Can tree-ring density data reflect summer temperature extremes and associated circulation patterns over Fennoscandia?

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.