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Monday, July 16, 2018

Rain check: June nearly normal

After an extended dry spell, late rainfall helped the first month of the monsoon end well. A look at what had caused the dry spell, what broke it, and what the weather office expects in the near future.

Written by Amitabh Sinha | Pune | Updated: July 3, 2018 8:00:37 am
Thanks to the formation of a low-pressure area over the north Bay of Bengal in the last few days of June, there were widespread rains. (Express photo by Bhupendra Rana)

Good rainfall in most parts of the country in the last week of June ensured that the first month of the monsoon season ended with near-normal rains. But not before an extended dry phase of almost two weeks that saw a complete halt in the progress of the monsoon. Until June 27, the country had a more-than-10% deficiency in rainfall, and the monsoon had barely reached central India. Thanks to the formation of a low-pressure area over the north Bay of Bengal in the last few days of June, there were widespread rains. The monsoon quickly progressed its northward journey, and covered the entire country in a matter of two days, at least 15 days ahead of schedule.

Depressions absent

Scientists say the defining characteristic of monsoon behaviour in June was the lack of development of adequate “depressions” or “low-pressure areas” over the Bay of Bengal region. Only one such depression was formed in the entire month when at least two to three, sometimes four, are expected.

Low-pressure areas over the Bay of Bengal, near the Bengal and Odisha coasts, help in pulling the monsoon winds through to most of north and northwest India. The first seasonal rains in Bihar, Jharkhand, Uttar Pradesh, most parts of Madhya Pradesh, Delhi, Rajasthan, Punjab and upwards happen due to this activity, when the rain-bearing winds coming the Bay of Bengal are driven inside due to prevailing low pressure.

“Especially, during the northward advance of the monsoon, the formation of low-pressure areas is very important. In June, we saw only one such event taking place. Till that happened, around June 27, most of eastern, central and northern India was starved of rains. Lack of depressions was the most standout feature of the monsoon in June,” said Govindasamy Bala of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, Bengaluru.

Mruytunjay Mohapatra of India Meteorological Department agreed. “We had just one low-pressure event. This is less than what is expected in June and was chiefly responsible for the break that the monsoon went into,” he said. After good rains for a few days, the monsoon seems to have gone into another break. Bala said current forecasts show that another depression was not developing till July 6 or 7. So, the next spell of good rainfalls, especially in northern plains, in the states of Bihar, Jharkhand or Uttar Pradesh, will not happen before that time, he said.

These breaks, or dry phases, are not uncommon. The monsoon oscillates every few weeks between wet and dry phases, sometimes referred to as active and inactive phases. As J Srinivasan of the Divecha Centre for Climate Change at the Indian Institute of Science said, in a good monsoon year there are more wet phases than dry phases, while in a drought year it is the dry phases that dominate.

Madden Julian Oscillation

The dry phase seen in June is also being attributed to what is known as Madden Julian Oscillation, or MJO, an air-ocean interaction that happens along the equator. MJO is an eastward-moving disturbance of clouds, wind and pressure, that travels around the globe at a speed of 4 to 8 metres per second, for between 30 and 60 days on an average. Sometimes, one revolution around the planet can even take 90 days. As it moves, strong MJO activity often splits the planet into two parts, one in which the MJO is in the active phase and results in enhanced rainfall, and the other in which it suppresses rainfall.

In 2015, the MJO activity in June was favourable for the Indian landmass and resulted in an unusually high rainfall. This year, the opposite happened. “This year the lower rainfall in India during June 15 to June 22 was associated with an MJO,” Srinivasan said. “The accurate forecast of the Indian monsoon rainfall beyond a few days depends critically upon the ability of the (climate) model to correctly simulate the phase of the MJO. The models have improved a lot during the past decade and we can expect more improvements in the future.”
Mohapatra of IMD said even the absence of low-pressure areas over Bay of Bengal could be attributed to MJO activity. “Formation of depressions is related to MJO activity. About 60% of the depressions formed in the northern Bay of Bengal during this time are influenced by the MJO,” he said.

Mohapatra said the June rainfall had been very good for sowing of crops, though. “Sowing requires optimum amount of rainfall. Excess rains can spoil the sowing season. The rainfall we got in June was good for agriculture,” he said.

Water in stock

By June 28, the 91 large reservoirs of the country had stored 29.668 billion cubic metres, or 18% of capacity. This, however, remains slightly less than what is expected during this time of the year — about 94% of the average of the last 10 years. Good rains over the last few days would have led to a further increase in storage levels.

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