The annual July SPI12 reconstruction for the period 1221–2019 based on the oak tree-ring width for Eastern Europe (Romania)

This dataset contains the annually resolved reconstruction of the 12-month Standardized Precipitation Index for July (July SPI12) forc, spanning the period 1221–2019 Common Era (CE). The reconstruction was developed using the Suceava oak chronology, a replication-rich network of 554 samples comprising 183 living trees and 371 historical construction timbers from Romania, the Republic of Moldova, and Ukraine, built on two dominant oak species (Quercus robur L. and Quercus petraea (Matt.) Liebl.).

The regional curve standardization (RCS) method was applied to preserve the full spectrum of climatic variability across interannual-to-multidecadal time scales. The reconstruction model was calibrated and verified against observed July SPI12 values from the Iași meteorological station (1895–2019), yielding robust statistical skill across both calibration and verification periods.

The reconstructed July SPI12 time series captures pronounced hydroclimatic variability across eastern Europe over the past eight centuries, including multiple megadrought episodes in the fourteenth, fifteenth, and eighteenth centuries, as well as prominent wet phases. These extremes align with documentary evidence of agricultural failures, famines, regional migrations, and plague outbreaks, providing a long-term environmental context for historical societal changes in the region.

Mechanistic analyses link hydroclimatic extremes to large-scale atmospheric circulation anomalies, particularly geopotential height patterns at 500 hPa and North Atlantic sea surface temperature (SST) anomalies, implicating internal climate variability as the dominant driver of precipitation variability in this region. No consistent volcanic forcing signal was detected in the reconstruction.

Comparison with bias-corrected EURO-CORDEX regional climate projections under RCP4.5 and RCP8.5 scenarios suggests that mean hydroclimatic conditions over Romania are unlikely to change substantially through the twenty-first century, while the uncertainty envelope of possible outcomes expands considerably, heightening the risk of persistent extremes.

Temporal coverage: 1221–2019 CE

Spatial coverage: Eastern Europe (Romania, Republic of Moldova, Ukraine)

Reconstruction target: July SPI12 (12-month Standardized Precipitation Index ending in July)

Primary proxy: Oak (Quercus robur and Quercus petraea) tree-ring width chronology

This dataset supports the findings reported in: Roibu et al. (2026), Natural Variability Dominates Precipitation Variability over the Past 800 Years in Eastern Europe, Journal of Climate, 39, 3117–3136. https://doi.org/10.1175/JCLI-D-25-0626.1