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A significant paper published in Nature’s Scientific Reports said that climate change has emerged as a dominant factor in controlling the Indian monsoon in terms of the core monsoon zone along central India. The study also concluded that the north Indian region where the dominance of the monsoon is reducing over the last few years is increasingly being dependent on the ENSO (El Nino Southern Oscillation) in terms of rainfall variability.
The paper, based on work led by the Indian Institute of Tropical Meteorology, found that the relationship between El Nino Southern Oscillation (ENSO) and the monsoon has not remained the same since 1901, with a weakening trend observed from 1981 .
Based on gridded monsoon rainfall data from IMD (India Meteorological Department), the Indian Institute of Tropical Meteorology team found that the relationship between El Nino Southern Oscillation (ENSO) and the monsoon became stronger from 1901 to 1940, stable from 1941 to 1980 and then weakened (1981 onwards) with regional variations.
“The monsoon rainfall exhibits a strong relationship with the El Niño/ENSO, but is different across regions over India and has varied over time frames during the last century. While the ENSO–monsoon relationship has stayed moderately strong and stable for south India, it has become exceptionally stronger for north India, and considerably weakened and non-existent over the central Indian region (core monsoon zone) in the recent decades,” Roxy Koll, lead IPCC author and co-author of the study posted on X.
The researchers involved in the story stated that the warming Indian Ocean, because of climate change, might be a factor behind the decreasing strength of the monsoon and its failing to reach the north Indian region in recent decades.
“The climate change impact is much more pronounced over the core monsoon zone along central India, where the dominance of El Nino has reduced in recent decades. Our earlier studies show that climate change and Indian Ocean warming is resulting in a reduction in the total amount of rainfall, and a concurrent increase in the number of short-spell extreme rainfall events,” Roxy Koll, co-author of the study, said while speaking to Hindustan Times, “This weakening is mostly due to the reduction in El Nino control over the core monsoon zone. Climate change impact, for example the Indian Ocean warming, has emerged as a dominant factor controlling monsoon dynamics.”
The study further explained that climate change induced factors such as increased surface warming over Eurasia, strengthening and poleward shift of the jet streams over the North Atlantic, increased greenhouse gas concentration and shifts in the surface wind circulation pattern over the Indo-Pacific region are leading to a weakening of ENSO and monsoon’s inverse relationship after the 1980s.
About ENSO
The equatorial Pacific Ocean experiences ENSO, a phenomenon when sea surface temperatures rise or fall above the long-term norm. It is an ocean-atmosphere coupled phenomenon characterized by a periodic fluctuation in the sea surface temperature (SST) as well as air pressure across the Equatorial Pacific Ocean.
ENSO has three phases: (i) El Nino is the warm phase, of the ENSO which is said to occur when the water in the central and eastern tropical Pacific Ocean warms up due to higher than normal SST; (ii) La Nina is the cool phase of ENSO which occurs when the water in the central and eastern tropical Pacific Ocean cools down due to lower than normal SST; (iii) Neutral: During the neutral state of the ENSO the waters in the tropical Pacific Ocean maintains nearly average SST. Rainfall during the monsoon is typically reduced during the warming phase known as El Nino and generally increased during the cooling phase, known as La Nina.
What the study entails
The study examines the variations in ENSO and Indian Summer Monsoon Rainfall (ISMR) relationship, during the summer monsoon months i.e. June, July, August and September. The entire period from 1901 to 2018 is divided into three periods: 1901–1940, 1941–1980, and 1981–2018 based on changes in the ENSO–ISMR relationship. Over northern, central, and southern parts of India, the ENSO-ISMR relationship shows distinct regional variability with a time-varying relationship.
It is known that there exists an inverse relationship between ENSO–ISMR. However, this inverse relationship shows a weakening trend in the recent decade after the 1980s due to several factors like, (a) to several factors such as increased surface warming over Eurasia, (b) strengthening and poleward shift of the jet streams over the North Atlantic, (c ) increased greenhouse gas concentration and (d) shift in the surface wind circulation pattern over the Indo-Pacific region.
- Between 1901 and 1940, the strength of the teleconnection between ENSO and ISMR steadily increased from moderate to significantly strong. Between 1941 and 1980, this improved link remained steady and strong. However, the teleconnection steadily weakened to a modest level in the more recent time between 1981 and 2018.
- El Nino has a significant impact on India’s southwest monsoon. There is a strong correlation between warmer summers and weaker monsoon rains in India during El Nino years, which are characterized by an unusual warming of waters in the eastern equatorial Pacific.
- Between June 1 and July 31, there was 5% more rainfall than normal across India, with 31% more in the northwest of India, 12% more in the center, 5% more in the peninsula, and 25% less in the east and northeast.
- There was 13% more rain than normal in July 2023, 25% more in northwest India, 32% less in east and northeast India, 22% more in central India, and 45% more in the southern peninsula. This scenario was witnessed despite the onset of El Nino conditions.
- With growing El Nino’s influence over north India, there will be a reduction in the rainfall due to the weather phenomenon. However, La Nina would result in increased rains.
What do the experts say?
In terms of monsoon forecasting, Dr Koll added that a stronger El Nino-monsoon correlation over north and south India could help in improving the forecasting in these regions. However, the weakening El Nino dominance in the central region has made predictions less predictable and factors such as warming of the Indian ocean must be taken into consideration in such cases. “The weakening El Nino dominance has, however, made seasonal predictions in this region less predictable. As a result, there is a need to monitor other factors, such as the Indian Ocean warming, that influences the strength of the monsoon trough and depressions,” Koll said.
Vineet Kumar, researcher, Typhoon Research Centre, Jeju National University, South Korea, and co-author of the paper said, “This year the impact of El Nino was limited till the end of July, which is mainly due to the lack of atmospheric response to the evolving conditions till date. It is in line with our study that the impact of El Nino is limited on the core monsoon zone rainfall. However, in the later half of the monsoon season some negative impact might be there on rainfall due to El Niño.”
M Mohapatra, director general, IMD acknowledged that the topic needs to be studied in detail. “Some research papers are showing that the ENSO relationship with the monsoon has weakened over central India. This is something that we need to study in detail,” Mohapatra said.
M Rajeevan, former secretary, ministry of earth sciences, however, said that El Nino should not be underestimated. “What is more important is the decrease in correlation over the monsoon core region. It is the region with maximum rains and variability. This could be due to changes in the monsoon trough and low pressure systems during the season. We should not underestimate El Nino,” said Rajeevan.
References:
https://www.nature.com/articles/s41598-023-38730-5
https://www.cell.com/the-innovation/pdf/S2666-6758(21)00027-8.pdf
Image: https://www.pexels.com/photo/rainfall-on-a-forest-2609106/
