Large-scale changes in the hydroclimate of the Last Millennium from reconstructions and simulations

The climate of the Last Millennium (LM) was characterised in many regions by long periods of warmer and cooler conditions, like the Medieval Climate Anomaly (MCA; ca. 950-1250 CE) and the Little Ice Age (LIA; ca. 1450-1850 CE). These changes in temperatures can be linked to changes in the Earth’s energy balance, being the solar and volcanic activity the main drivers during the pre-industrial period. Even if the MCA and LIA are typically defined in terms of temperatures, reconstructions of the hydroclimate conditions from regions of North America, Europe, Northern and Central Africa, Northern and Central South America, and Central and Eastern Asia also show relevant changes during these periods, suggesting a large-scale impact on the hydroclimate.

To analyse these large-scale changes in the hydroclimate, and whether similar changes also extend to other periods of the LM outside the MCA and the LIA, an exhaustive compilation of all the available reconstruction-based and model-based sources providing information about the hydroclimate of the LM has been considered, including the Drought Atlases for Europe (OWDA), North America (NADA), Asia (MADA), and Mexico (MXDA), the Paleo Hydrodynamics Data Assimilation product (PHYDA), the Last Millennium Reanalysis (LMR), and model simulations from the Community Earth System Model - Last Millennium Ensemble (CESM-LME) and the Coupled Model Intercomparison Project Phase 5 (CMIP5) and 6 (CMIP6).

This analysis has shown large-scale changes in the hydroclimate of the LM, particularly relevant in tropical areas around the Intertropical Convergence Zone (ITCZ), in the area of influence of the Northern (NAM) and Southern Annular Modes (SAM), and in the Indian Monsoon region, extending from the Middle East to Southeast Asia. The use of several reconstruction-based products allowed the identification with a robustness assessment of those regions more impacted by hydroclimatic changes, while the comparison between reconstruction-based products and model simulations from different ensembles allowed an assessment of the contributions of external forcing and internal variability on the hydroclimate of each region.