The interannual leading modes of the extratropical variability in the Southern Hemisphere simulated by the ECHAM-4 atmospheric model

An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability to simulate the princi pal modes of interannual variability. The integrations were performed under specified monthly SST between 1979-1993. The analysis was focused on the S outhern Hemisphere (SH) extratropics. Empirical orthogonal functions analys is (EOF) using seasonal anomaly fields has been performed to isolate the pr incipal modes that dominate the southern extratropical variability at the i nterannual time scale. Leading patterns of 500 hPa geopotential height (z50 0) have been compared with those estimated from the ECMWF re-analysis datas et. The model is able to adequately reproduce the spatial pattern of the an nular mode, but it represents the temporal variations of the oscillation le ss satisfactorily. The model simulation of the Pacific South American (PSA) pattern is better, both in the shape of the pattern and in the temporal ev olution. To verify if the capability of the model to adequately simulate th e temporal oscillation of the propagating patterns is related to the increa sed influence of the tropical external forcing, covarying SST-atmospheric m odes have been identified by singular value decomposition (SVD). In winter (July-August-September, JAS) the tropical SST variability is highly correla ted with the ENSO mode. In summer (January-February-March, JFM) the strengt h of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation near to South America. The large siz e of the ensemble was exploited by comparing the time-varying model spread and degrees of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees of freedom, the reproducibility is relatively low and the ensemble is governed by a fai rly robust zonally symmetric structure of dispersion.