The Earth Institute's International Research Institute for Climate and Society (IRI) presents a seminar, "Challenge of Predicting the Indian Summer Monsoon Rainfall," with Sulochana Gadgil, Indian Institute of Science, Bangalore, India.
The problem of simulating and predicting the inter-annual variation of the Indian Summer Monsoon Rainfall (ISMR) remains a challenge. During June-August 2009, the deficit in the observed rainfall over the Indian region was a massive 36%. However, the predictions generated with almost all the atmospheric and coupled models, were not even for deficit rainfall (leave alone a severe drought!), but rather for above average rainfall over the Indian region. In fact, deficit rainfall was also not predicted during the drought of 2004 and the severe drought of 2002. While the poor skill has been attributed to model deficiencies by some, another school attributes it to the special nature of the SST-rainfall relationship over South China Sea, Bay of Bengal and tropical West Pacific. It is suggested that atmospheric GCMs have a fundamental problem in simulating this relationship and hence in simulating the inter-annual variation of the Indian /Asian monsoon. Thus, according to this school, coupled models have to be used for simulation/prediction of the monsoon since the two-tier approach is not appropriate. After considering the observed SST-Convection/rainfall relationship, I discuss its simulation by atmospheric and coupled GCMs of IPCC-AR4 and suggest that AGCMs do not have a fundamental problem in simulating it.
Why then is the skill of simulation and prediction of ISMR is so poor? In order to gain an insight into this, I discuss briefly what we understand about the interannual variation of ISMR, the role of ENSO, EQUINOO. We find that AGCMs are better able to simulate the extremes linked to ENSO than those in which EQUINOO plays an important role. Finally I present an analysis of the retrospective forecasts by CFS for 1980-2009 and suggest why that the prediction for 2009 was wrong, and the way forward.