Second Revision

• Main Authors: A. W. Robertson, M. Ghil, M. Latif, J. Atmos Sci, Sub Judice, Dr. Andrew, W. Robertson
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1999
• Subjects: North Atlantic; North Atlantic oscillation
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.329.4673; http://iri.columbia.edu/~awr/papers/RGL_990521.pdf

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-todecade 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. 1 Preliminary results of this numerical experiment were reported at the Atlantic Climate