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Tree-ring maximum latewood density (MXD) records from Fennoscandia have been widely used to infer regional- and hemispheric-scale mean temperature variability. Here, we explore if MXD records can also be used to infer past variability of summer temperature extremes across Fennoscandia. The first principal component (PC1) based on 34 MXD chronologies in Fennoscandia explains 50% of the total variance in the observed warm-day extremes over the period 1901-1978. Variations in both observed summer warm-day extremes and PC1 are influenced by the frequency of anomalous anticyclonic pattern over the region, summer sea surface temperatures over the Baltic, North and Norwegian Seas, and the strength of the westerly zonal wind at 200 hPa across Fennoscandia. Both time series are associated with nearly identical atmospheric circulation and SST patterns according to composite map analysis. In a longer context, the first PC based on 3 millennium-long MXD chronologies in central and northern Fennoscandia explains 83% of the total variance of PC1 from the 34 MXD chronologies over the period 1901-1978, 48% of the total variance of the summer warm-day extreme variability over the period 1901-2006, and 36% of the total variance in the frequency of a summer anticyclonic pattern centered over eastern-central Fennoscandia in the period 1948-2006. The frequency of summer warm-day extremes in Fennoscandia is likely linked to a meridional shift of the northern mid-latitude jet stream. This study shows that the MXD network can be used to infer the variability of past summer warm-day extremes and the frequency of the associated summer anticyclonic circulation pattern over Fennoscandia. [ABSTRACT FROM AUTHOR]

In Europe, several extreme summer drought events have occurred the last two decades, with the 2018 Northern European drought as the most recent example. A better understanding of the combined characteristics and the large-scale atmospheric circulation driving such events is of high importance to enhance preparation and mitigation planning of drought. Initial analysis based on the E-OBS daily gridded observational datasets, showed record-breaking May and July temperature in large parts of northern Europe caused by blocking systems centred over Fennoscandia. Resulting high evapotranspiration combined with precipitation deficit caused dry conditions over large parts of northern Europe. The objective of this study is to investigate the hydrological aspect (streamflow and groundwater) of the 2018 drought, in a historical perspective, in the Nordic countries (Norway, Sweden, Finland and Denmark). The analysis is based on high quality long records (at least 30 years) daily streamflow data for near-natural catchments in the Nordic countries. For groundwater, daily (or weekly) reference station data for Norway will be analysed. Further, the study will investigate potential co-occurrence of, and changes in, driving mechanisms of northern Europe drought events using gridded historical datasets (e.g. E-OBS and NCEP reanalysis data). This study will highlight the historical distinctiveness of the combined drivers and hydrological characteristics of the 2018 drought in the Nordic countries. [ABSTRACT FROM AUTHOR]