Historic data suggests monsoon may be delayed by ten days

The India Meteorological Department (IMD) may have forecast an above-normal monsoon this year, but it is highly probable that its onset over Kerala will be delayed by more than 10 days.

There is a lot of physical evidence in support of such a delay this year, says PV Joseph, eminent monsoon researcher and former IMD director.

Normally, the date for the monsoon’s onset is June 1. But whenever the El Nino phenomenon takes place, not only is the monsoon delayed, but delays of 10 to 20 days have been reported in the following year as well.

For instance, 2002 had a strong El Nino. That year, the monsoon set in on June 9.

The following year, the onset was pushed to June 13.

Again, in the strong El Nino years of 1957 and 1982, the onset was around the normal date, but in 1958 and 1983, it made landfall on June 14 and June 12, respectively.

Given that 2015 was an El Nino year, and the monsoon’s onset was June 5, there is a high probability that it will be more than 10 days late this year, Joseph told BusinessLine .

A study of 100 years of monsoon onsets over Kerala by Joseph et al, published in 1994, showed that there is statistical significance for delays in the year immediately following an El Nino.

There have been exceptions, though not many, Joseph noted. In 1972, a strong El Nino year, the onset was on June 18. The following year, it hit Kerala on June 4.

Globally, the satellite-monitored warmest sea-surface temperature anomalies (areas of maximum surface temperatures in the sea) for January-March 2016 are close to but south of the Equator.

During this season, the overlying circum-global band of rain clouds associated with the Inter Tropical Convergence Zone (ITCZ, on which the monsoon rides) has produced unusually high rainfall.

In such a situation, past data has shown that the crossing of the ITCZ cloud band from the southern to the northern hemisphere in the Indian Ocean will be delayed, leading to delays in the onset of the monsoon over Kerala.

“We have to now monitor satellite pictures to look for the formation of the pre-monsoon rain peak in the Bay of Bengal (which is nothing but a part of the ITCZ displaced northward), a good prior indicator for the date of onset over Kerala,” Joseph said.

For a June 1 onset, the pre-monsoon rain peak occurs around April 21.

“When this gets pushed to early May, the monsoon onset over Kerala is delayed by as much as two weeks,” he said.