The total monsoon precipitation in 2020, as of 8.30 am Sunday, is the eleventh highest for this period since 1901. However, 2020 appears to be an outlier when read with long-term precipitation statistics. Monsoon rains in India have been gradually decreasing since the 1950s. This trend has been accompanied by increasingly skewed rains; most rains fall for a shorter period of time. The rains are also getting more intense, which means there are fewer episodes of rain, but when it does rain, it pours. In 2019, heavy and extremely intense rains were the highest since 1901.
The total monsoon rains in the country this year, at 8.30 am on Sunday, has been 771.1 mm.Total monsoon rainfall in the country this year, at 8.30am Sunday, has been 771.1mm, according to the Indian Meteorological Department (IMD) gridded rainfall dataset. This is the 11th highest monsoon precipitation for the period June 1 to August 30 since 1901. That makes 2020 an outlier. Decennial averages of monsoon rains through August 30 suggest that total monsoon rains in India have been decreasing. The period from June 1 to September 30 is considered the monsoon season.
The rains are not only decreasing, they are also skewing
This decrease in total rainfall has accompanied another trend: the decrease in the number of days in which at least half of the rain falls. In the past decade (2020 has been excluded to see the pattern throughout the monsoon season), 50% of the rain during the 122-day monsoon was recorded over an average of 40.4 days. This was the fastest in the 12 decades since 1901. Undoubtedly, the corresponding time for 75% of the rain and 90% was only the fourth highest (70.6 days and 94.3 days respectively), although the Long-term trend for those higher percentages of rain is also the same: it now takes fewer days.
Regional variations in the number of days it takes for rains to occur
The numbers for all of India hide significant regional variation in this pattern. For example, Delhi received 50% of its total monsoon rains in 2011-19 in just 83 days. This number was 30 for Karnataka.
When it rains it pours
Skewed rains could mean an increase in heavy and extreme rains. And this has happened in India as a whole since the second half of the last century. The combined heavy and extreme rains were the third highest this decade. In 2019, heavy and extreme rains were the highest since 1901. 24-hour rainfall between 35.5 mm and 244.5 mm on a grid (this analysis uses the IMD gridded rainfall dataset) is considered rain of great intensity and above 244.5 mm, extreme rain. A grid is a box bounded by two latitudes and longitudes 0.25 degrees apart.
Skewed rains do not guarantee heavy rains
The two trends – days that take a certain proportion of monsoon rain to fall and its intensity – can diverge when total monsoon rain is decreasing. This is exactly what has happened in north-central India, the Indo-Gangetic plains, and the northeastern states. With the decrease in total rainfall, the amount of heavy rain has decreased, although the number of days it takes for 50% of monsoon rains to fall has decreased.
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