Changing correlation structures of the Northern Hemisphere atmospheric circulation from 1000 to 2100 AD

Abstract

Abstract. Atmospheric circulation modes are important concepts to understand the variability of atmospheric dynamics. Assuming their spatial patterns to be fixed, such modes are often described by simple indices derived from rather short observational data sets. The increasing length of reanalysis products allows scrutinizing these concepts and assumptions. Here we investigate the stability of spatial patterns of Northern Hemisphere teleconnections by using the Twentieth Century Reanalysis as well as several control and transient millennium-scale simulations with coupled models. The observed and simulated centers of action of the two major teleconnection patterns, the North Atlantic Oscillation (NAO) and to some extent the Pacific North American (PNA), are not stable in time. The currently observed dipole pattern of the NAO with its center of action over Iceland and the Azores split into a North-South dipole pattern in the western Atlantic and a wave train pattern in the eastern part connecting the British Isles with West Greenland and the Eastern Mediterranean in the period 1940–1969 AD. The PNA centers of action over Canada are shifted southwards and over Florida into the Gulf of Mexico in the period 1915–1944 AD. The analysis further shows that shifts in the centers of action of either telconnection pattern are not related to changes in the external forcing applied in transient simulations of the last millennium. Such shifts in their centers of action are associated with changes in the relation of local precipitation and temperature to the overlying atmospheric mode. These findings further undermine the assumption of stationarity between local climate/proxy variability and large-scale dynamics inherent in proxy-based reconstructions of atmospheric modes and call for a more robust understanding of atmospheric variability on decadal time scales.