Prince K. Xavier and Jean-Philippe Duvel
Laboratoire de Météorologie Dynamique, Ecole Normale Superieure
24 Rue Lhomond, 75231 Paris, France
Francisco J. Doblas-Reyes
ECMWF, Shinfield Park
RG2 9AX, Reading, UK
The intraseasonal variability (ISV) associated with the Asian summer monsoon represented
in seven coupled general circulation models (CGCMs) as part of the European DEMETER
project is analyzed and evaluated against observations. The focus is on the spatial and seasonal
variations of 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) 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 the other. Most models simulate too weak sea surface temperature
(SST) perturbations and systematic phase quadrature between OLR, surface winds 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 dominant role of atmospheric models in fixing the nature of the intraseasonal
variability. It is, therefore, implied that improvements in the representation of ISV in coupled
models have to fundamentally arise from fixing problems in the large-scale organization of
convection in AGCMs.