Simulations of the interannual leading modes of the southern extratropial circulation

CARRIL A. F. AND A. NAVARRA

Niza, Francia

Conferencia; EGS General Assembly; 2000

EGS (European Geophysical Society)

An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined inorder to evaluate the capability of the model to simulate the principal modes of interannual variability. Theintegrations were performed under specified monthly SST for the period 1979-1993. The analysis wasfocused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis (EOF) usingseasonal anomaly fields has been performed to isolate the principal modes that dominate the southernextratropical variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500)have been compared with those estimated from the ECMWF re-analysis dataset. The model is able toadequately reproduce the spatial pattern of the annular mode, but it represents the temporal variations ofthe oscillation less satisfactorily. The model simulation of the Pacific South American (PSA) pattern is better,both in the shape of the pattern and in the temporal evolution. To verify if the capability of the model toadequately simulate the temporal oscillation of the propagating patterns is related with the increasedinfluence of the tropical external forcing, covarying SST-atmospheric modes have been identified by thetechnique of singular value decomposition (SVD). In Winter (July-August-September, JAS) the tropical SSTvariability is highly correlated with the ENSO mode. In Summer (January-February-March, JFM) the strengthof the teleconnections is related with strong westerly anomalies, disrupted by a meridional out of phaserelation in the neighbourhoods of South America. The large size of the ensemble was exploited bycomparing the time-varying model spread and degrees of freedom of the simulated extratropical circulation.Results show that in the years in which the extratropical circulation reside in a few degrees of freedom, thereproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structureof dispersion.