Poor July rain may lead to below normal monsoon overall, says study

Is July the most critical month for the South-West monsoon?

Probably yes, as by this time the monsoon has covered most parts of the country and the land-based monsoon trough is well established across the plains of north-west India.

A detailed study by K Srikanth who blogs at Chennaiyil Oru Mazhaikalam and made available to BusinessLine has brought out a linkage between July rains and performance of the monsoon as a whole.

July portends How does a bad July influence the season?

In 20 out of 30 subdivisions, every second time July has received less than 90 per cent rainfall, the seasonal rain too has clocked in at less than 90 per cent.

This goes to establish a strong relationship between the performance of July and the seasonal rainfall, says Srikanth (see tables).

It is very strong in certain divisions such as north interior Karnataka and West Uttar Pradesh. But the probability of monsoon failing a year when July rainfall has failed is only 37 per cent in Odisha and a few others.

Overall trend During El Nino Years, 21 out of 30 met subdivisions have seen seasonal rainfall fail twice in three times when July has failed. The probability is as high as 90 per cent for Bihar.

Overall, the trend is that during an El Nino year, a poor July results in failure of the overall monsoon.

Exceptions are Odisha, sub-Himalayan West Bengal, Assam and Meghalaya where the correlation is less than 50 per cent. In some subdivisions such as Bihar, nine out 10 El Nino years when July has failed the monsoon too has failed.

Seven others have a probability of more than 80 per cent of failure in case of a poor July.

Critical month El Nino years raise the probability of rains being less than 90 per cent in all subdivisions surveyed except Odisha, Konkan, Goa and Chhattisgarh.

The probability is at a critical 70 per cent for Haryana, Punjab and West Rajasthan – i.e. in seven out of 10 El Nino years, they have recorded less than 90 per cent rain.

Why is July the most critical month for monsoon?

Nearly one-third of the rain for the season falls during July. India receives almost 20 per cent more rains in July than August; about 40 per cent more than June; and 70 per cent more than September.

Highest rainfall At least 21 out of 30 meteorological subdivisions receive the highest monthly rainfall in July. This ranges between a high of 40 per cent of seasonal rainfall in Saurashtra and 21 per cent in Tamil Nadu.

July and August combine to contribute two-third of the seasonal rainfall in about 14 subdivisions. Composite Madhya Pradesh gets more than 70 per cent during this period.

The probability for less than 90 per cent rain during July ranges between 35 per cent for Jharkhand and west Madhya Pradesh and almost 50 per cent for Haryana, Saurashtra and Bihar.

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