Mediterranean climates during Dansgaard-Oechger cycles in the last glacial period

We investigate the impact of rapid climate changes during the Dansgaard-Oeschger cycles of the last glacial on the climate of the Mediterranean region. We reconstructed the temperature of the coldest and warmest months, growing season warmth, and plant-available moisture (MI) from 20 published pollen cores from Iberia to Iran, using frequency corrected tolerance-weighted weighted average partial least squares (fxTWA-PLS) method. We corrected the MI reconstructions to account for the direct physiological impact of changing CO₂ levels on plant water-use efficiency. We found warm intervals – probable GIs - by identifying potential D-O warmings using their signature asymmetrical rise and fall. Cold climate intervals – considered as the most extreme expression of the GSs - were defined as periods when growing season warmth was below the long-term average value for each individual record.

Warm intervals are characterised by a decrease in moisture in the western Mediterranean compared to the preceding cold interval. Sites in the eastern Mediterranean show either no change in moisture between the two states or are characterised by a slight increase in moisture during the warm intervals compared to the cold intervals. There is also a marked west-east difference in temperature seasonality, with warmer intervals showing increased seasonality in the western and northwestern region compared to the eastern Mediterranean. The increased seasonality is largely driven by changes in summer temperature since the degree of winter warming during warm intervals is similar across the whole region. Changes in all of the bioclimatic variables between cold and warm intervals show a strong relationship with latitude: the latitudinal gradient is steeper in cold climates than in warm climates. The relative homogeneity of changes during the cold intervals is consistent with a more zonal circulation pattern than during warm intervals. This change in circulation patterns could help to explain the west-east patterns in the changes in moisture between cold and warm intervals.