IMD’s Monsoon Forecast

The India Meteorological Department’s (IMD) forecast for this monsoon estimates 106 per cent of normal rainfall this season. After two successive years of drought, this seems to be an encouraging prediction to the Indian economy.

What exactly does the IMD’s monsoon forecast mean?

IMD has predicted India is expected to receive 106% of the Long Period Average rainfall this monsoon. Averages of rainfall received between 1951 and 2001 (50 years) are termed as the Long Period Average or LPA and are considered as normal. This is computed to be 89cm. So, accordingly the present forecast of 106% of the Long Period Average means 106% of the normal 89 cm of rain, which is 94cm of rain.

Annually, India receives about 116cm of rainfall, out of which 89cm is received during the 4 month monsoon season (June-September).

IMD’s classification of rainfall

• Rainfall between 96% and 104% of the LPA is classified as ‘normal’.
• Rainfall between 104% and 110% of the LPA is classified as ‘above normal’.
• Rainfall above 110% of the LPA is classified as ‘excess’.
• Rainfall between 90% and 96% of the LPA is classified as ‘below normal’.
• Rainfall below 90% of the LPA is categorized as ‘deficient’.

So based on the above categorization, rainfall forecast for this year’s monsoon falls on the ‘above normal’ category. For the past two years, India experienced ‘deficient’ rainfall.

Will all parts of the country receive normal rainfall during a normal monsoon year?

In a normal monsoon year, not all parts of the country get normal rainfall. Rainfall is not evenly distributed across the country. In fact, even during a year of excess rainfall some areas may experience drought. Similarly, some areas may experience very good rainfall despite an overall bad monsoon.

The calculated 89cm of rainfall in a normal monsoon season is called as the ‘area weighted average’. It simply means the amount of rainfall collected throughout the country during the 4 month monsoon period would fill a tank of 89cm height spread over the entire area of the country.

Is monsoon a stable system?

The monsoon is a stable system and about 70 per cent of times the ‘normal’ monsoon rainfall is experienced. And even in extreme drought years, rainfall never went below 70 per cent of ‘normal’. For example, in1918 which is often labeled as the worst drought of the last century in terms of its geographical spread, only 26% rainfall deficiency was recorded. Similarly, in 2009 the country experienced 22% below normal rainfall. Last year, the country faced 14% deficiency and in the previous year 12% deficiency.

Why the IMD did not come with the rainfall forecast for smaller areas like districts at present?

The presently issued forecast is the long range forecast. This is done for a prolonged period of time over a large geographical area. For smaller areas the forecast cannot be done two months in advance. Forecasts can only be done for shorter periods of time. It can be done only a few days before the start of the monsoon season (late May or early June). In that time, IMD releases its monthly predictions for every area.

What is the basis for IMD’s forecasts?

The monsoon gets affected by the interactions of a variety of land and ocean phenomena. During the late 19th century, IMD predicted monsoon based only on an assessment of the amount of snow cover over the Eurasian region with lesser snow cover meaning a better monsoon. In 1980s, the IMD used 16 predictors to make its forecast. Later it was realized that 16 predictors was too many. The models were refined and for the April forecast the IMD has used five predictors. It will rely on two more predictors for its second-stage June forecast. Those 5 predictors are-temperature difference between surface temperatures in the North Pacific and North Atlantic oceans; surface temperature of the Indian Ocean near the equator; mean sea level pressure in the Pacific Ocean near East Asia; land temperatures over northwest Europe; and the volume of warm water in the equatorial Pacific Ocean.

At present, the IMD uses statistical models to arrive at its forecast. ‘Statistical models’ match the prevailing conditions with historical records of those years when similar conditions had prevailed, to know how the monsoon had behaved during those years. With this, it tries to arrive at its forecast.

Another model with which the IMD has been experimenting is the ‘dynamical’ model and it is mainly used at present for research purpose. This model makes continuous observation of some selected physical phenomena and notes the behavior of the monsoon over a period of time. Then, following those changes it extrapolates its predictions for the future and arrives at a forecast.

What is the Monsoon Mission of India?

The Monsoon Mission of India comes under the aegis of the Ministry of Earth Sciences. It is a well known fact that Monsoon impacts Indian economy. But the present prediction capabilities are not adequate to forecast seasonal and intra-seasonal Monsoon patterns. The mission aims to improve the monsoon prediction by supporting focused research carried out by national and international research groups for generating dynamical forecasts and improving skill of forecasts.

The following are the objectives of this mission:

• To improve Seasonal and Intra-seasonal Monsoon Forecast.
• To improve Medium Range Forecast.

The mission participating institutions are:

• Indian Institute of Tropical Meteorology, Pune.
• National Centre for Medium Range Weather Forecast, Noida.
• Indian Meteorological Department, New Delhi.

Functions of IMD

IMD was established in the year 1875 as a principal government agency in all matters relating to meteorology, seismology and allied subjects.

Its mandate includes:

• To provide current and forecast meteorological information for weather sensitive activities like agriculture, irrigation, shipping, aviation, offshore oil explorations etc,
• To issue warnings against severe weather phenomena which cause destruction to life and property such as tropical cyclones, norewesters, duststorms, heavy rains and snow, cold and heat waves, etc,
• To locate and detect earthquakes and evaluate seismicity in various parts of the country for development projects,
• To provide meteorological statistics essential for water resource management, industries, agriculture, oil exploration and other nation building activities,
• To conduct and promote research in meteorology and other allied sectors.

El-Nino and its impact on Indian Monsoon

El-Nino is the global weather phenomenon characterized by abnormal and prolonged warming of the sea surface temperatures in the Pacific Ocean and is known to cause floods and famines across the world. This phenomenon usually lasts for 9 months to 2 years and occurs every few years. It weakens the trade winds which normally blow from South America towards Asia and causes changes in the Southern Oscillations and alters the rainfall patterns across the world.

El-Nino phenomenon is known to affect the Indian monsoon. There tends to be a reduction in the rainfall across the Indian subcontinent during an El-Nino year. However, not every El-Nino brings deficient monsoon. In the last 140 years, El-Nino Southern Oscillation events has resulted in over half of the major droughts in India. Recently, three of the four El-Nino years (2002, 2004, 2006 and 2009) resulted in drought in India. Out of the four years, El-Nino year 2006 was not a drought year in India.