A tree-ring field reconstruction of Fennoscandian summer hydroclimate variability for the last millennium
Hydroclimatological extremes, such as droughts and floods, are expected to increase in frequency and intensity with global climate change. An improved knowledge of its natural variability and the underlying physical mechanisms for changes in the hydrological cycle will help understand the response of extreme hydroclimatic events to climate warming. This study presents the first gridded hydroclimatic reconstruction (0.5° × 0.5° grid resolution), as expressed by the warm season Standardized Precipitation Evapotranspiration Index (SPEI), for most of Fennoscandia. A point-by-point regression approach is used to develop the reconstruction from a network of moisture sensitive tree-ring chronologies spanning over the past millennium. The reconstruction gives a unique opportunity to examine the frequency, severity, persistence, and spatial characteristics of Fennoscandian hydroclimatic variability in the context of the last 1,000 years. The full SPEI reconstruction highlights the seventeenth century as a period of frequent severe and widespread hydroclimatic anomalies. Although some severe extremes have occurred locally throughout the domain over the fifteenth and sixteenth centuries, the period is surprisingly free from any spatially extensive anomalies. The twentieth century is not anomalous in terms of the number of severe and spatially extensive hydro climatic extremes in the context of the last millennium. Principle component analysis reveals that there are two dominant modes of spatial moisture variability across Fennoscandia. The same patterns are evident in the observational record and in the reconstructed dataset over the instrumental era and two paleoperiods. The 500 mb pressure patterns associated with the two modes suggests the importance of the summer North Atlantic Oscillation.
KeywordsHydroclimate Tree-ring Standardized precipitation evapotranspiration index Field reconstruction Fennoscandia North Atlantic Oscillation
This research has been supported by the Swedish Research councils Vetenskapsrådet and FORMAS (grants to Hans W Linderholm). The paper contributes to the Swedish strategic research areas Modelling the Regional and Global Earth system (MERGE), and Biodiversity and Ecosystem services in a Changing Climate (BECC). This is Lamont-Doherty Earth Observatory Contribution No. 7801, and contribution No. 29 from the Sino-Swedish Centre for Tree ring Research (SISTRR). The authors wish to thank the Swedish county administrations of Kalmar, Kronoberg, Västra Götaland, Jönköping, Örebro, Östergötland, Södermanland, Uppsala, and Gävleborg for sampling permissions; Alexander Saplin, Peter Seftigen and Petter Stridbeck for help in the field. The authors also gratefully acknowledge the numerous researchers who have contributed their data to the ITRDB. Finally, we thank the two anonymous reviewers and the journal editor for providing constructive feedback on the initial submitted version of this manuscript.
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