In India, SW monsoon is the principal rainy season and it receives about 80 per cent of its total annual rainfall during the summer monsoon season, from June to September.

Indian agriculture is largely controlled by rainfall in this season. Small variations in the monsoon onset, in the spatio-temporal variability during the season and in the seasonal mean rainfall have a potential for significant economic and social impacts. Therefore, accurate forecasting of all India summer monsoon rainfall is beneficial to more than a billion people and has profound influence on agricultural planning and economic strategies of the country.

The year to year variation in the Indian Summer Monsoon Rainfall (ISMR) is primarily attributed to its association with the slowly varying boundary forcing such as sea surface temperature, snow cover, soil moisture etc. This is the predictable part of the inter-annual variability. The unpredictable part of the variability is due to the natural variability (internal dynamics) of the monsoon system.

Two main approaches are used for the long range forecasting (LRF) of the ISMR – based on empirical statistical method and on the dynamical method, which uses General Circulation Models (GCM) of the atmosphere and oceans to simulate the summer monsoon circulation and associated rainfall. The GCM simulation is primarily driven by the sea surface temperature (boundary) conditions provided in the models.

In spite of its inherent problems, at present, statistical models have shown better skill than the dynamical models for the seasonal forecasts of ISMR. The dynamical models have not shown the required skill to accurately simulate the salient features of the mean monsoon and its interring annual variability. Therefore, India Meteorological Department (IMD) currently uses statistical methods for issuing long range forecasts of monthly and seasonal monsoon rainfall over India.

The major thrust areas in R & D related to long range forecasting are -

• Development of a coupled ocean atmospheric model specifically for Indian conditions for seasonal prediction of monsoon rainfall.
• Development of high resolution Regional Climate Model for forecast over smaller regions like district, met subdivision, state etc.

The report notes that the present skills of dynamical as well as statistical models in predicting the monsoon rainfall have limitations. It agrees that, to improve the prediction skill of the models, more collaboration among operational long range forecasters of the South Asia among themselves and research institutes of the countries of the region are required. Active interaction with the concerned global and regional experts would be of the immense help. It strongly recommends the initiation of a capacity building/human resource development plan for the South Asian nations, particularly for seasonal prediction.

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