New research by UAE-based scientists has found that temperatures in the Atlantic are having a growing effect on the Indian monsoon, something that could prove crucial in predicting how severe rains are likely to be.
The New York University Abu Dhabi study comes as climate change alters the patterns of monsoon rainfall in India, with likely far-reaching effects on agriculture, the economy and society.
In the latest study, three NYU Abu Dhabi researchers and a colleague at the University of Wisconsin-Madison in the United States looked at the relationship between sea-surface temperature variability in the Atlantic and variability in the Indian summer monsoon.
Colder sea-surface temperatures in the eastern tropical Atlantic, linked to a weather system called the Atlantic Zonal Mode (AZM), tend to be associated with stronger monsoon rains.
The colder temperatures over the Atlantic strengthen the waves into the Indian Ocean, which increases the difference in upper atmospheric temperature between the Indian Ocean and the Indian continent. This causes more powerful moisture-laden winds to blow onto the land, leading to greater monsoon rains.
“Similarly, with strong warming [over the Atlantic], it weakens the waves and decreases the temperature gradient, which reduces the rainfall,” said Dr Thelliyil Sabeerali, an Indian postdoctoral associate in NYU Abu Dhabi’s Centre for Prototype Climate Modelling (CPCM) and the study’s first author.
This pattern was already known, but the NYU Abu Dhabi team were interested in how this relationship is changing over time as the Earth’s atmosphere warms.
They looked at the relationship between the AZM and the Indian monsoon throughout the 20th century, comparing statistics from 1901 to 1938 with results from 1939 to 1974 and from 1975 to 2000. Data analysis showed that the AZM’s effect on the Indian monsoon is growing.
“In earlier decades it was weak, but now it’s become stronger,” said Dr Ajaya Ravindran, an Indian researcher who works as a senior scientist at the CPCM and who is also an author of the new study.
The new findings suggest that the effect of the AZM should be factored into models that predict the scale of the annual monsoon.
Such forecasts are important because, as a previous study put it, annual variations in the strength of the monsoon have “huge socioeconomic impacts”.
“There’s more intense warming and more intense cooling in the Atlantic Ocean in recent decades. It produces very strong waves into the Indian Ocean,” said Dr Sabeerali.
Because multiple factors affect the strength of the monsoon, the impact of the AZM is, however, not thought in itself to be causing more extreme monsoon rains.
About 600 million people live in northern parts of India where the way of life depends upon the arrival of monsoon rains between July and September. They are particularly crucial to agriculture, which continues to employ half of India’s population.
More extreme monsoons are being seen as climate change exerts its effects and, as reported in The National last year, Dr Ravindran and his colleagues have found that climate change is likely to lead to a northward shift in the low pressure systems that create monsoons rains.
This, in turn, means that some areas will likely experience a reduction in the rainfall that they receive, which could impact agriculture and economic activity.
The dramatic effect on the way of life in an area if it loses its monsoon rains is demonstrated by a look back through the history of Arabia.
Periodically the Arabian peninsula was watered by monsoon rains, and when this happened, areas were green and lush where today they might be sandy and devoid of much vegetation. A study by Oxford University researchers looking at south-east Arabia found that the last wet period was about 55,000 years ago.
The new paper, entitled Atlantic Zonal Mode: An Emerging Source of Indian Summer Monsoon Variability in a Warming World, appears in Geophysical Research Letters, published by the American Geophysical Union.