Towards a Regional Synthesis of Mediterranean Climatic Change Using Lake Stable Isotope Records

fairly equally distributed over the year, with a slight maximum in early autumn. A contrasting situation exists under nearor full-glacial conditions, marked by reduced precipitation overall, and most particularly during winter, thus strongly biasing the water isotope signal towards that of the summer. This mechanism explains about 70% of the difference between the simulated spatial and temporal isotopetemperature gradients the latter being unreasonably low (in the order of 0.2‰/°C) in the absence of correction for seasonality, which increases the gradient to about 0.4‰/°C. This provides a graphic example of the utility of isotopicGCMs as “thinking tools”, since this profound shift in the temporal isotope-temperature relation can be related directly to the extreme southward shift of the winter polar front under glacial conditions and the concomitant suppression of meridional moisture transport to central Greenland, thus offering a clear mechanistic explanation for the apparent failure of the isotope paleothermometer across the glacial-interglacial transition (e.g., see Cuffey et al., 1995; Dahl-Jensen et al., 1998; Severinghaus et al., 1998; Lang et al., 1999).