Monsoon

According to the IMD, conditions are likely to become favourable for the withdrawal of monsoon from the next week. "Due to trough/ cyclonic circulation over central and peninsular India, scattered to fairly widespread rainfall and isolated thunderstorm/ lightning likely over central and peninsular India during most days between September 29 - October 5," the IMD said.

The latest forecast by the Monsoon Mission Coupled Forecast System (MMCFS) of the IMD said that the Indian Ocean Dipole (IOD) conditions, which is at present neutral with negative DMI index over the Indian Ocean, is likely to develop negative conditions during the upcoming season. DMI Index or Dipole Mode Index (DMI) means the difference between sea surface temperature (SST).

The 'normal' monsoon rainfall for the country, as a whole, would mean within a range of 96 to 104 per cent of the LPA, which is 87 cm or 870 mm. The IMD had, this April, introduced the new LPA, the new rainfall normal based on rainfall data from 1971-2020 for the southwest monsoon season, replacing the till then used rainfall normal that was based on data from 1961-2010.

La Nina is a phenomenon, wherein the sea surface temperatures along the equatorial Pacific Ocean are mostly below normal. It is the exact opposite of El Nino, when trade winds weaken, and warm water is pushed back east. The World Meteorological Organisation (WMO) says that the El Nino/La Nina Southern Oscillation (ENSO) has a major influence on climate patterns in various parts of the world and impacts the year-to-year variability of the monsoon over South Asia.

The weather department, in a statement, further said the southwest monsoon is likely to withdraw from the entire country around October 26 as the north-easterly winds are likely to set in the lower tropospheric levels over Bay of Bengal and extreme south Peninsular India. The process this year is delayed as the southwest monsoon normally withdraws from across India by October 15. The beginning of the withdrawal usually begins around September 26 every year but it got delayed this year and started from October 6.

"In view of the establishment of an anti-cyclonic circulation in the lower tropospheric levels over western parts of northwest India and substantial reduction in moisture content & rainfall, we can say the withdrawal of southwest monsoon has commenced today," the IMD said in a release. The IMD had revised the date of withdrawal of the southwest monsoon last year to September 17.

"The southwest monsoon seasonal rainfall between June 1 to September 30 for the country was normal, i.e., between 96-104 per cent of long period average (LPA). The rainfall quantum of 870 mm was 99 per cent of the LPA," it said. The long-period average is calculated based on the data from 1961 till 2010.

The India Meteorological Department (IMD) had, last year, revised the date of commencement of withdrawal of monsoon from northwest India from September 1 to September 17. Earlier last week, the IMD had said, the withdrawal process may not commence before September 28 or 29.

The all-India rainfall from June 1 till September 15 was 772.7 mm as against the 807.4 mm normal projection. For East and Northeast India on the one hand and the Southern Peninsula on the other, the departure from the LPA was by 27 and 26 per cent, respectively, for the week. In IMD parlance, LPA of rainfall is based on data between 1961 and 2010.