Intraseasonal variability of Asian summer monsoon in coupled seasonal hindcasts

The intraseasonal variability (ISV) associated with the Asian summer monsoon represented in seven coupled general circulation models (CGCMs) as part of the DEMETER project is analysed and evaluated against observations. The focus is on spatial and seasonal variations associated with summer monsoon ISV of outgoing long wave radiation (OLR). The large-scale organization of convection, the propagation characteristics and the air-sea coupling related to the monsoon ISV are also evaluated. A multi-variate Local Mode Analysis (LMA) that enables a detailed evaluation of individual intraseasonal perturbations reveals that most models produce less organized convection and ISV events of shorter duration than the observed. Compared to the real atmosphere, these simulated patterns of perturbations are poorly reproducible from one event to another. Most models simulate too weak SST perturbations and systematic phase quadrature between OLR and SST, indicative of a slab-ocean-like response of the SST to surface flux perturbations. The relatively coarse vertical resolution of the different ocean GCMs (OGCMs) limits their ability to represent intraseasonal processes, such as warm layer formation, which are important for realistic simulation of the SST perturbations at intraseasonal time-scales. Models with the same atmospheric GCM (AGCM) and different OGCMs tend to have similar biases of the simulated ISV indicating the important role of atmospheric models in fixing the nature of the intraseasonal variability. It is implied that improvements in the representation of ISV in coupled models have to fundamentally arise from fixing problems in the organization of convection in AGCMs. Results on the extended range predictability in these hindcasts in relation to the initial organization and phase of the convective perturbation will also be presented with focus on the regional characteristics of summer rainfall variability over southern Asia.