Interdecadal changes in atmospheric low-frequency variability with and without boundary forcing

The response of the Max Planck Institute's ECHAM3 atmospheric general circu lation model to a prescribed decade-long positive anomaly in sea surface te mperatures (SSTs) over the North Atlantic is investigated. Two 10-yr realiz ations of the anomaly experiment are compared against a 100-yr control run of the model with seasonally varying climatological SST using a model spati al resolution of T42. In addition to the: time-mean response. particular at tention is paid to changes in intraseasonal variability, expressed in terms of North Atlantic-European weather regimes. The model regimes are quits re alistic. Substantial differences are found in the 700-mb geopotential height held re sponse between the two decadal realizations. The rime-mean response in the first sample decade is characterized by the positive (zonal) phase of the N orth Atlantic oscillation (NAO); this response con be identified with chang es in the frequency of occurrence of certain weather regimes by about one s tandard deviation, (Preliminary results of this numerical experiment were r eported at thr: Atlantic Climate Variability Workshop held at the Lamont-Do herty Earth Observatory of Columbia University, Palisades, New York, 24-26 September 1997.) By contrast. the second SST anomaly decade shows a localiz ed trough centered over the British Isles; it projects less strongly onto t he model's intrinsic weather regimes. The control run itself exhibits prono unced decade-to-decade variations in the weather regimes' frequency of occu rrence as well as in its NAO index. The two 10-yr anomaly experiments are i nsufficient; in length and number, to identify a robust SST response above this level of intrinsic variability.