El Niño and the monsoon

For India, the weakening of El Niño conditions, and the forecast of the emergence of ENSO neutral conditions towards the end of the summer should bring some cheer ahead of the southwest monsoon

EARLIER THIS month, the India Meteorological Department (IMD) predicted a hot summer this year, with above-normal temperatures and an above-normal number of heatwave days from March to May.

On Tuesday, the World Meteorological Organization (WMO), a specialised agency of the UN, said in a media release that the 2023-24 El Niño, one of the five strongest on record, has peaked and is gradually weakening — but it will continue to impact climate around the world in the coming months.

What are the implications for India and the southwest monsoon?

First, how has the ongoing El Niño event played out?

El Niño refers to the abnormal warming of sea surface waters in the equatorial Pacific Ocean. El Niño episodes emerge naturally during autumn-summer in the northern hemisphere, typically once every 2-7 years.

It peaks during winter (Octoberfebruary) before weakening in the subsequent summer, making it a phenomenon that typically lasts for between 9 and 12 months. Occasionally, El Niño will last up to two years in a row.

El Niño episodes affect the global weather, lead to an increase in temperatures and large-scale dryness and droughts, and disrupt normal rainfall patterns globally. Large parts of East Africa have experienced multiple failed rainy seasons in recent years, in part due to El Niño conditions.

After the last episode in 2015-16, El Niño conditions emerged for the first time in June last year — and average temperatures breached new records across most parts of the world in every subsequent month. As the El Niño peaked between November 2023 and January 2024, recorded sea surface temperatures in the Pacific Ocean touched 2 degrees Celsius above the 1991-2020 normal, the WMO said. The 2023-24 El Niño played a significant role in 2023 ending as the warmest year ever recorded.

So what does the peaking and weakening of El Niño mean?

The intensity of an El Niño event is measured

Anomaly of heatwave duration(days) by the temperature values recorded over the sea along the equatorial Pacific Ocean.

One of the key indicators of the sea surface temperatures is the Oceanic Niño Indices (ONI) values (in degree Celsius). Data from the United States National Oceanic and Atmospheric Administration (NOAA) show that after peaking at 2 degree Celsius during November-january, the ONI value has started to decline. The timing of the fall in ONI is important because it suggests that the El Niño will not extend into another year.

What is the heat forecast for India?

The IMD has said that even as they weaken, El Niño conditions will push temperatures above normal in most of the country during March, April, and May (see map).

Above normal maximum temperatures are likely to be experienced over Ladakh, Himachal Pradesh, Rajasthan, Uttar Pradesh, Jharkhand, Odisha, Maharashtra, coastal Gujarat, Karnataka, Kerala, Tamil Nadu,

Lakshadweep and the Andaman and Nicobar islands, the IMD has said in its summer forecast. Nights too, shall remain warmer than usual over almost the entire country, except some regions in Eastern India.

Heat waves are common over Central and Northern Peninsular India during March to June, when day temperatures rise 4.5 degree Celsius above normal. The IMD has warned of longer and harsher heat wave spells this summer, mainly due to prevailing El Niño conditions.

Individual spells of heat wave are expected to last for as long as 12-15 days in Maharashtra, Telangana, and North Karnataka. Heat waves are also expected to sweep over Andhra Pradesh, Gujarat, Rajasthan, Madhya Pradesh, Uttar Pradesh, Jharkhand, Odisha, West Bengal, Bihar, Delhi, Haryana and Punjab in the coming months.

And how is the Indian summer monsoon likely to be?

The four-month southwest monsoon is

India’s economic lifeline. The country receives nearly 70% of its annual rainfall (880 mm) during the June to September period. A good monsoon is crucial for sustaining the agrarian economy and for replenishment of the country’s water reservoirs.

Although no direct one-on-one link has been established yet, El Niño episodes are correlated with a depressed southwest monsoon. Past El Niño episodes have coincided with India receiving below average rainfall during the four-month rainy season. The 2015-16 El Niño coincided with a deficient monsoon in 2015, at 84% below the Long Period Average.

However, climatologically and historically, about 50% of El Niño events are followed by ENSO (El Niño Southern Oscillation) neutral conditions, and in 4050% cases, La Niña conditions develop post an El Niño event.

This time, multiple global climate models have suggested that ENSO neutral conditions will emerge during the April-june period. India's Monsoon Mission Climate Forecast System (MMCFS) model has predicted ENSO neutral conditions to be established along the equatorial Pacific Ocean by May. The transition from El Niño to ENSO neutral will take some time after that.

If these conditions are realised, the desired establishment of favourable oceanic conditions could take place ahead of the onset phase of the southwest monsoon over the country.

The southwest monsoon arrives over the Andaman and Nicobar Islands around mid-may. Thereafter, it progresses and enters the mainland through the gateway in Kerala around June 1 (with a standard deviation of four days). By mid-july, it covers the entire country.

As the country awaits the Long Range Forecast (LRF) for the upcoming southwest monsoon, expected to be released by the IMD by mid-april, the prediction about the development of ENSO neutral conditions would suggest that the southwest monsoon may not be directly impacted by the ongoing El Niño episode.

There are chances of normal monsoon rainfall — provided other ocean-atmosphere parameters, wind conditions, formation of low pressure systems, etc remain favourable — this year.