Interdecadal Changes in Atmospheric Low-Frequency Variability With and Without Boundary Forcing

The response of the Max-Planck Institute's ECHAM3 atmospheric general circulation model (GCM) to a prescribed decade-long positive anomaly in sea-surface temperatures (SSTs) over the North Atlantic is investigated. Two 10-year realizations of the anomaly experiment are compared against a 100-year control run of the model with seasonally varying climatological SST, using a model spatial resolution of T42. In addition to the time-mean response, particular attention is paid to changes in intraseasonal variability, expressed in terms of North Atlantic/European weather regimes. The model regimes are quite realistic. Substantial differences are found in the 700-mb geopotential height field response between the two decadal realizations. The time-mean response in the first sample decade is characterized by the positive (zonal) phase of the North Atlantic Oscillation (NAO); this response can be identified with changes in the frequency-of-occurrence of certain weather regimes by about one standard deviation 1. By contrast, the second SST-anomaly decade shows a localized trough centered over the British Isles; it projects less strongly onto the model's intrinsic weather regimes. The control run itself exhibits pronounced decade-to-decade variations in the weather regimes' frequency of occurrence as well as in its NAO index. The two 10-year anomaly experiments are insufficient, in length and number, to identify a robust SST response above this level of intrinsic variability.