Nascent Arabian Sea cyclone may delay monsoon

A strong system could delay onset over Kerala next week

Uncertainty clouds the monsoon’s arrival over Kerala. While the official date this year remains the June 5, the India Meteorological Department (IMD) said on Thursday evening that “conditions were favourable for onset” on June 1.

The uncertainty is primarily due to a prominent weather model that hints at the development of a cyclone in the Arabian Sea after May 31; the strength and trajectory of this weather system could, in the worst case scenario, delay the monsoon’s progress.

“One model — of the European Centre for Medium Range Weather Forecasting — is showing a cyclone as developing in the Arabian Sea. However, our own models are showing that it wouldn’t be that intense and would likely become a Deep Depression, at most,” said Madhavan Rajeevan, Secretary, Ministry of Earth Sciences. “It could mean a few days delay if it's too strong. Once the monsoon season sets in, there’s a certain uncertainty too in all weather models,” he added.

Storms that develop in the sea begin as ‘Low Pressures’ and graduate to Depressions, Deep Depressions and then onto three categories of cyclone. The most recent being the Super Cyclone Amphan in the Bay of Bengal.

Another depression is likely to form in the Arabian Sea over the weekend but will turn towards Oman and not significantly influence the monsoon.

While such depressions are normal during the advent of monsoon and usually aid the monsoon’s advent and progress in India, a particularly strong storm can hinder the monsoon. Last year, cyclone Vayu that formed in the Arabian Sea stalled the monsoon after it entered Kerala on June 8.

Rainfall measure

A key criteria to declaring the monsoon as having arrived is that at least 8 of 14 meteorological stations in Kerala and Karnataka consistently measure more than 2.5 mm of rains for two continuous days. That such rains are already being registered in these States gives G.P. Sharma, president of Skymet Weather Services, confidence that the monsoon will have set in latest by May 31.

On May 15, he had forecast the monsoon’s arrival date as May 28. That same day, the IMD had forecast a four-day delay in the onset to June 5, largely because of the influence of Amphan.

“We have a 100 automatic weather stations in Kerala and the data clearly shows the rainfall requirement being met as well as the required windspeeds. But officially, monsoon arrives when IMD says it does,” Mr. Sharma added.

Windspeeds

Rainfall apart, sustained windspeeds of 30-40 kmph at 5 to 6 kilometre from the sea level were a key criteria. “These winds from the southern hemisphere have to cross the equator and only then can we consider the monsoon to have set in,” Dr. Rajeevan added.

“Primarily, it depends on how the low pressure system ( after May 31) will form which is why we now say that the conditions could be favourable for June 1 or 2 onset,” Director-General of IMD M. Mohapatra told The Hindu.

The IMD's onset date of June 5 is based on a weather model that has a built-in error margin of four days. Thus a June 5 onset can technically encompass all days from June1 to 9.