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

CARRIL A. F. AND A. NAVARRA

CLIMATE DYNAMICS

Springer

Año: 2001 vol. 18 p. 1 - 16

0930-7575

An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4GCM has been examined in order to evaluate the capability of the model tosimulate the principal modes of interannual variability. The integrationswere performed under specified monthly SST for the period 1979-1993. Theanalysis was focused on the Southern Hemisphere (SH) extratropics.Empirical orthogonal functions analysis (EOF) using seasonal anomaly fieldshas been performed to isolate the principal modes that dominate the southernextratropical variability at the interannual time scale. Leading patternsof 500 hPa geopotential height (z500) have been compared with those estimatedfrom the ECMWF re-analysis dataset. The model is able to adequatelyreproduce the spatial pattern of the annular mode, but it represents thetemporal variations of the oscillation less satisfactorily. The model simulationof the Pacific South American (PSA) pattern is better, both in theshape of the pattern and in the temporal evolution. To verify if the capabilityof the model to adequately simulate the temporal oscillation of thepropagating patterns is related with the increased influence of the tropicalexternal forcing, covarying SST-atmospheric modes have been identified bythe technique of singular value decomposition (SVD). In Winter (July-August-September, JAS) the tropical SST variability is highly correlatedwith the ENSO mode. In Summer (January-February-March, JFM) the strength ofthe teleconnections is related with strong westerly anomalies, disrupted bya meridional out of phase relation in the neighbourhoods of South America.The large size of the ensemble was exploited by comparing the time-varyingmodel spread and degrees of freedom of the simulated extratropical circulation.Results show that in the years in which the extratropical circulationreside in a few degrees of freedom, the reproducibility is relatively lowand the ensemble is governed by a fairly robust zonally symmetric structureof dispersion.