Photo: Dave Hoefler/Unsplash
- The delayed withdrawal of the monsoon has, over the past 50 years, become an established trend.
- While the current rain level – 99% of the long-term average – seems to be normal, the rainfall in reality varied widely across the four monsoon months.
- The combination of concentrated spells temporally as well as spatially has meant a highly erratic distribution in which rain is received in extremely heavy spells over a small area, or not at all.
Two weeks after the commencement of India’s second most-delayed monsoon withdrawal, rain has continued to linger in many parts of the country. According to the latest withdrawal charts prepared by the India Meteorological Department (IMD), the retreat from Maharashtra, delayed by about a week to 10 days compared to the long period average, is now nearing completion, while most of peninsular and coastal India is still under southwest monsoon circulation.
The withdrawal from mainland India commenced on October 6, 19 days later than normal. Incidentally, the delay means that the southwest monsoon will now effectively spill over into the northeast monsoon season, which is particularly active in the south of the country, and is officially set to begin on October 20. Kerala has already received 84% of the northeast monsoon rainfall in the first 17 days of October.
The delayed withdrawal of the monsoon has, over the past 50 years, become an established trend. While the normal withdrawal date of September 1 was revised to September 17 based on observations from 1971-2019, actual withdrawal dates have been at least a week late in four of the past five years, which indicates a continued elongation of this trend.
During the southwest monsoon this year, India received 874.6 mm rainfall, about 99.32% of the normal southwest monsoon rainfall. The country received “normal” rainfall during the four-month monsoon season from June to September, IMD said. September recorded a positive departure of 34.96% rainfall due to circulations contributed from the Bay of Bengal, ending with Cyclone Gulab, which helped to fill the deficit formed during the months of July and August.
According to IMD, the season also saw the formation of 12 cyclonic circulations – low pressure (LP), well-marked low pressure area (WML), depression, deep depression and cyclone—which are critical during the rainfall period. June saw one LP, July recorded two LP and one WML. August recorded two LPAs. However, September recorded one cyclone (Gulab), one deep depression and two WML and one LP.
A deep depression was formed during September 12-15 and cyclonic storm Gulab was formed during September 24-28.
Onset of southwest monsoon
The southwest monsoon made an onset over Kerala on June 3 and covered the entire country on July 13—five days later than usual. The monsoon normally covers the entire country by July 8. However, the trend was different last year, when the monsoon covered the entire country by June 26.
According to IMD, the formation and movement of Cyclone Tauktae over the Arabian Sea (during May 14-19) and severe cyclonic storm Yaas over the Bay of Bengal (during May 23 to 28) helped increase cross-equatorial flow and the onset of monsoon.
In the past five years, the maximum delay to cover the entire country was recorded in 2018. Moreover, the same year recorded the earliest onset of the monsoon. Apart from 2018, the early onset of the monsoon was also recorded in 2017 and for the rest, it was usually delayed (except 2020), during the past five years.
Three consecutive years
India received 867.8 mm of rainfall during the southwest monsoon, which is 99% of its long-period average (LPA). LPA is the average of rainfall received by India over a 50-year period between 1951 and 2001. The current LPA of all southwest monsoon rainfall is 880.6 mm.
While 2019 recorded the highest percentage (110%) of its LPA followed by 2020, this year’s LPA is the third-highest in the past five years.
According to the IMD, the country received “normal” rainfall during the four-month southwest monsoon season from June to September. This is for the third consecutive year that the country has recorded rainfall in the normal or above-normal category. Rainfall was above normal in 2019 and 2020.
Rainfall over the northwest and central region was normal, while the south peninsula region recorded above normal rainfall. However, it was below normal over the east and northeast regions.
The southern peninsula has been receiving normal or above normal rainfall for the past three years, while the east and northeast region has been receiving normal or below normal rain for the past five years.
Positive departure in September, again
While the current rainfall level (99% of LPA) seems to suggest normal rainfall, in reality, the rainfall varied widely across the four monsoon months.
After an active start to the season, monsoon rains remained deficient over the crucial core monsoon months of July and August. The rainfall over the country as a whole was 110% in June, 93%, and 76% in July and August respectively. However, the shortfall of July and August was compensated in September, which recorded rainfall 135% of the LPA.
India recorded the maximum departure in September (+34.96%) and the minimum in August (-24.13%). September has been showing a positive departure from the normal rainfall (170.2 mm) for the last three years, recording the maximum positive departure in 2019 (+52%).
On the other hand, August has recorded the maximum negative departure this year in comparison to the past five years. Last year, August recorded excess rainfall (+26.6%) and in the previous four years, it observed normal rainfall.
In terms of the spatial distribution, too, the monsoon this year was a season of extremes – with wild swings between extreme deficiency to surplus, culminating in an overall “normal”. Distribution charts show that typically deficit rain regions such as West Madhya Pradesh, East Rajasthan, and Maharashtra’s Marathwada and Vidarbha have all seen surplus rains, while the rainiest pockets in Kerala, Odisha and the north-eastern states have recorded deficits.
The combination of concentrated spells temporally as well as spatially has meant a highly erratic distribution in which rainfall is received over extremely heavy spells over a small area, or not at all. “Monsoon 2021 has recorded 874.6mm against the normal of 880.6mm. With this, we can say that rains are more, but rainy days are still less. There are large gaps between the rainy spells, increasing the dry spells,” said Mahesh Palawat, vice-president, meteorology and climate change at Skymet Weather.
Southwest monsoon trends
According to IMD, out of the total 36 meteorological subdivisions, 20 subdivisions received normal seasonal rainfall, 10 subdivisions received excess rainfall and six subdivisions received deficient seasonal rainfall.
Nagaland, Manipur, Mizoram and Tripura, Assam and Meghalaya, Arunachal Pradesh, Jammu & Kashmir and Ladakh, West Uttar Pradesh, and Lakshadweep are the six subdivisions that received deficient rainfall. Out of these six subdivisions, three lie in northeast India.
The number of rainfall-deficient regions has increased by 200% this year as compared to 2019. It has been increasing for three consecutive years, with 2018 observing the most rainfall-deficient regions (11).
Additionally this year, 10 regions received excess rainfall, which is more than that observed in 2016, 2017 and 2018. West Rajasthan, Haryana, Delhi, Andhra Pradesh, Telangana, north interior Karnataka, Gangetic West Bengal, Konkan and Goa, Marathwada and Andaman and Nicobar recorded excess rainfall during the monsoon season.
More global connections
While the el Nino Southern Oscillation (ENSO) in the Pacific Ocean has long been recognised as a key driver of the performance of India’s monsoon, new global connections to circulatory systems are becoming apparent as a result of climate change. Circulation changes in faraway Arctic and the Atlantic regions have revealed such connections, which left a significant imprint of the 2021 monsoon performance.
“Reduced sea-ice in the Arctic during summer, especially over the Kara Sea, leads to high sea-level pressure over Western Europe and Northeastern China, which steer planetary waves southeastward instead of their eastward trajectory. And these waves enter India late in the season to produce circulation anomalies in the upper atmosphere, resulting in heavy rainfall in September,” according to Raghu Murtugudde, an earth system scientist at CMNS-Atmospheric & Oceanic Science.
“The erratic evolution of the monsoon through July and August can be attributed to the little brother of the El Niño from the Atlantic. Atlantic Niño’s impact on the monsoon was established in 2014 when an INCOIS led study showed that the number of low-pressure systems is sharply reduced by the Atlantic Niño, leading to deficit monsoons. This season, the sea-surface temperatures over the tropical eastern Atlantic were warmer than normal. The strongest Atlantic Niño event of the past 40 years occurred during June-August. We can blame the significantly fewer low-pressure systems in 2021 on the Atlantic Niño. Monsoon 2021 is a clear example of this missed link,” he added.
This article was first published by CarbonCopy and has been republished here with permission.