Nature, not man, has a greater impact on monsoon, says Study

In a development that may improve our understanding of Indian monsoon, an international team of researchers has found that natural variability, and not anthropogenic changes, has a bigger influence on the annual summer monsoon that accounts for 70 per cent of rains received in India.

The study by a team of researchers from China and the UK that appeared in Monday’s edition of Journal of Climate may help solve an enigma that has troubled scientists studying Indian monsoon for decades. Even though it was well articulated that global warming would bring more showers to India, the quantum of average annual rainfall during the Indian monsoon – particularly over north central India – witnessed a slight decrease between 1950 and 1999, when measured on a decadal scale. Subsequently, between 2000 and 2013, the monsoon ‘recovered’ to register a small increase in rains received.

Now, scientists seem to be able to explain this anomaly by separating the influence that global warming — such as increase in greenhouse gases and land use change — and natural variability have on the monsoon.

“Increase in greenhouse gas concentrations in the atmosphere generally tends to increase rainfall over India. Up to the year 2000, however, it appeared that the natural variability had been able to override this effect, resulting in the overall decrease,” said Xin Huang, a scientist with Institute of Atmospheric Physics of the Chinese Academy of Sciences and leader-author of the study. “In addition to anthropogenic climate change, rainfall changes in recent decades are also influenced by natural sea surface temperature oscillation over the Pacific basin,” Huang said in a statement.

The IPO and its phases

This prominent natural variability in Pacific sea surface temperature on decadal timescales is known as Interdecadal Pacific Oscillation (IPO). Positive IPO phases — a warmer than normal sea surface in the tropical central-eastern Pacific — are associated with poor monsoon, and cooler than normal conditions, the opposite.

While the negative to positive IPO transition observed between 1950-1999 pulled down rainfall over the north central India by 0.16 millimetre day per decade, externally forced rainfall over the region increased by 0.1 mm per day per decade. During 2000-2013, these factors — the IPO which shifted from positive to negative and externally forced rainfall trend — combined forces to make it wetter, by 0.68 mm per day per decade, the scientists argued.

“While we have to accept the internal variability of the Indian monsoon is high, it’s hard to know when the signal of external forcing will emerge from the din of internal variability. In that sense, the so-called ‘recovery’ period is too short and we need to watch it for a while longer,” said Raghu Murtugudde, an earth systems professor at the University of Maryland and currently a visiting professor at the Indian Institute of Technology, Bombay.

He also questioned the effort to put them in water-tight compartments and said how can the authors be sure that the transition of IPO itself is not affected by the external forcing.

Though this a good process study, it’s a long way before it can be used to improve projections, let alone predictions, Murtugudde said.

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