Monsoon drop alarm

Rainfall has weakened over century: Study

By G.S. Mudur
  • Published 17.06.15
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Warm and dry

New Delhi, June 16: The summer monsoon weakened across large swathes of India over the past century, scientists said today, linking the reductions in rainfall to hitherto-unobserved trends that they say portend a dangerous drying of the Indian subcontinent.

Scientists at the Indian Institute of Tropical Meteorology, Pune, said they had identified a significant weakening trend in summer rainfall between 1901 and 2012 over central and northern India, the Ganga-Brahmaputra basins and the Himalayan foothills.

Their analysis suggests the Indian Ocean has been warming much faster than the Indian landmass, thus reducing the land-sea temperature contrast, a key factor that determines the strength of the summer monsoon that accounts for over 75 per cent of the country's rainfall.

"A reduced land-sea temperature contrast interferes with the main engine that drives the summer monsoon," Roxy Mathew Koll, a senior scientist at the Pune institute's Centre for Climate Research and the principal investigator of the study, told The Telegraph.

The study by Koll and his colleagues from institutions in India, France and the US was published today in the journal Nature Communications.

The scientists say they observed a 10 to 20 per cent reduction in summer rainfall over parts of Bihar, Chhattisgarh, Jharkhand and eastern Uttar Pradesh, and five to 10 per cent reductions over parts of Bengal and Madhya Pradesh.

Rainfall has increased over a small region north of the Western Ghats but, the scientists said, this positive trend is confined to regions along India's west coast.

The findings are surprising because several previous studies had predicted that India would receive more rainfall under the influence of global warming. But Koll and his colleagues say the rapid warming of the Indian Ocean without a similar warming of the landmass could be expected to weaken the monsoon.

"The Indian Ocean has warmed much faster than any other ocean," said Koll. The increase in the average sea surface temperatures of the Indian Ocean ranges from 0.5°C to 0.7° C but the Indian landmass has warmed by less than 0.5°C.

Some scientists had earlier speculated that aerosols - tiny particles such as dust or soot in the atmosphere - could suppress surface warming over the land but it is still unclear why the landmass has not warmed as fast as or faster than the Indian Ocean.

The researchers say the weakening trend of the monsoon is a source of "grave concern" because agriculture, water resources and power generation critically depend on the amount of summer rainfall in India.

Their study is part of the Union earth sciences ministry's National Monsoon Mission, a multi-institutional research effort to better understand the complex land, atmosphere and ocean mechanisms that drive the monsoon.

The Pune scientists say their analysis also suggests that trends in sea surface temperature changes in the equatorial Pacific Ocean could explain the rapid warming of the Indian Ocean.

Over the past several decades, meteorologists have documented an increase in the frequency and magnitude of El Nino - a slight warming of the Pacific sea surface temperature - relative to La Nina, a slight dip in the Pacific sea surface temperature in other years.

"El Nino throws its heat from the Pacific into the Indian Ocean through complex ocean-ocean dynamics," Koll said. El Nino has been associated in the past with poor monsoon rainfall over the Indian subcontinent.