The hydroclimate of the Iberian Peninsula has been suggested to be persistently dry during Medieval times due to a persistently positive North Atlantic Oscillation (NAO) mode (Trouet et al., 2009). However, modeling results and proxy records from the region indicate that substantial wet intervals likely occurred. We present a sub-decadal stable isotope record from a calcite stalagmite (Buraca Glorioso, 39⁰N, 8⁰W) showing an inferred variable hydroclimate during Medieval times (~800 to 1300 AD) during the Medieval Climate Anomaly (MCA). Calcite δ18O values at this location are dependent on the relative amounts of precipitation received. Drier (wetter) conditions would lead to more (less) evaporation during precipitation events and higher (lower) δ18O values. Winter precipitation in Portugal is strongly dependent on the behavior of the NAO and this record has important implications for study of the NAO and its behavior in the past. In this study δ18O and δ13C records from Portugal are compared with other proxy records from Morocco, Spain, and Scotland. Although this δ18O and δ13C record implies positive NAO conditions throughout much of the MCA, precipitation appears to have been more variable at this location than would be suggested by the NAO reconstruction of Trouet et al. (2009).

Oxygen and carbon isotope data from calcite stalagmites from the Portuguese cave, Buraca Glorioso, show variability that is inferred to be due to hydroclimatic changes in the region. . Long-term (greater than millennial-scale) changes in calcite δ18O values at this location are dependent on long-term solar insolation changes due to orbital parameters, while shorter-term variability is a result of Total Solar Irradiance (TSI) variability, volcanic forcing, and temperature and precipitation changes, due, in part, to internal modes of variability such as the North Atlantic Oscillation (NAO). Variability in carbon isotopes (δ13C) appear to be dependent on volcanic forcing and temperature and precipitation variability. The δ18O and δ13C record from Portugal is compared with other relevant proxy records from stalagmites in Spain, Sweden, and Scotland, multi-proxy Northern Hemisphere temperature reconstructions, and pollen records from the Iberian Peninsula. The δ13C record from Buraca Glorioso is strikingly similar to records of other locations in the region, suggesting that Buraca Gloriso is recording regional climate variability. The main driver of isotopic variability on long time scales is insolation changes due to variability in the Earth orbital parameters. The δ18O and δ13C records from Buraca Glorioso both respond to large volcanic forcing events. Some small-scale variability is linked to changes in TSI and changes in temperature and precipitation as a result of internal climate mode (e.g. NAO) variability.