Influence of atmospheric circulation patterns on the oxygen isotope ratio of tree rings in the Alpine region

The oxygen isotope ratio of precipitation and tree rings is a complex function of climate variables and atmospheric dynamics, which often makes the interpretation of delta O-18 for palaeoclimate research challenging. Here we analyzed monthly precipitation delta O-18 series for 1973-2004 and annually resolved tree ring delta O-18 chronologies for 1945-2004 for three sites in Switzerland: one north of the Alps, one at high-elevation within the Alps, and one south of the Alps. The goal of the study was to improve the understanding of the tree ring archive by a systematic analysis of nonlocal parameters related to atmospheric circulation, in particular, geopotential height field anomalies and the frequency of synoptic weather situations, in addition to the usual local climate parameters like temperature, sunshine duration, and relative humidity. We observed that on average high-pressure situations during summer were associated with relatively high delta O-18 and low-pressure situations were associated with relatively low delta O-18, for both the isotope ratio in precipitation and tree rings. However, correlations to the frequency of weather types were not higher than simple correlations to local temperature. Accordingly, we constructed a combined index from temperature and air pressure that proved to be a good predictor of delta O-18 in precipitation and used this as the source water term in a tree ring isotope fractionation model. This enabled us to use the model beyond the period where isotope values for precipitation are available, opening new perspectives in the interpretation of long tree ring delta O-18 chronologies.