The Indian Monsoon

Monsoon is a season of joy for many because monsoons provide relief from the sultry hot summer and lead the way for greenery all around us. Monsoons are very important for farmers as well as this is the season which determines how successful a crop will be. Let’s find out more about the monsoons in India.

The Indian Monsoon is perhaps the biggest phenomenon in the Indian sphere of life; discussions on the event are perhaps rivalled by discussions on Indian Politics only. Besides its vital impact on the country’s agricultural productivity; it also has immense influence on trade thus affecting the economy, not to mention the influence on direct and indirect livelihood of more than one billion people.

Quite simply, it is nothing but a sea breeze albeit on a very large scale. The word may have originated from Hindi mausam (weather) or from Arabic mawsam (season). The importance of the monsoon in the context of the Indian economy is colossal.

Cause of the Monsoon-

Monsoon is triggered by the heating of the land; the earth is tilted in relation to the sun. The sun’s rays heat up the air and the cooler air over water bodies rushes over. So Monsoon is generated due to the difference in the temperatures between Land and Seas. Since the land is much more in the northern hemisphere, the Monsoon is more prevalent here. Remember that due to the earth’s tilt, the sun’s rays will be more direct over the northern hemisphere in summers. During the winter, a winter monsoon occurs as a result of the opposite phenomenon (in Australia, Sri Lanka and East Indian coast).

 Present Forecasting-

The IMD is the supreme organization involved in monsoon forecasting for the country along with other supporting organizations including the Indian Institute of Tropical Meteorology (IITM- Pune), Indian Institute of Science (IISc), Bangalore; Space Applications Centre (SAC- Ahmadabad), National Aerospace Laboratories (NAL- Bangalore), Centre for Mathematical Modelling and Computer Simulation (CMMACS- Bangalore), and the National Centre for Medium Range Weather Forecasting (NCMRWF- Noida). IMD issues forecasts for the whole country and separate forecast for the four geographical regions of India.

 

Sr. No	Geographical Region	Including States
1	Northest India	Jammu and Kashmir, Himachal Pradesh, Punjab, Rajasthan, Haryana, Chandigarh, Delhi, Uttarakhand and Uttar Pradesh.
2	Northeast India	Arunachal Pradesh, Meghalaya, Assam, Nagaland, Manipur, Mizoram, Tripura, Sikkim, West Bengal, Bihar and Jharkhand.
3	Central India	Gujarat State, Madhya Pradesh, Chhattisgarh, Maharashtra, Goa and Orissa.
4	South Peninsula	Andhra Pradesh, Karnataka, Tamil Nadu, Kerala, Lakshadweep and Andaman and Nicobar Islands.

Since 2007, IMD is using the new ensemble technique which uses eight Predictors for forecast. The error ranges from 5% to 4% for the two forecasts. South-West monsoon rainfall is forecasted in two stages.  The first stage ‘forecast for the seasonal rainfall (June to September)’ over the country as a whole is issued in April.  Second stage is ‘update of the April forecast’ is issued in June. For example in 2011, forecast for the seasonal rainfall (June 2011 to September 2011) is already issued in the month of April 2011 and the ‘update of the April 2011 forecast’ is issued in June 2011. Forecast for seasonal rainfall over four broad geographical regions of India (mentioned above) and July rainfall over country as a whole are also issued with the ‘update of the April forecast’ in June.

Following table shows eight Predictors for forecast of the Indian south west monsoon-

Sr. No	New Predictors	Used for forecasts in
1	North Atlantic Sea Surface Temperature (December +January)	April and June
2	Equatorial SE Indian Ocean Sea Surface Temperature (February + March)	April and June
3	East Asia Mean Sea Level Pressure(February + March)	April and June
4	NW Europe Land Surface Air Temperatures (January)	April
5	Equatorial Pacific Warm Water Volume(February +March)	April
6	Central Pacific (Nino 3.4) Sea Surface Temperature Tendency(Mar+Apr+May) – (Dec+Jan+Feb)	June
7	North Atlantic Mean Sea Level Pressure(May)	June
8	North Central Pacific wind at 1.5 Km above sea level (May)	June

 

• For April forecast, the first 5 predictors given in this table are used.
• For the Update of the April 2011 forecast in June, 6 predictors that include 3 predictors (first 3 predictors in this table) are used.

The prediction models being used by India are statistical ones meaning they try and estimate the amount of rainfall based on historic estimates. It basically uses a determined atmospheric variable data set subjected to statistical correlation to the total rainfall the country will receive in the Monsoon season (June- September). (Please visit IMD web site for further details)

Forecast for the 2011 South-west monsoon rainfall-

April 2011: Rainfall for the country as a whole is most likely to be Normal (96-104% of Long Period Average (LPA)). There is very low probability for season rainfall to be deficient (below 90% of LPA) or excess (above 110% of LPA). Quantitatively, monsoon season rainfall is likely to be 98% of the LPA with a model error of ± 5%. The LPA of the season rainfall over the country as a whole for the period 1951-2000 is 89 cm.

No update of the June 2011 forecast was issued at the time of this post.

Ministry of Earth Sciences is planning an Indian Mission on Monsoon based on a dynamic model. At present, the IMD (Indian Meteorological Department) does use open source dynamic models which are the GFS (Global Forecasting System) and WFS (Weather Research and Forecasting System) of the USA.

The Indian monsoon shows variability and thus there is a need to constantly weed out existing models and develop new models. The ensemble model can be useful as it uses data from forecast available from different models to develop the final forecast. A dynamic prediction system has been implemented under collaboration between IMD and IISC (Indian Institute of Science) to bring more advancement in the forecasting system.

Normal rainfall maps of India: (Click on the map to enlarge)

 

 

(Source: IMD)

Article by

Puskar Pande, Shailesh Telang