Aerosol pollution offsetting expected enhancement of Indian summer monsoons: Study

New Delhi: Scientists say that the expected enhancement of the Indian Summer Monsoon (ISM) precipitation by heating effect caused by greenhouse gases (GHG) in the atmosphere has been offset by the anthropogenic aerosols, causing precipitation reductions post-1950.

The study by the researchers from the Indian Institute of Tropical Meteorology, Pune, also found that the forced weakening of the ISM was accompanied by a significant increase (13.5 per cent) in the forming of tropic cyclones over the Western North Pacific (WNP).

This accompaniment, the researchers said, is because the anthropogenically-forced, or human-induced, weakening of the ISM precipitation and circulation significantly altered the monsoon flow pattern over the near-equatorial and tropical Indo-Pacific Oceans in a manner so as to enhance the tropical cyclogenesis over WNP.

Their findings are published in the journal Frontiers in Earth Science.

Intergovernmental Panel on Climate Change (IPCC) has previously noted human-induced climate change to be a key driver of the observed changes in global land monsoon precipitation, since the second half of the 20th century.

The researchers observed the response of monsoon to human-induced climate change through simulations and found it to be characterised by a monsoon break-like situation involving a large-scale weakening of the ISM circulation, but an intensified transport of moisture towards the WNP by westerly wind anomalies from the tropical eastern Indian Ocean region.

Earlier climate model simulations had showed that the observed weakening trend of the ISM circulation and precipitation during the post-1950 is largely because of Northern Hemispheric (NH) anthropogenic aerosols, further supported by land-use land-cover changes and rapid warming of the equatorial Indian Ocean.

The researchers found it interesting that the pronounced weakening of the ISM circulation is associated with the formation of an equatorial shear zone and enhancement of precipitation over the equatorial eastern Indian Ocean, typically observed during 'breaks' in the Indian monsoon.

They said that the influence of anthropogenic GHG and aerosol forcing on the observed precipitation changes over the Asian monsoon region has been well-documented. They also said that there is evidence suggesting that the increasing burden of anthropogenic aerosol emissions from the NH has offset the monsoon precipitation enhancement to GHG forcing and has been a key driver of the observed decrease in summer monsoon rainfall over the Indian region since the mid-20th century.

The presence of the north-south asymmetric distribution of anthropogenic aerosol emissions results in an interhemispheric energy imbalance of the heat flows.

Previous studies have shown that aerosol-induced energy imbalance between the northern and southern hemispheres can cause shifts in the tropical rain belts and precipitation changes over the tropics and monsoon regions.

Basically, the researchers said, the intrusion of cold and dry westerly winds from the sub-tropics and mid-latitudes in the middle and upper troposphere, the lowest atmospheric layer, tends to suppress the monsoon convective activity, which is typically observed during 'breaks' in the Indian monsoon.

They said that this study has important key policy implications for near-term mitigation of the Asian monsoon hydrological response to climate change because near-term (2021–2040) monsoon precipitation response will be dominated by the effects of internal variability and future aerosol emission pathways.