An event-by-event assessment of tropical intraseasonal perturbations for general circulation models

We detect and characterize each large-scale intraseasonal perturbation in observations (1979-2009) and in Coupled General Circulation Models (CGCMs) of Institut Pierre Simon Laplace (IPSL) and of Centre National de Recherche en Météorologie (CNRM). These observed and simulated ensembles of intraseasonal events are used to assess the skill of the two models, with an emphasis put on the relation between the rainfall and the zonal wind perturbations. This assessment addresses: (1) the planetary-scale extent of the events and the reproducibility of the perturbation patterns for a given season; (2) the size and amplitude of local precipitation and wind anomalies at basin-scale and; (3) the evolution of the vertical structure of the perturbations (U, T and RH). These are important parameters considering that, due to their non-linear behavior, intraseasonal perturbations impact the average dynamic of CGCMs. Over the Indian Ocean during boreal winter, observed and simulated low-level wind perturbations are generally consistent with a Gill-type response to basin-scale convective heating. The CNRM model gives a realistic baroclinic perturbations structure for wind, moisture and temperature, but with too large amplitude. This large amplitude is due in part to a zonally extended rainfall anomaly for the phase of maximum rainfall over the eastern Indian Ocean, and in part to burst-like rainfall perturbations south of the Maritime Continent in the following phases. The IPSL model gives a realistic response for low-level wind only; temperature and moisture perturbations are barotropic with a wrong warm anomaly at rainfall maximum. The differences between the two CGCMs are interpreted considering the characteristics of their convective schemes. The event-by-event approach presented here can be used to assess the general skill of the GCMs used in the framework of the Cindy-Dynamo programme.