The interannual variability of the Madden-Julian Oscillation in an ensemble of GCM simulations

The interannual variability of the Madden-Julian Oscillation (MJO) is inves tigated in an ensemble of 15 experiments performed with the ECHAM4 T30 gene ral circulation model (GCM). The model experiments have been performed with AMIP conditions from January 1979 to December 1993. The MJO signal has bee n identified applying a principal oscillation pattern (POP) analysis to the 200-mb tropical velocity potential. The results obtained from the model en semble are compared with 15 y of ECMWF re-analysis and OLR observations. Th e results suggest that the warm and cold phases of El Nino have some influe nce on the spatial propagation of the oscillation. Both in the re-analysis and in the model ensemble, the results indicate that during La Nina conditi ons the MJO is mostly confined west of the date line, with the largest acti vity located over the Indian Ocean and the western Pacific. In warm El Nino conditions, the convective anomalies associated with the oscillation appea r to penetrate farther into the central Pacific. These changes in the MJO c onvective forcing seem to affect the zonal mean of the rotational component of the flow anomaly, which tends to weaken during warm El Nino periods. So me weak reproducibility of the interannual variability of the MJO activity is found. The results obtained from four-member and eight-member subsamples of the ensemble indicate that the reproducibility of the interannual behav iour of the MJO can be detected by choosing an ensemble of a larger size. C orresponding to the emergence of reproducibility with the increasing size o f the sample, the correlation between the MJO activity and the Nino-3 SST a nomaly appears to intensify.