Will the monsoon's retreat be delayed this year too?

The June-September Southwest Monsoon is expected to begin retreating from the Northwest on September 17. There is speculation over this due to the intensification of the current low-pressure system and the expected development of another one over the Bay of Bengal later in the month.

Consequently, while some experts are of the view that the four-month seasonal rains will extend their stay, others such as Mahesh Palawat of Skymet India believe it is “unlikely”. He says that the expected development of an anticyclone over Pakistan in mid-September will set in motion the process of the monsoon’s withdrawal from western Rajasthan.

The current system

The current system is expected to progress slowly towards east and central India. Under its influence, many parts of the coastal, eastern, central and parts of southern peninsula will get fairly widespread to widespread light/moderate rains over the next couple of days.

Meanwhile, widespread light to moderate rains are also very likely over parts of the Northwest — Uttarakhand, Rajasthan, Haryana and Uttar Pradesh — this week, the India Meteorological Department (IMD) says.

But according to some reports, the seasonal rains may extend until late September due to the development of another low-pressure system around the middle of the month. This could lead to above-normal rainfall and possible damage to summer-sown crops —paddy, cotton, soybean, corn, and pulses — that are normally harvested mid-September onward.

“There is an increased probability of a low-pressure system developing in the third week of September, and this could delay the withdrawal of the monsoon,” a senior IMD official said recently.

Above-normal, uneven rains

That the monsoon pattern is changing is a fact is borne out by the IMD’s records.

This year, the rainfall recorded so far is 7% above average — this is technically termed “good, above average rains”. However, the distribution was uneven, with some states receiving more than their normal share and some others landing into a deficit.

For example, in the Northwest, Rajasthan recorded excess rain this year but Punjab, Himachal Pradesh, Jammu & Kashmir and Ladakh received deficient rains. Parts of Gujarat, Madhya Pradesh, Telangana, Andhra Pradesh and Rajasthan experienced much more rain than the norm, leading to catastrophic deluges and flooding.

Effect of climate change

According to the IMD, the June-September rains start retreating around September 17 and the withdrawal is completed by October 15, but deviations are common.

For instance, last year, the start of the withdrawal was delayed until September 25; some experts believe that we could witness a similar pattern this year. When La Nina develops around the second half of the season, a delay in the monsoon’s withdrawal can be expected, they add.

The IMD says that like onset of the monsoon, the criteria for its withdrawal have also undergone change.

The current operational criteria used by the IMD for declaring the monsoon’s withdrawal from the extreme Northwestern parts of the country was adopted in 2006; it comprises synoptic features that are considered only after September 1.

These features are cessation of rainfall activity over the area for five continuous days; establishment of an anticyclone in the lower troposphere; and considerable reduction in moisture content, as inferred from satellite water vapour imageries and tephigrams. Spatial continuity is kept in view while declaring the monsoon’s withdrawal from the rest of the country.

The monsoon withdraws from the southern peninsula — and hence the entire country — around October 15, when the circulation pattern indicates a change from the southwesterly wind regime.

Several studies, however, say that climate change and natural variability are the reasons behind the rise in the frequency and magnitude of extreme rainfall events and decrease in moderate rainfall events over central Indian during the monsoon season. In recent times, there has also been a shift toward more frequent dry spells and more intense wet spells during the season, these studies add.