Pollution, tropical cyclones affect rains in India

Pollution and the rise in tropical cyclones over the West Pacific could be a cause for the decline in rainfall and Indian monsoons over the past decades. These are the findings of a new study led by researchers at the Indian Institute of Tropical Meteorology (IITM) in Pune, India, as reported by The Weather Channel — India (WCI).

The WCI report says that the summer monsoon in India, which typically lasts between the months of June and September, is responsible for over 75% of the country’s annual rainfall. These rains are critical for filling our stomachs owing to their role in agriculture. Last year, the India Meteorological Department (IMD) changed the country’s long-period average (LPA) for monsoon rainfall to 87 cm. This shift did not seem like much, being only a centimetre less than the previous 1961-2010 LPA. But it did confirm the receding trend in rainfall, given the LPA for 1951-2000 was 89 cm. Further, the regional distribution of rainfall has witnessed a clear decrease since 1950 in different parts of the country.

Researchers of the study believe that greenhouse gas emissions and other human-induced activities can significantly alter monsoon flow patterns over the tropical Indo-Pacific Oceans and also lead to an increase in tropical cyclones over the West Pacific. And a rise in the frequency of tropical cyclones over the Pacific, in turn, can cause a weakening of the Indian monsoon since it is linked with moisture depletion over the Indian subcontinent.

As highlighted by the WCI report, the researchers at IITM carried out two modelling experiments as part of the study. In the first one, they conducted human-induced pollution simulation by incorporating aspects like greenhouse gases, aerosols and land-use changes and another that incorporated natural aspects. And just as they had anticipated, the simulation with human-induced pollution showed a connection with the declining monsoon signals in India. The mechanism behind this is that high levels of aerosols in the atmosphere can deflect the Sun’s heat back into space rather than warming the Earth’s surface. This leads to a decrease in temperatures and ultimately reduces rainfall.

As for the connection between the cyclonic activity in the Pacific and the Indian monsoon, the study pointed out how periods of monsoon ‘breaks’ over India coincided with the movement of the cyclones over to the north of a certain latitude range in the west-north Pacific. Citing the example of the record-breaking number of 18 such cyclones that occurred over the west-north Pacific during 2018, researchers said that that India experienced deficient monsoon rains that season.

This study thus shows that any changes in the Indian summer monsoon can influence the west-north Pacific cyclone activity and vice versa, and highlights how human-induced pollution is changing our climate patterns.

Another IITM study has found that rising marine heatwaves in the Indian Ocean have impacted have impacted rainfall in the Indian monsoon, according to a Times of India report. The study published in the journal JGR Oceans, found a significant increase in marine heatwaves, aided by rapid warming in the Indian Ocean and strong El Ninos. Researchers said that marine heatwaves used to be rare in the tropical Indian Ocean, but now have become an annual occurrence. The experts also said that said marine heatwaves in the western Indian Ocean and the Bay of Bengal have resulted in drying conditions over the central Indian subcontinent. At the same time, there is a significant increase in the rainfall over south peninsular India in response to the heatwaves in the north Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the heatwaves. The researchers also said it is the first time that a study has demonstrated a close link between marine heatwaves and atmospheric circulation and rainfall.

An earlier WCI report says that in its long-range forecast released in April, the IMD has predicted the southwest monsoon rains to be ‘normal’. The seasonal precipitation during the four-month period between June to September is expected to be 96% (with a model error of ± 5%) of the long-period average, which amounts to 83.5 cm. The countrywide seasonal average, calculated for the period 1971-2020, is 87 cm. Interestingly, this is the fifth straight year of normal or above-normal rainfall predictions for the monsoon season. This is good news for India, as normal rain is crucial for sufficient agricultural yield and the replenishing of reservoirs critical for drinking water and power generation across the country.