El Niño and the Indian monsoon: Instead of panic, let’s be vigilant

El Niño events are known to have significant impacts on the Indian monsoon. During an El Niño event, sea surface temperatures in the equatorial Pacific rise, which can lead to changes in atmospheric circulation patterns in the region. (File image)

The US National Oceanic and Atmospheric Administration prediction of an El Niño after three years of La Niña have raised anxieties about the impact this could have on the southwest monsoon that contributes most of India’s rainfall. The worries are also heightened on account of the impact climate events are having on agriculture. Last year’s unusually high March temperatures had dented the rabi crop yield.

What Is El Niño?

El Niño refers to a warming of the central and eastern tropical Pacific Ocean that can have profound effects on global weather patterns. El Niño events are known to have significant impacts on the Indian monsoon. During an El Niño event, sea surface temperatures in the equatorial Pacific rise, which can lead to changes in atmospheric circulation patterns in the region.

El Niño intensity typically peaks near the end of the year. El Nino may result in a weakening of the Indian monsoon, leading to drier conditions and reduced rainfall. On the other hand, La Niña events, which bring cooling to the Pacific Ocean, can result in stronger monsoons and more precipitation in India.

An El Niño event can be identified by the variations in sea surface temperature (SST) over the equatorial Central Pacific (the Nino 3.4 region). El Niño events are classified as weak, moderate, strong, and very strong depending on the strength of the positive SST variations. An El Niño event is announced when monthly Nino 3.4 SST deviations reach +0.50 °C, along with consistent atmospheric features, and when these anomalies persist for three consecutive months.

Impact On Indian Monsoon

However, there is no one-to-one correspondence between El Niño and the Indian summer monsoon; the relationship is much more complex. The exact impact may vary depending on the strength and timing of the El Nino event as well as the transition phase. In addition, other important global influences from the Indian Ocean (Indian Ocean Dipole) and the Atlantic Ocean (Atlantic Multidecadal Oscillation) also play an important role in modulating the Indian monsoon.

The long-term average of seasonal monsoon rainfall (June to September) for the whole country is 88 cm. The Indian monsoon rainfall is known to show large variations from year to year (with a coefficient of variation of 10 per cent). Historical data shows that there were 15 moderate and strong El Nino events between 1951 and 2022.

Of these years, the Indian monsoon was deficient (rainfall deficit of more than 10 per cent) in 8 years and in another 3 years the monsoon rainfall was on the lower side of the norm (deficit of more than 5 per cent but less than 10 per cent). Therefore, in a moderate to strong El Nino year, there is a 73 per cent chance that monsoon rainfall in India will be below the long-term normal. The last major El Nino event was in 2015 and Indian monsoon rainfall was 13 per cent lower.

Predicting El Nino

Can we predict the development of El Nino in advance? Yes, thanks to systematic global research efforts, we are now able to reliably predict these events at least 5-6 months in advance. To this end, we use advanced climate models that explicitly address the physics and dynamics of the atmosphere and oceans and their interactions using high-end supercomputers.

Over the past three years, the La Nina event persisted over the Pacific Ocean, contributing to the above-average monsoon rains over India. Most climate models are now predicting the transition from La Nina to El Nino in the period February to April 2023. The probability of an El Nino remains low until May-July 2023 (only 44 per cent chance), but thereafter the probability of an El Nino increases to 55 per cent.

The probability of El Nino thus increases with longer forecast periods, although uncertainty remains high due to the spring predictability barrier in climate models, which is usually associated with reduced accuracy. The models suggest that the 2023 El Nino could be near the moderate category with SST deviations close to 1.0⁰C.

Better Forecasts Coming

A more reliable forecast will be available by May/June, which will give us more accurate information on the intensity and spatial structure that will determine the impact on the Indian monsoon.

As mentioned earlier, we should all take into account how other global forces from the Indian Ocean and Atlantic behave, and whether they are unfavourable in nature. Climate models are currently predicting the development of a positive dipole in the Indian Ocean later this year, which should favour a normal monsoon.

In summary, we should expect an El Nino event by August-September. Predicting El Nino and taking measures to prepare for it can help mitigate the impact on the population and ensure food security in times of drought. However, we are not very confident at present about the characteristics of the event, such as its intensity and spatial structure.

The India Meteorological Department (IMD) will issue its first official assessment of the 2023 monsoon outlook in mid-April and an update in late May. They use coupled climate models that explicitly take into account El Nino development to predict the outlook for the Indian monsoon rainfall. In the official statistical prediction model, El Nino indices are considered.

Let us not panic, but be vigilant.

M Rajeevan is an MoES Distinguished Scientist and former Secretary of the Union Ministry of Earth Sciences. Views are personal and do not represent the stand of this publication.