AGCM experiments on the Younger Dryas climate

The Younger Dryas (YD) cooling (#propor to#12.5-11.5 thousand years before present) signified an interruption of the warming during the transition from the last glacial to the present interglacial. Since the mechanism responsible for this cooling is still uncertain, a numerical simulation of the YD climate has been undertaken to obtain some additional insight into this event. Three experiments with the ECHAM3 atmospheric general circulation model at T42 resolution were carried out to study the YD climate. In a first experiment only the North Atlantic surface conditions were altered according to geological evidence. In a second and third experiment additional boundary conditions were altered, as ice-sheets were introduced in North America and Scandinavia and the insolation and CO_2 concentration were changed. The third simulation was identical to the second, except for an additional North Pacific cooling. The experimental outcomes were compared with that of a control simulation of present climate. The main deviations in the results were found around the North Atlantic Ocean. In the YD experiments the mean annual temperature was a few degrees lower over the continents, although the difference was larger in the vicinity of newly defined ice-sheets. An intensified atmospheric circulation was simulated. Deviations from the control climate were compared with climate reconstructions based on geological evidence. The reconstructed cooling in Europe during YD was larger than in the simulations, especially during the northern hemisphere (NH) winter. During the NH summer the reconstructions showed a large temperature gradient going inland which was consistent with the experiments. Improvements in the N-Atlantic sea surface conditions were proposed, since they were the most important factor controlling deviations. (orig.)Available from TIB Hannover: RR 1347 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman