Large-scale fluctuations in a long-range integration of the ECMWF spectral model

The ECMWF spectral model with a triangular T21 truncation has been integrated for a 6-year period. The chronology of the atmosphere simulated by the model is studied through Principal Component Analysis of the 500 mb geopotential field. The annual cycle is found to be predominant, but the interannual variability is shown to be large and the model proves able to generate long and persistent anomalies, even though sea-surface temperatures change only with a climatological annual variation.