A host of factors including temperature anomalies, the Madden Julian Oscillation and the Indian Ocean Dipole are responsible
The southwest monsoon, one of the most stable weather systems on the planet, has gone for a toss in 2021. This is a major cause of concern for the millions of farmers in India who are still dependent on the annual rainfall season for their agricultural activities.
As on July 12, the country-wide deficit of monsoon rainfall stands at seven per cent below normal. This is when 75 per cent of meteorological sub divisions till June 16 had reported normal, excess or large excess rains.
“I think we are holding on to definitions that may not be relevant anymore in the context of pre-monsoon activities, especially the cyclones and their impact on the progression of the monsoon trough over the Andaman Seas, the Bay of Bengal and onto Kerala,” Raghu Murtugudde, a climate scientist at the University of Maryland in the United States told Down To Earth (DTE).
When the monsoon onset happened almost on time (June 3) and it made swift progress, with ample rains all over the country in the first three weeks of June, everyone rejoiced at the performance.
“I would worry about the patterns of the rainfall deficits. The core monsoon zone actually has excess rain thus far but that’s because of the cyclones,” Murtugudde said.
He highlighted that this is a key understanding that everybody is missing.
“We have had an on-time ‘onset’ because the cyclone dragged the monsoon trough onto Kerala and it met the definition of the onset. But were the large-scale conditions really favourable to keep the trough moving? Clearly, not. Didn’t this happen last year as well? Overall rainfall was normal or above normal but what does that even mean?” he added.
Mutugudde said attention should be paid to the large-scale dynamics that stretch northwestward from the northwestern tropical Pacific. These are often conducive to the 10-20 oscillations that come from the east towards India, he added.
These oscillations determine the further progress of the trough. A repeat of last year, with excess rain later this month and into September and even a delayed withdrawal into October, may happen again.
As of July 12, the monsoon winds over India have remained stalled for 24 days. This is one of the longest such hiatus in the progress of the monsoon season over India in recent time and even if the monsoon revives, it may remain weak for the next week or so.
According to IMD’s own monsoon progress map, the northern limit of the monsoon trough still remains stationary. There are a multitude of factors that may have led to this long hiatus.
“The monsoon is delayed due to lower-than-usual temperatures in northern Pakistan and northern India because it takes a longer time to heat these regions to monsoon temperature,” Elena Surovyatkina, a climate scientist at the Potsdam Institute for Climate Impact Research in Potsdam, Germany and a principal researcher at Space Research Institute of Russian Academy of Sciences in Moscow, Russia, told DTE.
“It led to the absence of the formation of the low-pressure system over the north Bay of Bengal. However, nature abhors a vacuum. As a result, suitable conditions occur for the appearance of synoptic-scale Rossby waves in between the African Jet and mid-latitude westerly winds that led to the longest break and heatwaves in Delhi. When temperatures in northern Pakistan reach the monsoon temperature in Delhi, these Rossby waves disappear, and the monsoon begins in Delhi,” she added.
Murtugudde points out that “the midlatitude Rossby waves are unlikely to have affected the Indian monsoon but the overall warming of the northern hemisphere will have to be looked at carefully to see if it affected the monsoon trough.”
Apart from the temperature anomalies in north Pakistan, another cause for the stalling is pre-monsoon rainfall in May, which has disorganised the onset of the monsoon and caused alternating premature rainfall and dry spells, in turn, leading to a longer transition to the monsoon. When one part of the Indian subcontinent is warming, another is cooling, which leads to an erratic transition to the monsoon and a risk of dry spells.
The monsoon stalling is also one of the reasons for the spate of heat waves in north and north west India for the past few weeks.
“While on the eastern side, there have not been any low pressure areas in the Bay of Bengal which usually aid the progress of the monsoon season, on the western side there is a constant flow of westerlies flowing in from North Africa and the Arabian Peninsula.
These westerlies are dry and contain dust which does not allow the formation of clouds which means that the sun has a direct line of sight towards the land, heating it up at leisure, which in turn causes heat waves,” J R Kulkarni, a climate scientist at the Indian Institute of Tropical Meteorology in Pune, said.
Kulkarni further added that the westerly winds coming in are due to an active Atlantic Intertropical Convergence Zone (ITCZ) which is a monsoon-like trough that exists between the eastern Atlantic Ocean and North Africa.
The ITCZ may have itself been related to the Rossby wave activity and resulting heating events in Libya in North Africa and in eastern Europe, United States and Canada, forming a series of events with impacts over large regions spread across the northern hemisphere.
Another reason for the break in monsoon progress that all scientists have highlighted is the unfavourable Madden Julian oscillation and Indian Ocean Dipole. The Madden Julian Oscillation is an eastward moving pulse of cloud and rainfall that recurs every 30 to 60 days. It causes major fluctuations in the monsoon over India.
The Indian Ocean Dipole is a climatic system formed by the difference in temperatures in the two different sides of the Indian Ocean. In its negative phase, when the eastern Indian Ocean is warmer and has more rainfall, it sometimes stalls the progress of the monsoon winds.
Taking all these factors into account, Kulkarni predicts that the monsoon progress may begin around July 13 but it will remain sluggish for another week as the Arabian and North African dusty winds continue to impact northern India.
Surovyatkina predicts that the monsoon will reach Delhi after July 11, a prediction she had made at the beginning of May, but the rains may begin in earnest only after July 19.
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