Unveiling climate signals and long-term variability from 30 years of precipitation isotopes in Pisa (Italy)

Stable isotopes (δ¹⁸O and δ²H) and the radioactive isotope tritium (³H) in precipitation are sensitive indicators of spatiotemporal changes in climate and the hydrological cycle, reflecting both large-scale atmospheric oscillations and local hydroclimatic processes. They are widely used to study atmospheric, hydrological, and hydrogeological processes, and the δ¹⁸O is a well-established proxy for long-term hydroclimatic variations in paleoclimatic archives like speleothems, ice cores, and tree rings. Thus, stable isotopes in precipitation serve as key tracers of past and present hydroclimatic conditions, providing valuable insights into global climate changes and future trajectories.

Long isotope time series are essential for assessing the climatic dependency of isotopic variability in precipitation, detecting multi-decadal climatic oscillations and identifying trends. In this study, we present the first results from 30 years of isotopic precipitation monitoring in Pisa, Italy (1993-2023), a region characterized by Mediterranean climate. The dataset spans 327 monthly samples and is one of the most extensive and continuous records in the Mediterranean.

Seasonal isotopic variability reflects seasonal climatic fluctuations, with a positive correlation between δ¹⁸O and temperature (temperature effect) and a negative correlation with precipitation amount (amount effect). These climatic factors also partly drive sub-seasonal isotopic variability, especially in summer. In winter, higher precipitation is associated with negative phases of the Mediterranean Oscillation Index (MOI) and North Atlantic Oscillation (NAO), and positive phases of the Western Mediterranean Oscillation (WeMO). In spring and autumn, it correlates with MOI and WeMO. These patterns result in significant correlations between δ¹⁸O, MOI, and NAO in winter, and δ¹⁸O and MOI in autumn, which reveals the influence of large-scale and regional atmospheric patterns in these seasons. Deuterium excess shows negative correlations with temperature and positive correlations with precipitation amount.

Mann-Kendall and Theil-Sen tests reveal no significant trends in δ¹⁸O and deuterium excess series over the 30-year period or within individual seasons, except for a positive trend of deuterium excess in winter. Seasonal-Trend Decomposition (STL) also shows no significant trends in deseasonalized data, except for the radioactive isotope tritium, which exhibited a clear decreasing trend both in the raw and deseasonalized data.

These findings highlight the importance of isotopic precipitation studies in understanding hydrological and atmospheric processes and their potential for tracking present and past hydroclimatic changes.