New Delhi. Aug 25: The spells of weak rainfall during the monsoon season have lengthened since 1950, scientists at the Cochin University for Science and Technology (Cusat) said today.
The total annual monsoon rainfall over India has not changed over the past 100 years, the researchers said, but the shorter spells of active monsoon and longer spells of weak monsoon could harm agricultural output.
“Long spells of dry weather followed by short spells of intense rainfall might not change total rainfall, but could prove disastrous for crops,” Porathur Joseph, professor emeritus at Cusat’s Centre for Atmospheric Sciences, told The Telegraph.
The 122-day June-September wet season has always been marked by cycles of high rainfall and low rainfall ' the active and weak phases of the monsoon. The Cusat investigations ' based on an analysis of wind patterns and rainfall from 306 sites across India ' show that the number of days of weak monsoon is up by 45 per cent while that of active monsoon has fallen 77 per cent between 1950 and 2002.
“These are alarming findings for a country whose food production and economy depend heavily on the monsoon rainfall,” Joseph and his colleague Anu Simon said in a research paper published in the journal Current Science.
Senior meteorologists said it is unclear whether these observations point to real trends or natural fluctuations.
The monsoon depends on a wind called the low-level jetstream that flows eastward from the Arabian Sea over the Indian peninsula. “The low-level jetstream is the main artery that brings moisture from the oceans to the mainland for rain,” said Simon, a research associate at Cusat.
During active monsoon spells, the jetstream passes over peninsular India; but during the weak phases, it bypasses India and flows south across Sri Lanka and onward to the western Pacific. The jetstream flow over India during the wet season has shown a “significant decreasing trend” since 1950, the Cusat researchers said.
Meteorologists had earlier observed a 30-year cycle in the monsoon and some believe that the changes in wind and rainfall may be part of a natural fluctuation that has been seen for over a century.
Rainfall was poor between 1901 and 1930 but good from 1931 to 1960. Then, 1961-1990 witnessed weak rainfall with nine droughts. Under this cycle, India should have received good rainfall since 1990.
“Unfortunately, this has not happened,” said Dev Raj Sikka, head of a panel monitoring climate research.