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Here is how the IMD predicted ‘near normal’ monsoon rains for 2019

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15 Apr 2019

Southern India, now battered with the scorching heat, is awaiting the monsoon showers that bring some respite during the month of June. It looks like the wait would not be too long and well worth it. In a press release issued today, the Indian Meteorological Department (IMD) has announced that the southwest monsoon rainfall over the country, on the whole, is likely to be normal. It predicts that quantitatively, the rainfall is expected to be about 96% of the ‘Long Period Average (LPA)’ of 89 cm during June-September. This number is the average amount of rainfall received in the country between 1951-2000, and in 2019, this amount is not likely to deviate much from the mean.

“Overall, the country is expected to have well-distributed rainfall scenario during 2019 monsoon season, which will be beneficial to farmers in the country during the ensuing kharif season”, read a statement from the press release.

Each year, the Indian Meteorological Department issues two stages of forecasts for the southwestern monsoon rains—one in April and the other in June. But, how does it predict the intensity of these rains? Here lies an interesting bit of science.

Weather phenomena, like monsoon rains, are calculated using a set of algorithms and climate models, both analytical and numerical, that can predict the conditions of the future based on what conditions prevail in the present. Monsoon Mission, an initiative launched by the Ministry of Earth Sciences for this purpose, has two state-of-the-art dynamical prediction systems for short range to medium, extended range and seasonal forecasts. It was launched in 2017, and its predictions have since been used for many purposes. 

A variety of factors across the globe influence the rains in India. Meteorologists keep a track on five important parameters that can dictate the fate of India monsoon—the gradient in the sea surface temperatures between North Atlantic and North Pacific Oceans, the sea surface temperature over Equatorial Indian Ocean, sea-level pressure in East Asia, air temperature of the land surface in Northwest Europe, and the heat content over Equatorial Pacific measured by its warm water volume. 

In India, scientists have been using advanced climate models that can predict monsoon rains a season in advance. Studies have proposed using various indicators, such as surface pressure over the Arabian Sea, which can be included in such forecasting models, and ways to eliminate biases to make these predictions accurate.

For the current year, the global Climate Forecasting System, one of the forecasting systems used by the IMD, has predicted that the monsoon rainfall would be about 94% of the LPA. The second forecasting system, called the Statistical Ensemble Forecasting System, has predicted 96%. Monsoon rains are considered ‘Deficient’ if they are below 90% of the LPA,  ‘Below Normal’ if they are between 90%-96%, ‘Near Normal’ when they are between 96%-104% and ‘Above Normal’ when they are between 104-110 and ‘Excess’ for anything above this range.

Besides, weak El Niño conditions are currently prevailing in the Equatorial Pacific Ocean, and their intensity could reduce later in the season, the prediction notes. During an El Niño, the ocean temperatures rise in the central and eastern tropical Pacific Ocean, resulting in a change in the wind and rainfall patterns over the Pacific. In the Indian Ocean, the sea surface temperature oscillates aperiodically in a phenomenon called ‘Indian Ocean Dipole (IOD)’ or ‘Indian Niño’. When it is ‘positive’, the sea surface temperatures are high, resulting in good rains in the Western Indian Ocean and droughts in parts of Indonesia and Australia. If it is ‘negative’, the conditions are just the opposite. This year, however, the IOD is predicted to be ‘neutral’ now and could turn ‘positive’ during the monsoon season, which could bring good rains to India.