Study maps link between El Nino and monsoon

The paper explained that the monsoon trough and formation of monsoon depressions emerged as the primary cause of rainfall variability over central India, surpassing the ENSO effect

The relationship between the El Nino and the monsoon has weakened over the monsoon core zone or central India over the years, but strengthened in north India, a new paper published in Nature’s Scientific Reports said on Wednesday.

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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 team found that the relationship became stronger from 1901 to 1940, was stable from 1941 to 1980, then weakened (1981 onwards). But even within these, there were regional variations.

“Over south India, there is no considerable variation in the ENSO–monsoon relationship. Whereas over north India the ENSO–monsoon relationship is becoming strong in recent decades. On the contrary, association between the rainfall over central India (core monsoon zone) and ENSO has diminished in the recent decades,” the authors of the paper said in a note.

The strengthening of El Nino’s influence over north India means that the weather phenomenon will usually lead to reduced rainfall over this region, while a La Nina on the other hand would lead to enhanced rains, said Roxy Mathew Koll, scientist and co-author of the paper.

El Nino has a strong influence on the southwest monsoon in India. El Nino years are characterised by an unusual warming of waters in the eastern equatorial Pacific, which has a high correlation with warmer summers and weaker monsoon rains in India. La Nina is the reverse.

The paper explained that the monsoon trough and formation of monsoon depressions emerged as the primary cause of rainfall variability over central India, surpassing the ENSO effect. For rainfall over south India, the influence of ENSO and the strength of the monsoon trough and depressions have been consistent since 1901. Over north India, rainfall variability is increasingly dependent on the ENSO, while the role of the monsoon trough and depressions is decreasing.

“This may be due to the decreasing strength of the monsoon due to Indian Ocean warming, and the weakening reach of the monsoon depressions into the north Indian region in recent decades,” the note added.

“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,” Koll said.

“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,” he added.

ENSO and the monsoon’s inverse relationship is showing a weakening trend in recent decades after the 1980s owing to several 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, the paper explained.

“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,” said M Mohapatra, director general, IMD.

“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 M Rajeevan, former secretary, ministry of earth sciences.

There was 5% excess rainfall over the country between June 1 and July 31 with 31% excess over northwest India, 12% excess over central India, 5% excess over peninsular India, and 25% deficiency over east and northeast India. In July alone, there was 13% excess rain over the country, 25% excess over northwest India, 32% deficiency over east and northeast India, 22% excess over central India and 45% excess over South Peninsula. This was despite the onset of El Nino conditions.

Weak El Nino conditions are prevailing at present, which are expected to intensify and become moderate during the next two months. This is likely to impact monsoon rainfall during the remaining period of the season, IMD said on July 31. Monsoon rains over the country during the second half of the season are expected to be on the negative side of the ‘normal’ category, ranging between 94% and 99% of the long-period average (LPA). Rainfall during August is also likely to be ‘below normal’, ranging between 90% and 94% of LPA, IMD has forecast.

“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 Nino,” said Vineet Kumar, researcher, Typhoon Research Centre, Jeju National University, South Korea, and co-author of the paper.