Monsoon 2022 was statistically normal, not normally normal
An HT analysis of IMD’s gridded dataset – it differs slightly from IMD’s published statistics although it is directionally similar– shows that this year’s rainfall is more a statistical normal than what the term would suggest in practice.
This year’s monsoon season – June 1 to September 30 – is officially over. According to the India Meteorological Department (IMD), 2022 saw 6% more than rainfall than the long period average (LPA). Under IMD’s classification, this quantum of rainfall is defined as a normal monsoon. An HT analysis of IMD’s gridded dataset – it differs slightly from IMD’s published statistics although it is directionally similar– shows that this year’s rainfall is more a statistical normal than what the term would suggest in practice. Here is why.
(PTI)
40% of the country in deficit, Gangetic plains lagged rest of India the most since 1901
A normal monsoon rainfall at the all-India level does not mean all parts of the country experienced the same. This is best seen through the accompanying map. Of the 4,685 grids in IMD’s gridded dataset – one grid captures the area between two latitudes and longitudes 0.25 degrees apart in this data – 40% have a deficit and 22% (1,041 grids) have a deficit of 20% or more. Most of the latter are in the four states of the Gangetic plains -- Uttar Pradesh, Bihar, Jharkhand, and Bihar – that produce one-third of the country’s rice. The rain the four states received towards the end of the season did nothing to fix this geographical skew that HT first pointed out in August (https://bit.ly/3Rmg9O5). Instead of getting 22.7% more rain than the rest of the country as in the 1961-2010 average, the four states combined received 22.2% less rain than the rest of the country, the most they have lagged since 1901. To be sure, although this may not have implications for crop output, Ladakh (30.8% deficit), Delhi (26.6% deficit) and Manipur (24.3% deficit) also fall in the category.
See Map 1
One of four days was very dry, one of three, very wet
India received 931 mm of rain in the official summer monsoon , according to IMD’s gridded dataset. This is the 24th highest since 1901, and 9.1% more than the average 853.7 mm rain in the 1961-2010 period . This surplus, however, was built from several very dry and very wet patches. On a quarter of days (31 of 122 days of monsoon), there was a deficit of 20% or more; and on a third (41 days), there was a surplus of 20% or more. Only on half of the days was the rain was within 20% of the 1961-2010 average.
See Chart 1
It is heavy intensity rain that accounts for the surplus
What was the quality of rain this monsoon? As rain and temperature trends suggest, the 9.1% surplus was not because of regular drizzle. Compared to the overall surplus of 9.1%, light rain – rain less than 7.5 mm in a grid in 24 hours – has only a 3.8% surplus. Even moderate rain (7.5 mm to 35.5 mm rain in a grid in 24 hours) has a smaller surplus than the overall rain: of 6.9%. But heavy rain – 35.5 mm to 244.5 mm rain in a grid in 24 hours – has a surplus of 12.7%. Extreme rain – more than 244.5 mm in a grid in 24 hours – has an even bigger 20.1% surplus. To be sure, heavy and extreme rainfall numbers referred to here may not necessarily mean flooding or similar disasters. For example, 1,000 mm rain distributed over three days and 1,000 mm rain concentrated in a single day would both be extreme rain in IMD’s gridded dataset. The 2022 monsoon was more like the latter. A month-by-month breakdown of shows that the surplus in heavy and extreme rain was also concentrated largely in July and August.
See Chart 2
Among the 10 hottest monsoons for eight states
As expected from the rainfall trends, temperatures were also extreme this monsoon. By maximum temperature, about 19% (58 of 300 grids in IMD’s gridded temperature dataset) of the country was 1°C warmer than the 1981-2010 average (used as the “normal” for temperature). Around 15% of the country (44 of 300 grids), most in the western and peninsular regions, was also cooler by 1°C or more. Just how unusual is this much deviation? For around 5% of the country (16 of 300 grids), this was the hottest monsoon since 1951, the earliest year for which IMD has grid-level data; and for another 6% (18 of 300 grids), this was the 2nd or 3rd hottest monsoon season. At the state level, this was among the top 10 hottest monsoons in eight states: Uttar Pradesh, Bihar, West Bengal, Kerala, Assam, Manipur, Mizoram, and Tripura. Delhi, which has been blowing hot and cold this season in different parts of the season (https://bit.ly/3LWji5Q), has ended up in the middle for the season as a whole: 32nd hottest since 1951. This “part hot, part cold” trend being also the geographic trend for India, the average maximum temperature for India as a whole is also ranked in the middle: 35th hottest since 1951.
E-Paper
