Explained: How do we define withdrawal of monsoon?

Monsoon's withdrawal begins from the extreme northwest by the beginning of September, progressively receding southwards.

New Delhi, May 9: A monsoon is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation. It usually arrives in Kerala in the first week of June, but this year it might arrive any time after May 20, as India Meteorological Department (IMD).

Monsoon's withdrawal begins from the extreme northwest by the beginning of September, progressively receding southwards, but how do we define withdrawal of monsoon?

"As per the four-week extended range forecast for May 5-June 1, the monsoon onset over Kerala could take place any time after May 20. In the last ERF too, which was released on April 28, the May 19-25 period was showing an increase in rainfall activity over Kerala. If the ERF next week also shows similar conditions over Kerala after May 20, one could say that the onset over the coastal state could take place before time," The Times of India quotes a top expert from IITM as saying.

Monsoon's withdrawal begins from the extreme northwest by the beginning of September, progressively receding southwards.

How do we define withdrawal of monsoon?

According to the IMD, like the onset criteria, the criteria for the withdrawal of monsoon have also undergone changes. The current operational criteria used by IMD for declaring the withdrawal from extreme north-western parts of the country was adopted in 2006 and consist of the following major synoptic features which will be considered only after 1st September,

A) Cessation of rainfall activity over the area for continuous 5 days.

B) Establishment of anticyclone in the lower troposphere (850 hPa and below)

C) Considerable reduction in moisture content as inferred from satellite water
vapour imageries and tephigrams

Further withdrawal from the country is declared keeping the spatial continuity, reduction in moisture as seen in the water vapour imageries and prevalence of dry weather for 5 days. SW monsoon is withdrawn from the southern peninsula and hence from the entire country around 15 th October, when the circulation pattern indicates a change over from the south-westerly wind regime.