Southwest Monsoon has been extremely weak this year with rain deficiency mounting across the country. Southwest Monsoon this year apparently ushered in a little early around 28-29th of May as monsoonal rain was visible around this date. However, Monsoon made a sluggish start this year with subdued activity in the initial phase and made a very slow advancement. The onset of Monsoon phase should normally witness significant rain but this year the actual outburst of Southwest Monsoon was missing.
The Monsoon line (Northern limit of Monsoon), is way behind the normal course and after passing through Veraval, Surat, Nashik, Wasim, Damoh and Sultanpur on 21st June, continues to be stagnant. Monsoon will cover Punjab, Haryana, Delhi and west Uttar Pradesh by the end of June or the first week of next month but rain will remain subdued. Progress of the line does not look bright and this month will leave us with an alarming deficiency.
Weak Monsoon Performance
As of now the national cumulative average Monsoon rain is deficient by a whopping 37%. While Northwest India is deficient by 45%, Central India is even more than 50%. These regions performed rather poorly as compared to southern peninsula and East & Northeast India. The state of Gujarat is reeling under extreme dry weather conditions and the deficit here has mounted to 84% from 78% till 17th June. Similarly, even Rajasthan is facing a deficit of about 60%.
Among the 36 sub-divisions in India, only 9 places have received normal rainfall (± 20%) till date. With 8% surplus rain, Sub Himalayan West Bengal has shown the best performance followed by Andaman and Nicobar Islands. Kerala and Karnataka coasts have been among the main centres of rain activity in the last few days. But the Monsoon surge has been week over Mumbai for over a week now and the last time the city observed some good showers was on 16th June when 32 mm of rain was recorded at Santa Cruz.
Reasons cited for mounting rain deficit
El Nino
El-Nino is a phenomenon which emerges after a gap of every 3 to 7 years and affects rainfall in India during the Monsoon. Due to more heating, warm waters off Western coast of South America increase the sea surface temperatures above normal by 0.5⁰C. This process leads to diversion of moist winds from the Indian Ocean towards the western coast of South America, thus reducing the amount of rainfall in Indian sub-continent during the year of its emergence.
El Nino in India may not just affect the overall Monsoon performance, increasing or decreasing rainfall in a region, but also impacts the date of onset.
Monsoon trough
With the onset of Monsoon in India, a trough extending from Ganganagar across Allahabad to the Bay of Bengal makes an appearance. We call this a Monsoon trough as the Southwest Monsoon covers the entire country. Surprisingly, the trough gave a miss this year.
Nanauk
Monsoon systems, like Nanauk in the Arabian Sea, possess fundamental role, particularly during the initial days of the onset of Monsoon in India. As the tropical cyclone was punched with energy drawn from the Ocean, it brought torrential rain over the peripheral of the system in the Arabian Sea. The system was outward bound and took away all the energy from the area leading to a delay of the actual outburst of Monsoon in South India.
Whether in the Arabian Sea or the South China Sea, such a system mops up all the moisture and carries it away from the Indian coast. The effects of such a system can be offset only if there is a strong system in the Bay of Bengal. On the other hand, systems that come up in the Bay of Bengal holds moisture and drifts them towards the eastern coast.
Cyclonic circulations in Bay of Bengal-
The Bay of Bengal is supposed to be more active during the onset phase of Monsoon. Any activity over the Bay enhances the overall performance of Monsoon in India.
Only one system brewed up this month over Bangladesh and adjoining West Bengal but remained active only for a day and became insignificant thereafter. Normally, such systems travel north westwards carrying moisture across Odisha, Bihar and Uttar Pradesh.
According to the latest weather update by Skymet Meteorology Division in India, another cyclonic circulation can be seen appearing over the Bay, causing clouding over the region and bringing good Monsoon rain over Northeast India and adjoining Sub Himalayan West Bengal.
Depressions and Lows-
Monsoon is driven by low pressure areas and depressions and on an average from June to September at least 6 depressions and lows, which may not manifest into depressions, develop in the Bay of Bengal.
In 2013, 16 low pressures were formed while in the previous year there were 10. 2011 and 2010 observed 11 and 14 low pressure areas, respectively during the Monsoon season. Around 20 to 25% of these systems occur in the month of June but this year there has been no good depression or low pressure area to take the Monsoon current further.
The Monsoon season also witnesses frequent appearances of cyclonic circulations in the Bay of Bengal and Arabian Sea, originating both over the land, i.e. in-situ, and the sea.
