Decadal Variability in North Atlantic Weather Regimes A.W. Robertson 1 , M. Ghil 1 and M. Latif 2 1 Department of Atmospheric Sciences and Institute of Geophysics and Planetary Physics University of California, Los Angeles Los Angeles, California 2Max-Planck Institute for Meteorology Hamburg, Germany The concept of persistent and recurrent large-scale atmospheric flow regimes, or "weather regimes", is used to investigate the response of an atmospheric general circulation model (GCM) to a prescribed decade-long anomaly in sea-surface temperature (SST) over the North Atlantic during winter. We define firstly intraseasonal atmospheric variability in terms of persistent and recurrent, large- scale atmospheric flow regimes with fixed spatial patterns (Dole and Gordon, 1983; Cheng and Wallace, 1993; Kimoto and Ghil, 1993; Michelangeli et al., 1995). Within this framework, interdecadal variability can manifest itself either as a change in the classifiable number of regimes, or as a change of the spatial patterns and frequency of occurrence of the same identifiable regimes, or as some combination of these possibilities. We test these ideas by (1) estimating weather regimes robustly from a 100-year control run of the Max-Planck Institute's atmospheric GCM (ECHAM3) at T42 horizontal resolution; (2) testing the extent to which changes of regime pattern and frequency occur when a broad-scale observed SST anomaly pattern is prescribed over the North Atlantic in a 10-year response experiment; and (3) reconstructing the GCM's response in terms of weather-regime changes.