TROPICAL INTRASEASONAL OSCILLATION APPEARING IN OPERATIONAL ANALYSES AND IN A FAMILY OF GENERAL-CIRCULATION MODELS

The statistics of tropical intraseasonal variability are studied using European Centre for Medium-Range Weather Forecasts analyses and sever al ECHAM General Circulation Model experiments made with different mod el versions (ECHAM2 and ECHAM3, which have different convection scheme s) and different horizontal resolutions (T21, T42, and T106). The stud y applies the principal oscillation pattern technique to the 200-mb eq uatorial velocity potential. Associated patterns of tropical outgoing longwave radiation, equatorial zonal wind, and equatorial divergence a re also presented. The intercomparison of ECHAM2 and ECHAM3 simulation s at low (T21) resolution shows that the improved model physics has a beneficial impact on the simulated Madden-Julian oscillation (MJO). Th e MJO produced by the ECHAM2 model has an unrealistic spatial distribu tion of convection, whereas the MJO simulated by the ECHAM3 model appe ars to be related to convective activity over the Indian Ocean and the West Pacific, which is consistent with the observed MJO. An increase of the horizontal resolution of the ECHAM3 model seems to actually deg rade the results. At T42 and T106, the ECHAM3 MJO exhibits too much co nvective activity over central and equatorial America, with only a mar ginal effect of the MJO on the West Pacific-Indonesian region.