Rainy Septembers point to monsoon shifts
Changing patterns of low pressure, fluctuations in moisture distribution may cause rainfall variation
For the first time since 2010, India is poised to see three consecutive Septembers with excess rainfall. Experts say that this is a sign of a change in monsoon patterns though it is too early yet to demonstrably prove that is a lasting consequence of global warming.
As of September 24, monsoon rainfall for the month is nearly 19 cm. The normal for the entire month is 17 cm, and there is still a week’s worth of rainfall to come.
Spikes in rain
While September is usually the month that marks the beginning of the end, for the monsoon’s four month sojourn over India, both 2020 and 2019 have seen spikes in rain. In 2019, September rain was a staggering 152% or close to 25 cm. To put that in perspective, that is close to what the country gets in August (26 cm), usually the second rainiest of the monsoon months. That year saw India get the highest monsoon rainfall since 1994. Last September’s rainfall was 17.7 cm which was not too high but more than what is normal.
From 2013 to 2018, September rainfall has been less than normal save for 2014, when it was 18 cm, or 1 cm above normal.
From 2010–2012, the three years of extra-rainy Septembers did not do much to boost the total rainfall India received. India only got 2% more monsoon rain in 2010 and 2011, and ended up with an overall 7% deficit in 2012 (largely due to weak rains in June and July).
However, both in 2019 and 2020, India received close to 10% more monsoon rain than normal.
When August ended, India appeared to be dangerously close to a rainfall deficit and almost 9% short of what is normal from June–August end.
This year, with weak August rainfall, the India Meteorological Department said that September rainfall would be stronger than usual but India would still end up with only around 96% of what's normal. However, rainfall this month has significantly narrowed the deficit from 9% at the beginning of the month to 2% at the last week of September.
A rain-bearing circulation is forming in the Bay of Bengal—unusual for this time of the year—and is expected to bring significant rain over several parts of India for most of the coming week.
“The most obvious reason is that the monsoon is withdrawing much later than usual,” said Mahesh Palawat, Chief Meteorologist, Skymet, a private weather forecast agency, “This means more September rainfall. But why it's staying on for longer is still to be understood.”
The monsoon normally starts to withdraw by the 1st of September and completely exits by October. Last year, to reflect the increasingly delayed withdrawal of the monsoon, the IMD updated the beginning of the withdrawal date to September 17th. In both 2019 and 2020, the monsoon began its withdrawal in October and the same is expected this year too.
“Overall global warming is increasing moisture levels during the monsoon period but this is affecting the distribution of the rain,” said KJ Ramesh, former Director General, IMD, “July and August see periods of active rains and breaks. June and September because they have low base rainfall, even the slightest increase shows up big percentage gains.”
A study by scientists at the Central Water Commission published in Theoretical and Applied Climatology, 2018 analysed changes in monsoon patterns and concluded that the monthly variability of the summer rainfall was due to changing patterns of low pressure over the Indian landmass as well as fluctuations in moisture distribution.