Grantham PhD student Dilshad Shawki explores the latest research unpicking the influence of human activity across the globe on the South Asian monsoon.
Each summer the South Asian monsoon drenches the Indian subcontinent, as strong moisture-laden winds from the Indian Ocean deliver over 70% of the region’s annual rainfall in just 3 months. As such, the monsoon’s bountiful rain is crucial to the economy and to livelihoods in the region.
In recent decades however, rising pollution levels and increases in global surface temperatures have influenced atmospheric circulation patterns in the tropics, in turn affecting monsoon rainfall patterns. The challenge for scientists, including myself, is to gain a better appreciation of these relationships in order to build more accurate forecasts. Understanding and predicting monsoon rainfall is of huge importance to those societies that have developed following its rhythms.
Sharing the latest knowledge in this field was the ambition of the second Local and Remote Rainfall Influences (LORRI) workshop, held at the Indian Institute of Science (IISc) in Bangalore, India last month. The workshop brought together leading scientists from across India and the UK to discuss new findings and common goals.
How does air pollution affect the South Asian monsoon?
Aerosols are liquid and solid particles suspended in the atmosphere that have the ability to reflect or absorb solar radiation. They also influence clouds by making them brighter and even supressing rainfall. Aerosols can also occur naturally from sea salt spray and dust plumes driven by winds in desert regions, but synthetic aerosols make up a major part of global air pollution. Whether manufactured or natural, aerosols remain in the atmosphere for days to weeks, which makes their direct influence on the climate more localised. Interestingly though, local differences in temperature and pressure can influence circulation – and hence the climate – in areas far away from the emission source.
The South Asian monsoon’s interaction with aerosols has been studied extensively in recent years, with many researchers concluding that manufactured aerosols may be responsible for weakening the circulation of monsoon winds and precipitation in recent decades. These changes matter to people on the ground. In the second half of the twentieth century, drier conditions in central India have led to more frequent and intense droughts, and a devastating effect on crop yields.
A global system
While South Asia creates its own share of air pollution, aerosols from regions thousands of kilometres away also indirectly influence local conditions. My own research explores how the South Asian monsoon changes in response to emissions in regions such as the United States, Europe and China.
Aerosols of differing kinds from a variety of distant regions can have a strong effect on rainfall in India, and on dust kicked up into the atmosphere around South Asia. To explain, these effects occur as air moves between the zones of warmer and cooler temperature that are thought to be indirectly caused by different aerosols. Having short lifetimes in the atmosphere, aerosols tend to be unevenly distributed and are mostly concentrated around their source region. As they scatter and reflect solar radiation, the different aerosols create localised cooling effects and can set up temperature contrasts that influence the circulation of air.
During the workshop, I was impressed to see colleagues present climate models able to simulate the effect of localised cooling across large distances. This means that the models can establish links between changes in the monsoon and aerosols thousands of miles away all via the changes in circulation.
However, aerosols’ influence on the monsoon is also seen in other ways. Our discussions at the workshop raised the spectres of aerosol-induced shifts in rainfall during the course of a day, and fluctuations in patterns of rainfall within the season. We also saw increasing evidence that aerosols affect the vertical structure of clouds and their microphysical properties, thereby leading to further changes in climate, atmospheric stability and rainfall over the Indian region.
A cautious approach must be taken when studying the interactions between pollutants and the monsoon since much work is still needed to validate the complex and interconnected processes. Deciphering the root causes of monsoon changes in the past and the various scenarios of monsoon change in the future will be an ongoing challenge for climate researchers in this field.
With emails exchanged and plans for further collaboration in unpicking the myriad processes that influence the monsoon, the LORRI workshop came to a close after two days of fruitful talks and discussions. This UKIERI funded project has now come to a close, but the ambition to shed light on the interplay between pollution and South Asia’s life-giving monsoon continues.
One thing that can be said for sure is that, from this project as well as others, we progressively have increasing evidence that our activities can influence the lives of people and the health of ecosystems very far from ourselves. I’d personally like to see our understanding of these effects grow in order to inform policy regarding a country’s contribution to changes in the climate beyond their own borders.
Written in collaboration with Apostolos Voulgarakis.
Find out more
For more information on the workshop, please contact the organisers:
- J. Srinivasan (Chairman of the Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India)
- Apostolos Voulgarakis (Lecturer in the Space and Atmospheric Physics Group, Imperial College London, U.K.)
- Arindam Chakraborty (Lecturer at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India)
The LORRI project is funded by UKIERI.