Background
During the four months of southwest monsoon season, June to September, India receives on an average about 88 cm of rainfall, which amounts to 80% of its annual rainfall. The monsoon seasonal rainfall for the country as a whole is defined as normal when it is within +/- 10% of its long period average. This definition of normal rainfall is based on the nature of the statistical variation of monsoon rainfall from year to year.
The India Meteorological Department (IMD) has been issuing every year the long range forecast for monsoon rainfall on the basis of the statistical correlation between the rainfall and certain preceding meteorological conditions, both within the country and over distant regions. Beginning with the monsoon season of 1988, a new long range forecast model, termed as 'Parametric and Power Regression Model', was developed by IMD scientists led by Dr. V.R. Gowariker, then Secretary, Department of Science and Technology. This model is being used operationally by IMD since then. The model uses 16 regional and global land-ocean-atmospheric parameters which are physically related to the Indian monsoon rainfall. Each model parameter is defined in terms of observations over a specific location and period, which in some cases extends upto the end of May. Thus IMD's monsoon forecast can be generated only by May end.
The long range monsoon rainfall forecasts issued by IMD since 1988 with the help of this model have proved to be reasonably accurate. The model error is about +/- 4%. However, it will have to be borne in mind that in view of the statistical nature of the model, the predicted value could, on some occassions, have a wider deviation compared to the model error.
Forecast for 1997 Monsoon
In 1997, 9 out of the 16 model parameters have been found to be favourable and considering similar situations in the past, there is a very good probability of a normal monsoon this year.
On the basis of detailed analysis of the parameters, both individually and collectively, and computation with the help of the Parameteric and Power Regression Model, IMD has arrived at the following conclusions about the 1997 southwest monsoon rainfall and these constitute IMD's official forecast :
a) In 1997, the southwest monsoon rainfall over the country is likely to be normal, the normal being defined as rainfall being within +/- 10% of its long period value, thus making the year 1997 the tenth normal monsoon year in succession.
b) Quantitatively, the rainfall for the country as a whole for the entire monsoon season (June to September) is likely to be about 92% of its long period average value within an estimated model error limit of +/- 4%. Thus it will be on the negatiave side of the long period average.
It may be mentioned that it is not unusual for the monsoon rainfall to be normal every year over long periods. For example, the monsoon was normal evaery year during 1921-40 (20 years) and 1952-64 (13 years).
The monsoon rainfall over India is the result of a complex interaction of various land-ocean-atmospheric parameters, not only antecedent to the monsoon season but also during the monsoon season itself. The El Nino/Southern Oscillation (ENSO) paramters are known to exert a strong concurrent influence. These parameters, which have been near-normal since 1995, are exhibiting an unfavourable trend this year. They need to be carefully monitored in view of their likely adverse impact on the monsoon rainfall.
Onset of Monsoon
The southwest monsoon set in over south Andaman Sea on 16 May 1997 which is around its normal date. Its onset over Kerala is likely to be around its normal date of 1 June 1997 as per present indications. It may, however, be mentioned that the date of onset of the monsoon does not significantly influence the seasonal rainfall performance.
This information was produced by the Indian Meteorological Department and more details can be found by contacting directly:
The Director General
India Meteorological Department
Mausam Bhavan
Lodi Road
New Delhi 110 003
India