A long-term drought reconstruction based on oxygen isotope tree ring data

Abstract. This study investigates the relationship between oxygen isotope ratios (δ¹⁸O) in oak tree ring cellulose and past drought variability in Letea Forest, Romania. A δ¹⁸O site chronology spanning 1803–2020 was compiled from seven individual time series. δ¹⁸O values exhibited a significant negative correlation with moisture-related variables (cloud cover, relative humidity and precipitation) and a positive correlation with temperature and sunshine duration. This confirms that δ¹⁸O from tree rings can be a good proxy for moisture availability. The strongest correlation was found between δ¹⁸O and the August Standardized Precipitation Evapotranspiration Index for an accumulation period of 9-months (SPEI9) for central and eastern Europe. This highlights SPEI9 as a superior indicator of drought compared to individual parameters like temperature or precipitation. Using a linear regression model, we reconstructed August SPEI9 variability for the past 200 years. The reconstruction captured interannual and decadal variations, with distinct wet and dry periods. Analysis of large-scale atmospheric circulation patterns revealed a link between high δ¹⁸O values (indicating dry conditions) and a high-pressure system over the North Atlantic. Conversely, low δ¹⁸O values (indicating wet conditions) corresponded to negative pressure anomalies over Europe. Moreover, extreme values of δ¹⁸O are also associated with the prevalence of a hemispheric teleconnection pattern, namely wave number 4. This δ¹⁸O chronology and the corresponding August SPEI9 reconstruction offer valuable tools for understanding past climate variability and its relationship with large-scale atmospheric and oceanic circulation patterns.