Abstract
The statistics of tropical intraseasonal variability are studied using European Centre for Medium-Range Weather Forecasts analyses and several ECHAM General Circulation Model experiments made with different model versions (ECHAM2 and ECHAM3, which have different convection schemes) and different horizontal resolutions (T21, T42, and T106). The study applies the principal oscillation pattern technique to the 200-mb equatorial velocity potential. Associated patterns of tropical outgoing longwave radiation, equatorial zonal wind, and equatorial divergence are also presented. The intercomparison of ECHAM2 and ECHAM3 simulations at low (T21) resolution shows that the improved model physics has a beneficial impact on the simulated Madden–Julian oscillation (MJO). The MJO produced by the ECHAM2 model has an unrealistic spatial distribution of convection, whereas the MJO simulated by the ECHAM3 model appears 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 degrade the results. At T42 and T106, the ECHAM3 MJO exhibits too much convective activity over central and equatorial America, with only a marginal effect of the MJO on the West Pacific–Indonesian region.
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