Three Indian Institute of Technology (IITs) — Chennai, Bombay and Mandi — have data mined over 100 years of the Indian monsoon, to find some interesting observations that can have impact on agriculture and water resources.

India’s Southwest monsoon rainfall exhibits a periodicity of 2.85 years, during which it tends to switch between ‘strong and weak monsoon years’. This 2.85 year period is termed as the ‘triennial oscillation’.

In determining the quantum of rainfall that occurs during the monsoon period between June-September 30, the triennial oscillation along with the global climate phenomena such as El Nio Southern Oscillation (ENSO) that recurs in a 3-5 year period play crucial role, according to Sarita Azad of the IIT, Mandi.

Algorithms for effective prediction

Studies have shown that the periodicity of the ENSO is reducing, most likely due to global warming, and this would have a direct impact on the ‘strong-weak’ periodicity of the Indian Summer Monsoon (ISM).  

“Understanding the relationship between triennial oscillation, its spatial distribution, and how it is likely to change in future is important for reliable monsoon prediction,” says the IIT researcher whose team has developed an algorithm to process 100 years of data of the ISM Rainfall.

The algorithms analyse the changes in the periodicity of the monsoon. It also predicts the decreasing intensity of rainfall in most parts of the country, which covers 41 per cent area.

According to Pravat Jena, the lead researcher, the team examined the spatial distribution of the triennial oscillations using rainfall data of 1,260 months between1901–2005. They analysed the power spectrum of the observed data and showed that the 2.85-year periodicity was present at 95 per cent confidence level over more than half of the 354 grids across the country.

While the monsoon itself is a stable phenomenon, arriving almost like clockwork every year in the first week of June in Kerala, the short term fluctuations in the annual rainfall are unpredictable and pose a greater challenge to meteorologists. “Despite complexity, monsoon rainfall seems to show a well defined pattern,” said Pravat.

Impact on agriculture

“A weakened triennial monsoon cycle will have a severe impact on agriculture and water resource management, particularly over the southwest-coastal, northern and northeast-central parts of India,” he said, explaining the significance of the findings.

They would be useful for policy makers, farmers, water managers etc. especially in the predominantly agricultural situation. Weak and strong monsoons substantially alter agricultural output, and thus the economy, and it is important to understand the patterns and variations for better planning and management, the researchers said. 

Their work has recently been published in the reputed American Geophysical Union (AGU) peer-review International journal Earth and Space Science . 

The research team has projected the data into a collaborative framework-based simulation called the Coupled Model Inter comparison Project (CMIP) to ascertain the future pattern of the 2.85-year period oscillation. The projections showed that there is a weakening of this oscillation by the year 2100.

Rainfall pattern

In another recent study on monsoons, researches from the IIT Chennai and IIT Bombay who also analysed rainfall pattern data of the last 100 years found some interesting changes.

“Our study shows that the amount of rains has decreased in river basins with surplus water, and has increased in basins with deficit water,” said Sachin Gunthe of the IIT Madras.

Similarly, the study made a significant observation that contradicted the traditional notion of dry areas becoming drier and wet ones becoming wetter in response to climate change.

The geographic variations of extremes in rainfall occurs due to convection: the movement of moisture laden hot air upwards, followed by cooling at higher altitudes and shedding of the moisture as rain.

Convection-based rains would mean that regions where there is excess moisture in the air should experience more rainfall. This, however was not seen in the rainfall pattern in the analysis, according to Subimal Ghosh of IIT Bombay, the first author.

Recent observations, both at the meteorological level and from local perceptions, that monsoon has grown more unpredictable than before, bodes ill for a country where the economy is critically linked to seasonal rains. Extreme events such as the floods in Kerala and the ongoing zero-water situation in the adjoining Tamil Nadu stand testimony to the recent vagaries of the Indian summer monsoon, they observed.