Hide and seek — reading the monsoon

Aarati Krishnan | Updated on January 17, 2018





Our analysis of rainfall data during 1901-2015 points to certain trends and the possibility of copious rains in the next few months

Forecasters struggle to predict the whimsical Indian monsoon even for one season. But there have been attempts, from the times of Kautilya, to make multi-year forecasts of the monsoon by discerning long-term patterns in the rains hitting the Indian sub-continent. Vedic texts mention five- and seven-year patterns in rainfall that play out time and again. More recent academic studies have used complex statistical analysis to suggest a 30-year cycle.

We decided to take an independent look at rainfall data from 1901 to 2015, to arrive at key trends that can help get a grip on the monsoon.

The 30-year cycle

It has been suggested that the South-West monsoon follows a 30-year cycle, where three decades of bountiful rains tend to alternate with a similar stretch of sub-par monsoons. Well, past data provides no clear evidence of this cycle.

The last 30 years (1985-2015) saw the South-West monsoon average 865 millimetres (mm) of rainfall in each season. This was certainly lower than the 891 mm in the preceding 30 years (1950-1985). But then, the 30 years before that registered an even higher 917 mm, providing not much evidence of a yo-yo movement.

But the recent 30-year period has certainly been among the driest on record.

Measuring actual rainfall in the 1985 to 2015 period against the Long Period Average (890 mm) used by the IMD, India was the recipient of normal or above-normal rains in only nine of the last 30 years. That’s less than a one-in-three chance of a normal monsoon. Between 1950 and 1985, as many as 19 of the 30 years brought normal to above-normal rains. The years from 1920 to 1950 were even better, as 20 of the 30 years saw bountiful South-West monsoons. For the 30 years before, the number was 16.

Going by these trends, therefore, India has gone from an over 50 per cent probability of a normal monsoon to a one-in-three probability in the last 30 years. But if you are an optimist, this may be reason to hope rather than despair. By the law of averages, you can expect the South-West monsoon to revert to long-term averages in the next 30 years. That would mean going back to 16-20 years of normal to excess rains.

Relying on the South-West

Different parts of India receive their annual quota of rains in different parts of the year. With the post and pre-monsoon months also delivering an unexpected deluge in recent years (like Chennai last year and Delhi this year), is India’s dependence on the South-West monsoon diminishing? Not by a long chalk! Trends over the long-term suggest that the contribution of the South-West monsoon (June to September) in the country’s total annual rainfall remains undiminished at about 75 per cent. In the last 50 years, the South West monsoon has accounted for 862 mm or three-fourths of the annual rainfall of 1150 mm recorded across India. The 50 years before that witnessed more copious rains (1223 mm a year), but the contribution from the South-West monsoon was exactly the same. This makes the South-West monsoon critical to agricultural prospects as well as the outlook for the rural economy.

Critical July

For policymakers or farmers tracking the monsoon, there’s no month that is as important as July. In the preceding decade ended 2005, there appeared to be a shift from the long-term patterns, with pre-monsoon months of May and June registering copious rains. But the pre-monsoon months have lost importance again between 2005 and 2015, with July regaining its pole position. July contributed 25 per cent to the yearly rainfall in the last decade. This reversion to the past is welcome because Indian farmers plan their cropping cycles around peak monsoon rains in July. Bountiful July rains have a disproportionate impact on Kharif output too. This trend has been very evident this year, excess rainfall in July translating into sizeable increases in the area sown for rice and kharif pulses and oilseeds.

The El Nino factor

If there’s one factor that introduces a wild card element to the Indian monsoon, it is the El Nino, a weather phenomenon characterised by unusual warming of the ocean temperatures in equatorial Pacific, observed to cause droughts in Asia. An analysis in the Economic Survey 2016 observes that there have been 22 El Nino episodes since 1950 of which eight were rated moderate to strong. The strongest El Nino years have invariably coincided with deficient monsoons or droughts in India.

Taking stock since 1980, moderate to strong El Nino years have seen Indian farm output shrink by an average of 2.1 per cent, compared to normal growth rates of 3 per cent. But the good news is that the most debilitating El Ninos are often followed by La Nina, a cooling of ocean temperatures, which brings bountiful rains. The La Nina years have seen India’s farm sector expand by an average 8.4 per cent. As 2015-16 saw one of the strongest El Ninos on record, there is a live possibility of a La Nina developing in the coming months and bringing copious rainfall to India.

Published on August 28, 2016

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