Global changes in the hydroclimate of the Last Millennium: Evidences from reconstructions and simulations

Global changes have been found in simulated and reconstructed temperatures during the Last Millennium (LM), mainly associated to changes in the Earth’s energy balance like those during the periods of the Medieval Climate Anomaly (MCA; ca. 950-1250 CE) and the Little Ice Age (LIA; ca. 1450-1850 CE), respectively characterized by warmer and cooler conditions over many regions. Even if the impact of these changes in the hydroclimate is not fully understood, evidences of coordinated changes in the hydroclimate of distant regions can be also found in reconstructions from areas of North America, Europe and the Mediterranean basin, South America, Eastern Africa and Monsoon Asia. The timing of such changes suggests a link to the MCA and the LIA, while other areas like Southeast Asia and the Indo-Pacific also show coordinated changes in the hydroclimate of the LM, but without a patent link to the periods of MCA and LIA.

To better assess these coordinated changes in the hydroclimate of distant regions, and whether they also extend to other periods of the LM different from the MCA and the LIA, evidences from reconstructions and simulations have been compiled, including: a compilation of 92 reconstructions reporting changes from wetter to drier or from drier to wetter conditions in the transition from MCA to LIA; reconstruction-based products like the Drought Atlases for Europe (OWDA), North America (NADA), Asia (MADA), Mexico (MXDA) and Eastern Australia and New Zealand (ANZDA); hybrid products like the Paleo Hydrodynamics Data Assimilation product (PHYDA) and 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).

The comparison between proxy-based datasets and model simulations has allowed to obtain robust conclusions regarding the behavior of the hydroclimate of different regions and to deeply analyze the impact of external forcing and internal variability on hydroclimate changes. In tropical areas, changes have been mainly associated to alterations in the position and intensity of the Intertropical Convergence Zone (ITCZ), while in extratropical areas the alteration of variability modes like the Northern (NAM) and Southern Annular Modes (SAM) may have had a major role in the response of hydroclimate to changes in external forcing.