‘Cyclonic circulation’ in Arabian Sea may cloud monsoon’s progress

This is an additional challenge for the monsoon which has already missed its onset date over Kerala

Updated - June 06, 2023 07:54 am IST

Published - June 05, 2023 06:44 pm IST - New Delhi

The southwest monsoon normally sets in over Kerala on June 1 with a standard deviation of about seven days. In mid-May, the India Meteorological Department (IMD) said monsoon might arrive in Kerala by June 4. | representative image

The southwest monsoon normally sets in over Kerala on June 1 with a standard deviation of about seven days. In mid-May, the India Meteorological Department (IMD) said monsoon might arrive in Kerala by June 4. | representative image | Photo Credit: G. Moorthy

The monsoon, having already missed its onset date over Kerala, faces an additional challenge from a ‘cyclonic circulation’ in the Arabian Sea. However, as of June 5, it is unclear if this system will help or hinder the monsoon’s advent and ascent along India’s coast.

“A cyclonic circulation lies over southeast Arabian Sea... under its influence, a ‘low pressure area’ is very likely to form...during the next 24 hours. It is likely to move nearly northwards and intensify into a ‘depression’ (in) southeast and adjoining east-central Arabian Sea during subsequent 48 hours,” the India Meteorological Department (IMD) said in a statement on June 5.

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‘Low pressure area’ and ‘depression’ represent rising degrees of strengthening winds and a ‘cyclone’ is essentially a stronger ‘depression’ which results from the warming of the ocean surface and gusts of wind. The IMD’s current weather models are not unanimous on whether this cyclonic circulation will strengthen and move along the western coast or turn westwards towards Yemen and Oman.

“If it moves along the western coast, it will push the monsoon system upwards but if it turns, then it will further take away moisture and stall the monsoon. We must wait and watch to see what happens,” D. Sivananda Pai of the IMD who is closely involved with monsoon forecasts told The Hindu.

Four-day margin of error

Last month, the IMD had predicted an onset date of June 4 over Kerala. This date is arrived at from a customised weather model and has a four-day margin of error. “There were favourable clouds as well as good rains in Kerala on the 4th of June. This has however weakened. It is not unusual for such systems to form around this time, however, the monsoon system needs a slight push and that’s what we are waiting for,” he added.

Rainfall alone isn’t the criteria for the IMD to declare the monsoon’s advent. Windspeeds of a minimum strength and depth in the atmosphere, the Outgoing Longwave Radiation — a measure of cloudiness — and consistent rainfall over two days in Kerala, all combine to inform the agency’s declaration of the monsoon’s onset.

Also read: Southwest monsoon onset in Kerala likely to be weak

While monsoon’s onset doesn’t influence the quantity of rain during the June-September months, scientists say that El Nino years are linked to a delayed monsoon onset. “What we many times see is that in El Nino years, we have weak, delayed onset as well as an early withdrawal. We do see a weak onset this time and if a cyclone were to form, that would again stall the monsoon,” Roxy Mathew Koll, scientist at the Indian Institute of Tropical Meteorology, Pune and an expert in climate and ocean-atmospheric interactions, told The Hindu.

The IMD has said that June-September monsoon rains are likely to be 96% of the Long Period Average (a 50-year mean) of 87 cm. This is at the lowest end of what is considered ‘normal.

The key factor influencing the quantum of monsoon rains this year is the development of an El Nino, a cyclical phenomenon of warming in the central Pacific Ocean that is linked — in six out of 10 years — to diminished rainfall in western and northwestern India, as well as the western parts of central India. Since 2019, India has been under the influence of the converse La Nina, which is a cooling in those regions, and therefore linked to substantial monsoon rains.

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