Now that the India Meteorological Department (IMD) has predicted the onset of the monsoon in India on May 27, almost five days earlier than the scheduled onset date of June 1, it may be interesting to know that the scheduled date of June 1 has been met only three times in the last 25 years. Monsoon onsets in May and post-June 1 have happened 11 times each during the last two decades and half.
An analysis of monsoon onsets during 1997-2021 shows that all the three scheduled onset dates of June 1 have been met in the last decade (2013, 2016 and 2020). May 31 (2000, 2008 and 2010) has emerged as the most preferred date for onset of the monsoon, while May 29 (2011 and 218) and May 26 (2001 and 2006) are the next most preferred monsoon onset dates.
For predicted onsets post-June 1 , June 5 (2012 and 2015) and June 9 (1997 and 2002) are some of the preferred dates.
Onset of the monsoon has either been early by a week or has been delayed by a maximum of two weeks. In 2009, the onset over Kerala happened eight days in advance on May 23. It was delayed by 12 days in 2003 when it set in over Kerala on June 13.
Depressions
, Depressions are a feature of the Indian monsoons. Interestingly, a look at the onset pattern in the last 25 years shows that no depressions were formed in 2002, 2010 and 2012.
‘Monsoon 2012: A report’ by the National Climate Centre says no depressions formed over the Indian monsoon region during the season as against the normal of four-six monsoon depressions per season.
IMD’s Southwest Monsoon End of Season Report for 2010 says that the entire monsoon season was devoid of any monsoon depressions. Thus 2010 has been the only year in the recorded history after 2002 to have no depressions during the entire season. However, there were two intense systems -- the Severe Cyclonic Storm (Laila, May 16-21) over the Bay of Bengal and the Very Severe Cyclonic Storm (Phet, May 31 to June 2) over the Arabian Sea during the advance phase, it said.
Data available with the Climate Data Service Portal (CDSP) of IMD shows that a majority of depressions formed in the Bay of Bengal during the period between 1997 and 2020. In fact, the Bay of Bengal has been witness to depressions and cyclonic storms in 1997 and 2003.
However, the Arabian Sea witnessed more depressions during 1998, 2001 and 2019 when compared to the Bay of Bengal.
Rainfall
According toCDSP data, rainfall figures crossed the 300 mm mark during four years – 2003 (306.1 mm), 2005 (333.7 mm), 2013 (311.5 mm), and 2016 (308.4 mm) -- in July. Rainfall figures touched 327.8 mm in August 2020.
The total rainfall crossed 900 mm mark during June-September only during 10 years between 1997 and 2020. The June-September rainfall was the lowest at 695.7 mm in 2002 and the highest at 971 mm in 2007 during these 24 years.