climate dipole between northwestern and southeastern Europe that can result in contrasting extreme weather conditions in the two regions. To study long-term variability in the EU jet configuration, as well as its potential impact on past climate extremes and human systems, we have reconstructed EU jet variability over the past 800+ years (1200-2005 CE). To accomplish this, we have combined five European tree-ring chronologies to reconstruct the July-August jet stream latitude for the EU domain (30°W - 40°E; EU JSL). Our reconstruction explains 40% of summer EU JSL variability over the instrumental period (1948-2005 CE) with strong skill.</p><p>We find that, over the past 800 years, opposite phases of EU JSL variability have consistently resulted in contrasting climate extremes, including heatwaves, droughts, floods, and wildfires, between northwestern Europe, specifically the British Isles, and southeastern Europe, specifically the Balkans and Italy. This EU JSL-driven summer climate dipole is captured in a network of historical documentary data that further document the societal impacts of EU JSL-related climate extremes on both sides of the dipole.</p><p>Our summer EU JSL reconstruction shows a century-long negative phase from ca. 1355-1450 CE, corresponding to anomalously wet and cool summers over the British Isles and dry and hot conditions over the Balkans. This negative phase is comparable to the recent (1970-present) EU JSL configuration. We also found a positive phase, with opposite summer climate dipole conditions, from ca. 1812-1861 CE. Our results thus suggest that<strong> </strong>the EU JSL<strong> </strong>has been a long-term primary driver of the European summer climate dipole, as well as of the associated climate extremes and societal impacts.</p>