IMD Forecasts for Monsoon 2021

About India Meteorological Department

• India Meteorological Department was established in 1875. It is the National Meteorological Service of the country and the principal government agency in all matters relating to meteorology, seismology and allied subjects.
• To take meteorological observations and to provide current and forecast meteorological information for optimum operation of weather-sensitive activities like agriculture, irrigation, shipping, aviation, offshore oil explorations, etc.
• To warn against severe weather phenomena like tropical cyclones, norwesters, duststorms, heavy rains and snow, cold and heat waves, etc., which cause destruction of life and property.
• To provide meteorological statistics required for agriculture, water resource management, industries, oil exploration and other nation-building activities.
• To conduct and promote research in meteorology and allied disciplines.
• To detect and locate earthquakes and to evaluate seismicity in different parts of the country for development projects.’

Note: Long Period Average (LPA) -refers to the average monsoon rainfall from 1961-2010, which is 88 cm (880.6 mm to be precise). Until 2019, the LPA stood at 887.5 mm considering the average from 1951-2000. Monsoon rainfall between 96 and 104 per cent is considered as the normal monsoon. The current forecast of 100 per cent means a total of around 88 cm rainfall is likely during the monsoon months from June to September.

El Nino

• El Nino refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific. It is associated with high pressure in the western Pacific.
• El Nino adversely impacts the Indian monsoons and hence, agriculture in India.
• The cool surface water off the Peruvian coast goes warm because of El Nino. When the water is warm, the normal trade winds get lost or reverse their direction.
• Hence, the flow of moisture-laden winds is directed towards the coast of Peru from the western Pacific (the region near northern Australia and South East Asia).
• This causes heavy rains in Peru during the El Nino years robbing the Indian subcontinent of its normal monsoon rains.
• The larger the temperature and pressure difference, the larger the rainfall shortage in India.

La Nina

• La Nina means ‘little girl’ in Spanish and is also known as El Viejo or ‘cold event’.
• Here, the water temperature in the Eastern Pacific gets colder than normal.
• As a result of this, there is a strong high pressure over the eastern equatorial Pacific.
• Now, there is low pressure in the Western Pacific and off Asia.
• La Nina causes drought in Peru and Ecuador, heavy floods in Australia, high temperatures in Western Pacific, Indian Ocean, off the Somalian coast and good monsoon rains in India. A La Nina is actually beneficial for the Indian monsoon.
• Generally, El Nino and La Nina occur every 4 – 5 years. El Nino is more frequent than La Nina. Typically, the episodes last for nine to twelve months.

What is in the news?

• India is likely to receive “normal” monsoon rainfall this year, the India Meteorological Department (IMD) has said as part of its official April forecast. Except for parts of eastern and northeastern India, many parts of the country are expected to get “above normal” rainfall, the IMD’s models show.
• “Normal” rainfall refers to a range — 96%-104% of the Long Period Average (LPA) of 88 cm.

• Earlier this week, private weather forecasting company, Skymet Weather, too, said it expected India to get normal rainfall, but said this was likely to be 103% of the LPA, whereas the IMD, on Friday, has estimated it to be 98% of the LPA.
• The April forecast, which is based on an analysis of select meteorological factors in March, is updated in May, along with estimates of how the monsoon will perform in different geographical regions. In 2019, the IMD forecast 96% LPA in April, but India ended up with record excessive rainfall of 110%.
• In 2020, it said 100% LPA, but India wound up with 109%. IMD officials, however, said it was unlikely there would be such excessive rain this year.
• In 2019, the Indian Ocean Dipole (IOD) — defined as a swing in temperatures in the western and eastern sections of the Indian Ocean, where a positive phase usually corresponds to good rains over India — contributed to the excess rains.
• Last year it was La Nina — the converse of an El Nino and associated with a cooling of the equatorial central Pacific — again a feature correlated with heavy India rains.
• “This year, the models show a weak IOD and neutral conditions in the central Pacific. It’s unlikely there will be over 105% LPA this year,” D.S. Pai, a meteorologist with the IMD’s National Climate Centre, Pune, told The Hindu.
• Though the IMD now issues short-term and extended range forecasts — that is, an estimate of rainfall in time frames of three days to 15 days — for the first time this year, it will begin giving monthly forecasts for all months.
• It has so far refrained from giving a forecast for June and September, months that are known to be erratic as those are the months when the monsoon enters and exits the country, respectively, posing a challenge to meteorologists. Dr. Pai, however, said the IMD’s work for several years in honing the skills of a class of models, called dynamical models, has improved their forecasting abilities over three weeks.
• “So we can now extend it to monthly forecasts. The overall aim of these models is to be able to accommodate changes in the global weather that will influence the monsoon. This is more useful for planning,” he said.
• The IMD also said it was developing a separate forecast for the Monsoon Core Zone (MCZ), which represents most of the rain-fed agriculture region in the country.
• “A separate forecast for the MCZ will be more useful for agricultural planning and crop yield estimation, etc. In the second stage forecast in May, IMD will issue a separate probabilistic forecast for the MCZ, based on MME [Multi Model Ensemble] system and a new statistical model,” said a statement.
• Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology, Pune, tweeted that the reduction in rainfall in eastern India has been consistently decreasing. For an adequate monsoon, it is important for a temperature gradient to be present between the ocean and the land.
• In recent years, however, the India Ocean has been warming faster than the land surface, reducing this temperature differential and affecting the monsoonal flow.