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Holocene winter precipitation reconstructions

Due to the highly advective nature of winter-precipitation in Europe, precipitation-reconstructions are potentially a sensitive indicator of past atmospheric synoptic variations, including the North Atlantic Oscillation and other modes. In Scandinavia, a number of published glacier records are analyzed, and winter-accumulation signals have been extracted from these records by using the relationship between ablation-season temperature and winter-accumulation at Norwegian glaciers (Liestøl in Sissons, 1979). This has allowed the amount and relative precipitation to be temporally and spatially described in western Scandinavia.

At the start of PACLIVA, NAO was regarded as the most important atmospheric mode. Recent reanalysis data have provided insight into a more complex non-stationary behaviour of high and low-frequent atmospheric variability (Jacobeit et al., 2003; Cassou et al., 2003; Hurell et al., 2004). Analysis of measured glacier and pressure-field data over Scandinavia has shown how different geostrophic vectors and sub-systems affect different regions during winter (Nordli et al., 2003; 2005). Taking these findings into account and considering the spatial patterns in the reconstructed winter-precipitation over western Scandinavia it has become evident that on low frequencies spatial non-stationarity may be more significant than variations in the NAO-associated meridional pressure-gradient in the North Atlantic (Lie et al., 2003, 2004, 2005). As spatial variations include several independent time-series, the temporal precision of these analyses is below the original PACLIVA-objective.

Therefore we have calculated indices for a continuous time-series of non-stationarity over the last 6000 years. The Continental Index is the standardised glacier-size gradient between two glaciers in southern Norway, 90km apart along a strong precipitation gradient; we interpret differences between the sites as a winter-precipitation signal. The Coastal Index is the standardised winter-precipitation gradient between SW Norway and NW Norway (Bakke et al., submitted). The two indices capture the same low-frequent variability over the last 6000 years. These spatial variations in relative precipitation indicate that the atmosphere varies spatially as well as in its strength-domain during the Holocene, and suggest that the circulation-patterns vary on millennial time-scales.

To test if similar low-frequent variations occur in instrumental time-series, 4 meteorological stations, with precipitation data available for the 20th century, proximal to the 4 glaciers were selected. The normalized instrumental data and the southerly geostrophic pressure-field centred over southern Norway (60°N, 5°E), both smoothed by a 31 yr running mean, are highly correlated and display multi-decadal variability. The Atlantic Multidecadal Oscillation (AMO) leads variations in the pressure-field and relative precipitation. As our precipitation-data does not include variations in total precipitation, known to be associated with the NAO, it is spatial variations of atmospheric circulation changes that dominate this picture. Recently, the AMO has been explained by variations in the overturning circulation, and shifts in atmospheric circulation patterns were suggested as a response (Sutton et al., 2005). To test if this assumption holds on millennial time-scales, we compared our data with a SST-reconstruction from the Vøring-plateau (MD95-2011; Risebrobakken et al., 2003), a record of North Atlantic deep-water ventilation (ODP-980; Oppo et al., 2003) and the PCI of geochemical analyses from GISP2 (Mayewski, 1997). These records behave similarly to the instrumental time-period on millennial time-scales.

To examine variations near the second NAO centre of action, we collected 6 cores were retrieved from three lakes in the Grandes Rousses massif in the south-eastern French Alps. A core from Lac Blanc (2470 m asl) in the catchment of glacier St. Sorlin was chosen for analyses and subjected to a range of sedimentological analyses, including classical bulk-parameters, GEOTEC analyses and grain-size analyses. The top 100 cm has been analysed by micro-XRF. The data show evidence for a climatic optimum lasting from 325-375 cm, when little or no clastic glacier flour was deposited in the lake. There is a period of minor glaciation from 325-300 cm. Well-defined maxima in carbonate input are evidence for at least 8 distinct glacier maxima. The source of this carbonate is a carbonate-rich outcrop currently outside the glacier margin, but within the Little Ice Age maximum extent. This carbonate may affect bulk radiocarbon-dating through the hard-water effect. Wet-sieving the entire core recovered no terrestrial macrofossils for dating. We have postponed the dating of the core, and hope to date pollen extracted from the sediments. This work presents a continual record in which the Alps episodic moraine-derived glacier chronologies may be evaluated.

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Bjerknes Centre for Climate Research
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