Climate Analysis Section Seminar
Date: June 10, 2015
Time: 11:00 am
Location: Mesa Lab Main Seminar room (ML132)
Speaker: Christoph C. Raible, University of Bern, Switzerland
Atmospheric circulation modes such as the North Atlantic Oscillation (NAO) or the Pacific North America pattern (PNA) are important concepts in understanding the variability of atmospheric dynamics. Assuming their spatial patterns to be fixed, such modes are often described by simple indices from rather short observational data sets. The increasing length of reanalysis products but also millennium-scale simulations with complex Earth System Models allows these concepts and assumptions to be scrutinised. Using both data sources the stability of the spatial patterns of Northern Hemisphere teleconnections are analysed in more details. The results show that the observed and simulated locations of the centres of action of the NAO and to some extent of the PNA, are not stable in time. The currently observed dipole pattern of the NAO, its centre of action over Iceland and the Azores, split into a north–south dipole pattern in the western Atlantic with a wave train pattern in the eastern part, connecting the British Isles with West Greenland and the eastern Mediterranean during the period 1940–1969 AD. The PNA centres of action over Canada are shifted southwards and over Florida into the Gulf of Mexico during the period 1915–1944 AD. The analysis further shows that shifts in the centres of action of either teleconnection pattern are not related to changes in the external forcing applied in transient simulations of the last millennium. Additionally, CMIP5 simulations show also no systematic change in the centres of action the Northern Hemisphere teleconnection patterns. Thus, we conclude that the variations are mainly internal driven. Besides the relevance of these variations is investigated in the context of proxy reconstructions as such shifts in their centres of action are accompanied by changes in the relation of local precipitation and temperature.