India’s monsoon is getting more erratic by the year

A cloud burst in Leh; then in Uttarakhand; incessant rains in north Pakistan, and drought in the Gangetic plains — the monsoon had always been unpredictable, but never so much!

“It is raining where it should not, and not raining where it should,” says a meteorological department scientist at Nagpur, asking not to be named. “It could be a natural deviation, but the cloud burst events are unusual.”

Several parts — including western Rajasthan and Saurashtra — received record rainfall this year. And the Gangetic plains (Eastern UP, Bihar, Jharkhand and parts of West Bengal) are in drought. Even as the monsoon begins to recede, parts of India are facing excessive rains, parts drought, and a third has witnessed normal rains.

The Indian monsoon, a dynamic system, is perceptibly changing and, as several studies point out, behaving more and more erratically. Climate scientists are trying to answer how and why.
Met officials say it’s too early to comment if these deviations are linked to climate change, but numerous studies confirm a steady weakening of India’s summer monsoon.

What it means, is an open research question. But the Indian Met department director general Ajit Tyagi was quoted by agencies earlier this year as saying that “there is a structural shift in the pattern of the annual rainfall that could yet force a change in cropping patterns in the country.”

Studies by the Indian Institute of Tropical Meteorology, Pune, confirm that the southwest monsoon rain has decreased across the country by 4.7 per cent. The frequency and magnitude of extreme rainfall events (of greater than 10 cm/day) is increasing, while the frequency of moderate events is waning.

In their August 2009 report, the IITM researchers said they scanned the daily rainfall data of 165 stations across the region to ascertain their extreme point rainfall events (EPREs) — highest rainfall in 24 hours — and studied whether there was any change in the number and intensity of such events during the past four decades.

They discovered that the frequency of such events had gone up considerably after 1960, with an alarming rise in the intensity of the rains. They noticed that major cities, hill stations and islands were affected by heavy downpour.

Two monsoon systems prevail over the Indian subcontinent: the summer or southwest (SW) monsoon, and the winter or northeast (NE) monsoon (retreating southwest monsoon).

India gets rains in all the seasons due to both tropical and extra-tropical weather systems: monsoon low pressure areas, depressions, thunderstorms, tropical cyclones, and western disturbances.

The summer or the southwest monsoon season (June-September), the main rainy season, contributes to about 75-80 per cent of the total annual rainfall, with high average rainfall in the east.

The contribution from the winter (January-February), pre-monsoon (March-May) and the post or north-east monsoon (October-December) is not very significant, but important to particular regions. Any major structural shift in the pattern would impact the country’s economy that relies heavily on the agriculture output, which in turn depends on the monsoon.

Persistent weakening
Earlier this year, the earth sciences ministry launched a national mission on monsoon, to put in place a dynamic model framework to predict the monsoon. The mission aims to bring together all the relevant research and academic institutions in the country for improving the dynamical prediction of this complex system.

More than 40 scientists have been analysing the weather data since 1951 in an IMD study, whose outcome will lead to revision of the dates for the onset of the annual rainy season.

A team from Andhra University said last December in its research published in the Journal of Geophysical Research that the weakening of monsoon activity over India had persisted over three decades, unlike in the previous centuries when rainfall rose and fell between, and even within, decades.

The university scientists analysed the records of rainfall from 30 meteorological subdivisions during the four monsoon months and compared the trends over two time scales: from 1871-2005 and from 1970-2005, with the latter marking the “global warming era.” Unlike the late 19th century, when all the subdivisions recorded active rains, 19 out of the 30 subdivisions showed decreased rainfall in the later time scale.

The team detected a general trend in the shifting rainfall pattern. They found that the regions below 20 degrees north latitude (India lies between 8.7 and 37.6 degrees north) were experiencing lesser rainfall.

Another study released this year by Purdue University, US, indicated that the summer monsoon could significantly weaken by the end of this century.

Researchers from the Centre for Atmospheric Sciences at the Indian Institute of Technology (IIT), Delhi, also found a similar trend. In their analysis of the rainfall data for the period 1951-2004, they found that overall, long rainy spells (more than 2.5 mm of rain daily for more than four consecutive days) decreased over the last 50 years while, short and dry spells (less than 2.5 mm rain daily for a day or so) increased.

“Significant decrease in the number of long spell rain events and simultaneous increase in the short and dry spells over India suggest that monsoonal systems have weakened,” the scientists said in their paper published in the May 29, 2010 issue of the Journal of Geophysical Research.