Monsoon onset may be delayed by a week

The onset of the South-West Monsoon is likely to be delayed by a week this year, according to PV Joseph, eminent scientist and researcher and former director of the India Met Department (IMD).

Joseph bases the timeline of onset on the ‘pre-monsoon rain peak’ (PMRP) phenomenon. It is represented by a band of raining cloud extending from Kerala eastwards into the Bay of Bengal. Joseph’s pioneering work on the PMRP is among the key parameters considered by the IMD for its monsoon onset forecasts.

The long period average date of monsoon onset over Kerala is June 1. “In the strong El Nino year of 1972, we had an extreme case. The monsoon onset that year was on June 18 and the PMRP was delayed into the second week of May,” he told BusinessLine.

For a normal monsoon onset that occurs on June 1, the raining band of cloud in the Bay should form around April 21. The rain and clouds occurring now in Kerala and in the South Bay represents PMRP with a delay of 10 days. Cyclone Fani had its genesis in this cloud cluster, Joseph explained.

The Inter Tropical Convergence Zone (ITCZ), a belt of low-pressure near the equator, is located south of the equator in tropical Indian Ocean from December to March.

In a normal year, it begins to move northwards in April and sets up the PMRP. In an El Nino year, the northward movement of the ITCZ is delayed, which delays the PMRP, as has happened this year, Joseph said.

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