A model of the coupled ocean–atmosphere system, the climate forecast system (CFS), from the National Centre for Environmental Prediction (NCEP), USA, has been ported onto the PARAM Padma parallel computing system at the Centre for Development of Advanced Computing (CDAC), Bangalore and retrospective predictions for the summer monsoon (June–September) season of 2009 have been generated, using five initial conditions for the atmosphere and one initial condition for the ocean for May 2009.

Whereas a large deficit in the Indian summer monsoon rainfall (ISMR; June–September) was experienced over the Indian region (with the all-India rainfall deficit by 22% of the average), the ensemble average prediction was for above-average rainfall during the summer monsoon. The retrospective predictions of ISMR with CFS from NCEP for 1981–2008 have been analysed.

The retrospective predictions from NCEP for the summer monsoon of 1994 and that from CDAC for 2009 have been compared with the simulations for each of the seasons with the standalone atmospheric component of the model, the global forecast system (GFS), and observations. It has been shown that the simulation with GFS for 2009 showed deficit rainfall as observed.

The large error in the prediction for the monsoon of 2009 can be attributed to a positive Indian Ocean Dipole event seen in the prediction from July onwards, which was not present in the observations. This suggests that the error could be reduced with improvement of the ocean model over the equatorial Indian Ocean.

The study concludes that -

• Of the two major modes involved in the interannual variation of the Indian monsoon, viz. ENSO and EQUINOO, there has been remarkable progress in the understanding of ENSO in the last three decades and reasonably accurate predictions about the transitions to El Niño and La Nina have been generated.
• However, further work is required to understand in depth the transitions to strong positive and negative phases of IOD and EQUINOO. Different hypotheses have been proposed for triggering of positive IOD events such as El Niño or severe cyclones over the Bay of Bengal in April/May.
• However, which mechanism operates in nature is not clear. Given the high propensity for the genesis of positive IOD events, the CFS model could prove to be an extremely useful tool in understanding the triggering of these events and hence lead to improvement in the model for more realistic predictions of the transitions of EQUINOO, IOD and hence the Indian monsoon.

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