ENSO has had a greater impact on north India, lesser on central India and relatively constant impact on south India; one of the reasons cited by the study is global warming
Rain in Jaipur. Monsoon depressions now move towards Rajasthan and Madhya Pradesh. Photo: iStock The El Nino Southern Oscillation (ENSO) has had a greater impact on northern parts of India, lesser impact on the central parts and relatively constant impact on the southern parts of the country in recent decades, according to a research paper published in the journal Nature Scientific Reports in August 2023. One of the reasons for this is the impact of global warming.
ENSO occurs due to the warming or cooling of sea surface temperatures over the equatorial Pacific Ocean beyond long term average.
The warming phase known as the El Nino is generally known to suppress monsoon rainfall while the cooling phase known as the La Nina generally enhances monsoon rainfall.
“El Nino causes subsidence or downward movement of air which in turn suppresses monsoon rainfall”, Vineet Kumar Singh, one of the co-authors of the paper told Down To Earth (DTE).
On July 4, the World Meteorological Organization had announced the prevalence of El Nino conditions and the India Meteorological Department announced the same on July 31. A three-year La Nina phenomenon had ended in March 2023.
Monsoon rainfall in India gets influenced by both global factors such as the ENSO and localised factors such as the strength of the monsoon trough, which is a line of low pressure areas, and the frequency of low pressure areas that generally form in the Bay of Bengal, known as monsoon depressions, and cause rainfall during the monsoons.
The authors from various institutions in India and South Korea analysed data from 1901 to 2018 and found that both strength of the monsoon trough measured in terms of vorticity or the favourable conditions required for rainfall to occur and low pressure areas show declining strength across India over the period.
“In recent decades, the El Nino is also forming much later in the year than before. Earlier, it would begin around February or March but now it begins around June or July like it did this year,” said Singh, who is also research scientist at the Typhoon Research Center, Jeju National University, South Korea.
This means that the impact of the El Nino on monsoon rainfall is also delayed and over central India, this has meant the lack of rainfall being compensated by rainfall happening due to warming of the Arabian Sea.
A warm Arabian Sea, due to global warming, sends moisture across to central India with east-moving winds, according to Singh. The compensation is leading to a weakening of the relationship between monsoon rainfall over central India and the El Nino.
On the other hand, in recent decades, the monsoon depressions have been moving in a more southward direction.
“The monsoon depressions used to move more towards Uttar Pradesh but now they move more towards Madhya Pradesh and Rajasthan. This has decreased their influence over northern India which, in turn, has increased the influence of El Nino on the region,” says Singh.
The reason for the southward movement of the monsoon depressions is currently unknown, according to Singh.
Between 1901 and 1940, the relationship between ENSO and monsoon rainfall increased in strength. The strength was maintained between 1941 and 1980, and started decreasing around 1981 and came to a moderate strength until 2018, according to the paper.
How the relationship between ENSO with monsoon rainfall over different parts of India would change with further global warming is quite uncertain as the ENSO itself is changing with the changing climate.
“Along with the change in the onset of the ENSO, the region where it forms has also shifted from the extreme eastern Pacific to the east central Pacific Ocean, a change of 5-7 degrees of longitude,” said Singh.
He added that this differential impact of the El Nino on various regions of India should be incorporated into climate modelling for forecasting monsoon rainfall. This would improve regional rainfall forecasts during ENSO events.
We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.
.png)
Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.