Reconstruction of decadal to centennial Holocene variability of the North Atlantic Oscillation using speleothems from Morocco

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The North Atlantic Oscillation (NAO; the dominating atmospheric pressure mode in the North Atlantic region) has large ecological and socio-economical impact by affecting both marine and continental winter climate in the Northern Hemisphere. Understanding the mechanisms influencing the NAO as well as its relationship to other climate parameters is, thus, of great importance. Links between the NAO, solar activity and the ocean-atmosphere-sea ice system have been proposed. Since instrumental climate datasets do not extend back beyond the Anthropocene, the natural variability of the NAO can only be assessed by paleoclimate reconstructions. However, only a few continental NAO reconstructions extending beyond the instrumental data period are currently available. The Holocene evolution and forcing factors of the NAO are, thus, a matter of debate in the scientific community. In the framework of the proponents PhD thesis, stalagmites from caves in the Middle Atlas, NW Morocco, were used to reconstruct rainfall variability during the Holocene. NW Morocco is very sensitive to changes in the state of the NAO, and the speleothem data indicated significant past variability of the NAO on multi-centennial timescales. However, information on decadal to centennial timescales requires (near-) annual resolution climate reconstructions. At present these are not available. The proposed project focuses on stalagmites from two caves in Morocco, which have provided climate records with a relationship to the NAO. The aims are (i) to construct one master record based on the common rainfall signals of different stalagmite records from the same cave, and (ii) to provide a Holocene reconstruction of effective rainfall with annual resolution. Frequency analysis is employed to detect significant periodic components. Time intervals dominated by specific decadal to centennial frequencies or a persistent positive/negative state of the NAO will be compared with potential NAO forcing factors (i.e. solar activity, oceanic parameters, sea ice extension).