Abstract
Abstract. Climate reconstructions from central Scandinavia suggest that annual and summer temperatures were rising during the early Holocene and reached their maximum after 8000 cal yr BP. The period with highest temperatures was characterized by increasingly low lake-levels and dry climate, with driest and warmest conditions at about 7000 to 5000 cal yr BP. We compare the reconstructed climate pattern with simulations of a climate model for the last 9000 years and show that the model, which is predominantly driven by solar insolation patterns, suggests less prominent mid-Holocene dry and warm period in Scandinavia than the reconstructions. As an additional explanation for the reconstructed climate, we argue that the trend from the moist early Holocene towards dry and warm mid-Holocene was caused by a changing atmospheric circulation pattern with a mid-Holocene dominance of summer-time anticyclonic circulation. An extreme case of the anticyclonic conditions is the persistent blocking high, an atmospheric pressure pattern that at present often causes long spells of particularly dry and warm summer weather, or "Indian summers". The argument is tested with daily instrumental temperature and precipitation records in central Sweden and an objective circulation classification based on surface air pressure over the period 1900–2002. We conclude that the differences between the precipitation and temperature climates under anticyclonic and non-anticyclonic conditions are significant. Further, warm and dry combination, as indicated by mid-Holocene reconstructions, is a typical pattern under anticyclonic conditions. These results indicate that the presented hypothesis for the mid-Holocene climate is likely valid.
Highlights
It is well established that atmospheric circulation plays an important role in shaping the regional climate and influencing the marked intra-seasonal climatic variability over Scandinavia (e.g. Busuioc et al, 1999; Chen, 2000; Slonosky et al, 2000, 2001; Moberg and Jones, 2005; Achberger et al, 2007)
As an additional explanation for the reconstructed climate, we argue that the trend from the moist early Holocene towards dry and warm mid-Holocene was caused by a changing atmospheric circulation pattern with a midHolocene dominance of summer-time anticyclonic circulation
The pollen-based quantitative annual mean temperature records from Lakes Lilla Gloppsjon, Gilltjarnen and Trehorningen in central Sweden indicate coherent trends (Fig. 2a). They show that the early-Holocene temperature was low but steadily rising until about 8000 to 7000 cal yr BP, when it rose to the maximum Holocene level
Summary
It is well established that atmospheric circulation plays an important role in shaping the regional climate and influencing the marked intra-seasonal climatic variability over Scandinavia (e.g. Busuioc et al, 1999; Chen, 2000; Slonosky et al, 2000, 2001; Moberg and Jones, 2005; Achberger et al, 2007). Maximum temperatures above 30◦ and low precipitation are associated with clear skies and stable lower part of the atmosphere with minimal horizontal exchange This kind of weather is often termed an “Indian summer” and it can persists several weeks with major impact on, for example, ground-water level, hydrological conditions, plant growth, and agricultural productivity (Johannessen, 1970; Lupo et al, 1997). The occurrence of these contrasting weather types is associated with changing dominance of atmospheric circulation patterns over northern Europe. Blockings are an inherent and frequent feature in the European climate system, and their occurrence and prolonged duration has a strong influence on climate in northern and north-western Europe (Rimbu et al, 2007; Carril et al, 2008)
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