KJ Ramesh, IMD, Rainfall,

K J Ramesh I Bettering Monsoon Predictions

By: Staff Reporter
K J Ramesh, the Director General of the India Meteorological Department, Government of India.

G’nY. Would you say that the accuracy of monsoon prediction has improved over the years? Monsoon rainfall predictions.

Yes indeed. We have been able to improve the monsoon rainfall predictions on all temporal and spatial scales as far as rainfall distribution over the last 5 years is concerned. Coupled ocean-atmospheric model is run on a 37 km grid scale for monthly and seasonal rainfall predictions. These dynamic model rainfall predictions are compared with the traditional statistical suite of models over India and over the four homogeneous regions of northwest, central, northeast and peninsular India. On a five year scale, the quantum of all-India long range rainfall predicted versus realised shows significant improvements during 2013-18, as against the 2008-13 period. Additionally, rainfall forecasts at a 12 km grid scale, which is generated for identifying zones of heavy rainfall across India for five days in advance, are disseminated daily. These predictions have helped the IMD to issue heavy rainfall warnings at district and sub-district levels. Moreover, a 3 km grid scale rainfall prediction assists cluster level understanding of heavy rainfall zones and intensities. These predictions are updated twice a day. This helps various sectors to plan and develop strategies related to water resources such as deciding the quantum of inflow into rivers/reservoirs and rainfed agricultural activities focused on pulses, cereals and oilseeds. Monsoon rainfall predictions.

G’nY. How powerful would be the impact of the El Niño this season?
As assessed in our first and second stage long range forecast for 2019, rainfall in India as a whole will be within the normal range of 96 per cent, with an error range of 4 per cent from June to September. El Niño is found to be persisting and confined to a neutral or weak warming (below 1oC warming) over the Niño 3.4 zone of the tropical equatorial Pacific till the end of May 2019. It is predicted to remain so till the end of September which is when the season will end. Accordingly, IMD has forecasted that India will have a favorable kharif season for the third consecutive year.

G’nY. Does the Indian Ocean Dipole impact the monsoon?
Indian Ocean Dipole (IOD) is associated with an above-normal rainfall in India. It is based on positive IOD which is characterised by an above-normal sea surface temperature anomaly. This above normal sea surface temperature over the western-equatorial Indian Ocean—off the Somalian Coast—is opposed to a below-normal sea surface temperature over the eastern-equatorial Indian Ocean—off the Indonesian coast. Most of the climate models have indeed predicted a positive IOD for this season. Along with a well distributed monsoon rainfall, it is also likely to offset the minor impacts of a weak El Niño over the Pacific.

G’nY. Is El Niño the biggest determinant of the amount of rainfall received during monsoon? What are the other chief determinants?
El Niño and IOD are the most significant drivers of monsoon variability in India. Other factors that impact rainfall include, sea surface temperatures in the north Atlantic and its gradient in the Pacific (December and February), sea surface temperature in the south equatorial Indian Ocean (February), East Asian mean sea level pressure (February and March), land surface air temperature in northwest Europe (January) and equatorial Pacific warm water volume (February and March). monsoon rainfall predictions

G’nY. Is there a relationship between Arctic melting and the monsoon?
Although, a direct relationship has not clearly emerged so far, it is largely believed that the induced changes in ocean current and circulation changes of heat transfer between tropics and poles get affected due to melting of the Arctic ice. This will certainly change the atmospheric circulation in the northern hemisphere. This can already be witnessed in the polar vortex extension to the south and the impact it has had in the USA—triggering severe weather phenomena. This modified circulation is expected to manifest itself during the western disturbance and in its interaction with the monsoon air mass. However, these processes are yet to be monitored and noticed. monsoon rainfall predictions

G’nY. What makes the northeast monsoon tougher to predict as compared to the southwest monsoon?
Spatial scale of the northeast monsoon is restricted to the southern most regions of India that include Tamil Nadu, parts of Rayalaseema, Andhra Pradesh, Kerala, south interior Karnataka. This is a much smaller area in relation to the extent of regions covered under southwest monsoon. As the uncertainty of prediction increases over smaller spatial domains, the error margin in variability ranges of prediction changes from 4-5 per cent to 16-17 per cent. Additionally, the range of uncertainty in the long period average values of seasonal rainfall increases exponentially for the northeast monsoon. However, rainfall predictions for up to 15 days in advance can be accurately prepared. IMD is confident that in a couple of years, this bottleneck can also be overcome.

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