Tropical intraseasonal variability in seasonal hindcasts and climate simulations

The tropical intraseasonal variability is a source of predictability at extended time scales in the tropics. Its correct representation in coupled GCMs used to perform extended and seasonal forecasts is thus an important issue. Using a new metric assessing the realism and the reproducibility of the spatiotemporal characteristics of intraseasonal convective perturbations, differences appear between ENSEMBLES seasonal hindcasts and longer simulations done with the same coupled GCMs. This is examined for different models and different initialization procedures. For some configurations, the ISV perturbation patterns appear to be more reproducible and realistic in hindcasts compared to longer simulations. This apparent improvement may however hide intraseasonal adjustments due to initialization shocks. These shocks are for example related to systematic differences between the observed (initialized) SST and the climatology of the coupled GCM. Transient large-scale organized convective perturbations thus develop over relatively warm regions and evolve with a realistic and reproducible pattern (thanks to the generally well described Gill-type dynamical response) but at a given location and with a systematic phase in regard to the initialization date. For such a configuration, a realistic pattern does not guarantee a better skill for the extended range forecast.